Check libpng directly into third_party/

third_party/libpng/contrib/libtests/pngvalid.c

+
+/* pngvalid.c - validate libpng by constructing then reading png files.
+ *
+ * Last changed in libpng 1.6.22 [(PENDING RELEASE)]
+ * Copyright (c) 2014-2016 Glenn Randers-Pehrson
+ * Written by John Cunningham Bowler
+ *
+ * This code is released under the libpng license.
+ * For conditions of distribution and use, see the disclaimer
+ * and license in png.h
+ *
+ * NOTES:
+ *   This is a C program that is intended to be linked against libpng.  It
+ *   generates bitmaps internally, stores them as PNG files (using the
+ *   sequential write code) then reads them back (using the sequential
+ *   read code) and validates that the result has the correct data.
+ *
+ *   The program can be modified and extended to test the correctness of
+ *   transformations performed by libpng.
+ */
+
+#define _POSIX_SOURCE 1
+#define _ISOC99_SOURCE 1 /* For floating point */
+#define _GNU_SOURCE 1 /* For the floating point exception extension */
+
+#include <signal.h>
+#include <stdio.h>
+
+#if defined(HAVE_CONFIG_H) && !defined(PNG_NO_CONFIG_H)
+#  include <config.h>
+#endif
+
+#ifdef HAVE_FEENABLEEXCEPT /* from config.h, if included */
+#  include <fenv.h>
+#endif
+
+#ifndef FE_DIVBYZERO
+#  define FE_DIVBYZERO 0
+#endif
+#ifndef FE_INVALID
+#  define FE_INVALID 0
+#endif
+#ifndef FE_OVERFLOW
+#  define FE_OVERFLOW 0
+#endif
+
+/* Define the following to use this test against your installed libpng, rather
+ * than the one being built here:
+ */
+#ifdef PNG_FREESTANDING_TESTS
+#  include <png.h>
+#else
+#  include "../../png.h"
+#endif
+
+#ifdef PNG_ZLIB_HEADER
+#  include PNG_ZLIB_HEADER
+#else
+#  include <zlib.h>   /* For crc32 */
+#endif
+
+/* 1.6.1 added support for the configure test harness, which uses 77 to indicate
+ * a skipped test, in earlier versions we need to succeed on a skipped test, so:
+ */
+#if PNG_LIBPNG_VER >= 10601 && defined(HAVE_CONFIG_H)
+#  define SKIP 77
+#else
+#  define SKIP 0
+#endif
+
+/* pngvalid requires write support and one of the fixed or floating point APIs.
+ */
+#if defined(PNG_WRITE_SUPPORTED) &&\
+   (defined(PNG_FIXED_POINT_SUPPORTED) || defined(PNG_FLOATING_POINT_SUPPORTED))
+
+#if PNG_LIBPNG_VER < 10500
+/* This deliberately lacks the const. */
+typedef png_byte *png_const_bytep;
+
+/* This is copied from 1.5.1 png.h: */
+#define PNG_INTERLACE_ADAM7_PASSES 7
+#define PNG_PASS_START_ROW(pass) (((1U&~(pass))<<(3-((pass)>>1)))&7)
+#define PNG_PASS_START_COL(pass) (((1U& (pass))<<(3-(((pass)+1)>>1)))&7)
+#define PNG_PASS_ROW_SHIFT(pass) ((pass)>2?(8-(pass))>>1:3)
+#define PNG_PASS_COL_SHIFT(pass) ((pass)>1?(7-(pass))>>1:3)
+#define PNG_PASS_ROWS(height, pass) (((height)+(((1<<PNG_PASS_ROW_SHIFT(pass))\
+   -1)-PNG_PASS_START_ROW(pass)))>>PNG_PASS_ROW_SHIFT(pass))
+#define PNG_PASS_COLS(width, pass) (((width)+(((1<<PNG_PASS_COL_SHIFT(pass))\
+   -1)-PNG_PASS_START_COL(pass)))>>PNG_PASS_COL_SHIFT(pass))
+#define PNG_ROW_FROM_PASS_ROW(yIn, pass) \
+   (((yIn)<<PNG_PASS_ROW_SHIFT(pass))+PNG_PASS_START_ROW(pass))
+#define PNG_COL_FROM_PASS_COL(xIn, pass) \
+   (((xIn)<<PNG_PASS_COL_SHIFT(pass))+PNG_PASS_START_COL(pass))
+#define PNG_PASS_MASK(pass,off) ( \
+   ((0x110145AFU>>(((7-(off))-(pass))<<2)) & 0xFU) | \
+   ((0x01145AF0U>>(((7-(off))-(pass))<<2)) & 0xF0U))
+#define PNG_ROW_IN_INTERLACE_PASS(y, pass) \
+   ((PNG_PASS_MASK(pass,0) >> ((y)&7)) & 1)
+#define PNG_COL_IN_INTERLACE_PASS(x, pass) \
+   ((PNG_PASS_MASK(pass,1) >> ((x)&7)) & 1)
+
+/* These are needed too for the default build: */
+#define PNG_WRITE_16BIT_SUPPORTED
+#define PNG_READ_16BIT_SUPPORTED
+
+/* This comes from pnglibconf.h afer 1.5: */
+#define PNG_FP_1 100000
+#define PNG_GAMMA_THRESHOLD_FIXED\
+   ((png_fixed_point)(PNG_GAMMA_THRESHOLD * PNG_FP_1))
+#endif
+
+#if PNG_LIBPNG_VER < 10600
+   /* 1.6.0 constifies many APIs, the following exists to allow pngvalid to be
+    * compiled against earlier versions.
+    */
+#  define png_const_structp png_structp
+#endif
+
+#ifndef RELEASE_BUILD
+   /* RELEASE_BUILD is true for releases and release candidates: */
+#  define RELEASE_BUILD (PNG_LIBPNG_BUILD_BASE_TYPE >= PNG_LIBPNG_BUILD_RC)
+#endif
+#if RELEASE_BUILD
+#   define debugonly(something)
+#else /* !RELEASE_BUILD */
+#   define debugonly(something) something
+#endif /* !RELEASE_BUILD */
+
+#include <float.h>  /* For floating point constants */
+#include <stdlib.h> /* For malloc */
+#include <string.h> /* For memcpy, memset */
+#include <math.h>   /* For floor */
+
+/* Unused formal parameter errors are removed using the following macro which is
+ * expected to have no bad effects on performance.
+ */
+#ifndef UNUSED
+#  if defined(__GNUC__) || defined(_MSC_VER)
+#     define UNUSED(param) (void)param;
+#  else
+#     define UNUSED(param)
+#  endif
+#endif
+
+/***************************** EXCEPTION HANDLING *****************************/
+#ifdef PNG_FREESTANDING_TESTS
+#  include <cexcept.h>
+#else
+#  include "../visupng/cexcept.h"
+#endif
+
+#ifdef __cplusplus
+#  define this not_the_cpp_this
+#  define new not_the_cpp_new
+#  define voidcast(type, value) static_cast<type>(value)
+#else
+#  define voidcast(type, value) (value)
+#endif /* __cplusplus */
+
+struct png_store;
+define_exception_type(struct png_store*);
+
+/* The following are macros to reduce typing everywhere where the well known
+ * name 'the_exception_context' must be defined.
+ */
+#define anon_context(ps) struct exception_context *the_exception_context = \
+   &(ps)->exception_context
+#define context(ps,fault) anon_context(ps); png_store *fault
+
+/* This macro returns the number of elements in an array as an (unsigned int),
+ * it is necessary to avoid the inability of certain versions of GCC to use
+ * the value of a compile-time constant when performing range checks.  It must
+ * be passed an array name.
+ */
+#define ARRAY_SIZE(a) ((unsigned int)((sizeof (a))/(sizeof (a)[0])))
+
+/* GCC BUG 66447 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66447) requires
+ * some broken GCC versions to be fixed up to avoid invalid whining about auto
+ * variables that are *not* changed within the scope of a setjmp being changed.
+ *
+ * Feel free to extend the list of broken versions.
+ */
+#define is_gnu(major,minor)\
+   (defined __GNUC__) && __GNUC__ == (major) && __GNUC_MINOR__ == (minor)
+#define is_gnu_patch(major,minor,patch)\
+   is_gnu(major,minor) && __GNUC_PATCHLEVEL__ == 0
+/* For the moment just do it always; all versions of GCC seem to be broken: */
+#ifdef __GNUC__
+   const void * volatile make_volatile_for_gnu;
+#  define gnu_volatile(x) make_volatile_for_gnu = &x;
+#else /* !GNUC broken versions */
+#  define gnu_volatile(x)
+#endif /* !GNUC broken versions */
+
+/******************************* UTILITIES ************************************/
+/* Error handling is particularly problematic in production code - error
+ * handlers often themselves have bugs which lead to programs that detect
+ * minor errors crashing.  The following functions deal with one very
+ * common class of errors in error handlers - attempting to format error or
+ * warning messages into buffers that are too small.
+ */
+static size_t safecat(char *buffer, size_t bufsize, size_t pos,
+   const char *cat)
+{
+   while (pos < bufsize && cat != NULL && *cat != 0)
+      buffer[pos++] = *cat++;
+
+   if (pos >= bufsize)
+      pos = bufsize-1;
+
+   buffer[pos] = 0;
+   return pos;
+}
+
+static size_t safecatn(char *buffer, size_t bufsize, size_t pos, int n)
+{
+   char number[64];
+   sprintf(number, "%d", n);
+   return safecat(buffer, bufsize, pos, number);
+}
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+static size_t safecatd(char *buffer, size_t bufsize, size_t pos, double d,
+    int precision)
+{
+   char number[64];
+   sprintf(number, "%.*f", precision, d);
+   return safecat(buffer, bufsize, pos, number);
+}
+#endif
+
+static const char invalid[] = "invalid";
+static const char sep[] = ": ";
+
+static const char *colour_types[8] =
+{
+   "grayscale", invalid, "truecolour", "indexed-colour",
+   "grayscale with alpha", invalid, "truecolour with alpha", invalid
+};
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+/* Convert a double precision value to fixed point. */
+static png_fixed_point
+fix(double d)
+{
+   d = floor(d * PNG_FP_1 + .5);
+   return (png_fixed_point)d;
+}
+#endif /* PNG_READ_SUPPORTED */
+
+/* Generate random bytes.  This uses a boring repeatable algorithm and it
+ * is implemented here so that it gives the same set of numbers on every
+ * architecture.  It's a linear congruential generator (Knuth or Sedgewick
+ * "Algorithms") but it comes from the 'feedback taps' table in Horowitz and
+ * Hill, "The Art of Electronics" (Pseudo-Random Bit Sequences and Noise
+ * Generation.)
+ */
+static void
+make_random_bytes(png_uint_32* seed, void* pv, size_t size)
+{
+   png_uint_32 u0 = seed[0], u1 = seed[1];
+   png_bytep bytes = voidcast(png_bytep, pv);
+
+   /* There are thirty three bits, the next bit in the sequence is bit-33 XOR
+    * bit-20.  The top 1 bit is in u1, the bottom 32 are in u0.
+    */
+   size_t i;
+   for (i=0; i<size; ++i)
+   {
+      /* First generate 8 new bits then shift them in at the end. */
+      png_uint_32 u = ((u0 >> (20-8)) ^ ((u1 << 7) | (u0 >> (32-7)))) & 0xff;
+      u1 <<= 8;
+      u1 |= u0 >> 24;
+      u0 <<= 8;
+      u0 |= u;
+      *bytes++ = (png_byte)u;
+   }
+
+   seed[0] = u0;
+   seed[1] = u1;
+}
+
+static void
+make_four_random_bytes(png_uint_32* seed, png_bytep bytes)
+{
+   make_random_bytes(seed, bytes, 4);
+}
+
+#if defined PNG_READ_SUPPORTED || defined PNG_WRITE_tRNS_SUPPORTED ||\
+    defined PNG_WRITE_FILTER_SUPPORTED
+static void
+randomize(void *pv, size_t size)
+{
+   static png_uint_32 random_seed[2] = {0x56789abc, 0xd};
+   make_random_bytes(random_seed, pv, size);
+}
+
+#define R8(this) randomize(&(this), sizeof (this))
+
+static void r16(png_uint_16p p16, size_t count)
+{
+   size_t i;
+
+   for (i=0; i<count; ++i)
+   {
+      unsigned char b2[2];
+      randomize(b2, sizeof b2);
+      *p16++ = png_get_uint_16(b2);
+   }
+}
+
+#define R16(this) r16(&(this), (sizeof (this))/(sizeof (png_uint_16)))
+#define R16_1(this) r16(&(this), (size_t) 1U)
+
+#if defined PNG_READ_RGB_TO_GRAY_SUPPORTED ||\
+    defined PNG_READ_FILLER_SUPPORTED
+static void r32(png_uint_32p p32, size_t count)
+{
+   size_t i;
+
+   for (i=0; i<count; ++i)
+   {
+      unsigned char b4[4];
+      randomize(b4, sizeof b4);
+      *p32++ = png_get_uint_32(b4);
+   }
+}
+
+#define R32(this) r32(&(this), (sizeof (this))/(sizeof (png_uint_32)))
+#define R32_1(this) r32(&(this), (size_t) 1U)
+
+#endif /* READ_FILLER || READ_RGB_TO_GRAY */
+
+#endif /* READ || WRITE_tRNS || WRITE_FILTER */
+
+#if defined PNG_READ_TRANSFORMS_SUPPORTED ||\
+    defined PNG_WRITE_FILTER_SUPPORTED
+static unsigned int
+random_mod(unsigned int max)
+{
+   png_uint_16 x;
+
+   R16_1(x);
+
+   return x % max; /* 0 .. max-1 */
+}
+#endif /* READ_TRANSFORMS || WRITE_FILTER */
+
+#if (defined PNG_READ_RGB_TO_GRAY_SUPPORTED) ||\
+    (defined PNG_READ_FILLER_SUPPORTED)
+static int
+random_choice(void)
+{
+   unsigned char x;
+
+   R8(x);
+
+   return x & 1;
+}
+#endif /* READ_RGB_TO_GRAY || READ_FILLER */
+
+/* A numeric ID based on PNG file characteristics.  The 'do_interlace' field
+ * simply records whether pngvalid did the interlace itself or whether it
+ * was done by libpng.  Width and height must be less than 256.  'palette' is an
+ * index of the palette to use for formats with a palette otherwise a boolean
+ * indicating if a tRNS chunk was generated.
+ */
+#define FILEID(col, depth, palette, interlace, width, height, do_interlace) \
+   ((png_uint_32)((col) + ((depth)<<3) + ((palette)<<8) + ((interlace)<<13) + \
+    (((do_interlace)!=0)<<15) + ((width)<<16) + ((height)<<24)))
+
+#define COL_FROM_ID(id) ((png_byte)((id)& 0x7U))
+#define DEPTH_FROM_ID(id) ((png_byte)(((id) >> 3) & 0x1fU))
+#define PALETTE_FROM_ID(id) (((id) >> 8) & 0x1f)
+#define INTERLACE_FROM_ID(id) ((png_byte)(((id) >> 13) & 0x3))
+#define DO_INTERLACE_FROM_ID(id) ((int)(((id)>>15) & 1))
+#define WIDTH_FROM_ID(id) (((id)>>16) & 0xff)
+#define HEIGHT_FROM_ID(id) (((id)>>24) & 0xff)
+
+/* Utility to construct a standard name for a standard image. */
+static size_t
+standard_name(char *buffer, size_t bufsize, size_t pos, png_byte colour_type,
+    int bit_depth, unsigned int npalette, int interlace_type,
+    png_uint_32 w, png_uint_32 h, int do_interlace)
+{
+   pos = safecat(buffer, bufsize, pos, colour_types[colour_type]);
+   if (colour_type == 3) /* must have a palette */
+   {
+      pos = safecat(buffer, bufsize, pos, "[");
+      pos = safecatn(buffer, bufsize, pos, npalette);
+      pos = safecat(buffer, bufsize, pos, "]");
+   }
+
+   else if (npalette != 0)
+      pos = safecat(buffer, bufsize, pos, "+tRNS");
+
+   pos = safecat(buffer, bufsize, pos, " ");
+   pos = safecatn(buffer, bufsize, pos, bit_depth);
+   pos = safecat(buffer, bufsize, pos, " bit");
+
+   if (interlace_type != PNG_INTERLACE_NONE)
+   {
+      pos = safecat(buffer, bufsize, pos, " interlaced");
+      if (do_interlace)
+         pos = safecat(buffer, bufsize, pos, "(pngvalid)");
+      else
+         pos = safecat(buffer, bufsize, pos, "(libpng)");
+   }
+
+   if (w > 0 || h > 0)
+   {
+      pos = safecat(buffer, bufsize, pos, " ");
+      pos = safecatn(buffer, bufsize, pos, w);
+      pos = safecat(buffer, bufsize, pos, "x");
+      pos = safecatn(buffer, bufsize, pos, h);
+   }
+
+   return pos;
+}
+
+static size_t
+standard_name_from_id(char *buffer, size_t bufsize, size_t pos, png_uint_32 id)
+{
+   return standard_name(buffer, bufsize, pos, COL_FROM_ID(id),
+      DEPTH_FROM_ID(id), PALETTE_FROM_ID(id), INTERLACE_FROM_ID(id),
+      WIDTH_FROM_ID(id), HEIGHT_FROM_ID(id), DO_INTERLACE_FROM_ID(id));
+}
+
+/* Convenience API and defines to list valid formats.  Note that 16 bit read and
+ * write support is required to do 16 bit read tests (we must be able to make a
+ * 16 bit image to test!)
+ */
+#ifdef PNG_WRITE_16BIT_SUPPORTED
+#  define WRITE_BDHI 4
+#  ifdef PNG_READ_16BIT_SUPPORTED
+#     define READ_BDHI 4
+#     define DO_16BIT
+#  endif
+#else
+#  define WRITE_BDHI 3
+#endif
+#ifndef DO_16BIT
+#  define READ_BDHI 3
+#endif
+
+/* The following defines the number of different palettes to generate for
+ * each log bit depth of a colour type 3 standard image.
+ */
+#define PALETTE_COUNT(bit_depth) ((bit_depth) > 4 ? 1U : 16U)
+
+static int
+next_format(png_bytep colour_type, png_bytep bit_depth,
+   unsigned int* palette_number, int low_depth_gray, int tRNS)
+{
+   if (*bit_depth == 0)
+   {
+      *colour_type = 0;
+      if (low_depth_gray)
+         *bit_depth = 1;
+      else
+         *bit_depth = 8;
+      *palette_number = 0;
+      return 1;
+   }
+
+   if  (*colour_type < 4/*no alpha channel*/)
+   {
+      /* Add multiple palettes for colour type 3, one image with tRNS
+       * and one without for other non-alpha formats:
+       */
+      unsigned int pn = ++*palette_number;
+      png_byte ct = *colour_type;
+
+      if (((ct == 0/*GRAY*/ || ct/*RGB*/ == 2) && tRNS && pn < 2) ||
+          (ct == 3/*PALETTE*/ && pn < PALETTE_COUNT(*bit_depth)))
+         return 1;
+
+      /* No: next bit depth */
+      *palette_number = 0;
+   }
+
+   *bit_depth = (png_byte)(*bit_depth << 1);
+
+   /* Palette images are restricted to 8 bit depth */
+   if (*bit_depth <= 8
+#ifdef DO_16BIT
+         || (*colour_type != 3 && *bit_depth <= 16)
+#endif
+      )
+      return 1;
+
+   /* Move to the next color type, or return 0 at the end. */
+   switch (*colour_type)
+   {
+      case 0:
+         *colour_type = 2;
+         *bit_depth = 8;
+         return 1;
+
+      case 2:
+         *colour_type = 3;
+         *bit_depth = 1;
+         return 1;
+
+      case 3:
+         *colour_type = 4;
+         *bit_depth = 8;
+         return 1;
+
+      case 4:
+         *colour_type = 6;
+         *bit_depth = 8;
+         return 1;
+
+      default:
+         return 0;
+   }
+}
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+static unsigned int
+sample(png_const_bytep row, png_byte colour_type, png_byte bit_depth,
+    png_uint_32 x, unsigned int sample_index, int swap16, int littleendian)
+{
+   png_uint_32 bit_index, result;
+
+   /* Find a sample index for the desired sample: */
+   x *= bit_depth;
+   bit_index = x;
+
+   if ((colour_type & 1) == 0) /* !palette */
+   {
+      if (colour_type & 2)
+         bit_index *= 3;
+
+      if (colour_type & 4)
+         bit_index += x; /* Alpha channel */
+
+      /* Multiple channels; select one: */
+      if (colour_type & (2+4))
+         bit_index += sample_index * bit_depth;
+   }
+
+   /* Return the sample from the row as an integer. */
+   row += bit_index >> 3;
+   result = *row;
+
+   if (bit_depth == 8)
+      return result;
+
+   else if (bit_depth > 8)
+   {
+      if (swap16)
+         return (*++row << 8) + result;
+      else
+         return (result << 8) + *++row;
+   }
+
+   /* Less than 8 bits per sample.  By default PNG has the big end of
+    * the egg on the left of the screen, but if littleendian is set
+    * then the big end is on the right.
+    */
+   bit_index &= 7;
+
+   if (!littleendian)
+      bit_index = 8-bit_index-bit_depth;
+
+   return (result >> bit_index) & ((1U<<bit_depth)-1);
+}
+#endif /* PNG_READ_TRANSFORMS_SUPPORTED */
+
+/* Copy a single pixel, of a given size, from one buffer to another -
+ * while this is basically bit addressed there is an implicit assumption
+ * that pixels 8 or more bits in size are byte aligned and that pixels
+ * do not otherwise cross byte boundaries.  (This is, so far as I know,
+ * universally true in bitmap computer graphics.  [JCB 20101212])
+ *
+ * NOTE: The to and from buffers may be the same.
+ */
+static void
+pixel_copy(png_bytep toBuffer, png_uint_32 toIndex,
+   png_const_bytep fromBuffer, png_uint_32 fromIndex, unsigned int pixelSize,
+   int littleendian)
+{
+   /* Assume we can multiply by 'size' without overflow because we are
+    * just working in a single buffer.
+    */
+   toIndex *= pixelSize;
+   fromIndex *= pixelSize;
+   if (pixelSize < 8) /* Sub-byte */
+   {
+      /* Mask to select the location of the copied pixel: */
+      unsigned int destMask = ((1U<<pixelSize)-1) <<
+         (littleendian ? toIndex&7 : 8-pixelSize-(toIndex&7));
+      /* The following read the entire pixels and clears the extra: */
+      unsigned int destByte = toBuffer[toIndex >> 3] & ~destMask;
+      unsigned int sourceByte = fromBuffer[fromIndex >> 3];
+
+      /* Don't rely on << or >> supporting '0' here, just in case: */
+      fromIndex &= 7;
+      if (littleendian)
+      {
+         if (fromIndex > 0) sourceByte >>= fromIndex;
+         if ((toIndex & 7) > 0) sourceByte <<= toIndex & 7;
+      }
+
+      else
+      {
+         if (fromIndex > 0) sourceByte <<= fromIndex;
+         if ((toIndex & 7) > 0) sourceByte >>= toIndex & 7;
+      }
+
+      toBuffer[toIndex >> 3] = (png_byte)(destByte | (sourceByte & destMask));
+   }
+   else /* One or more bytes */
+      memmove(toBuffer+(toIndex>>3), fromBuffer+(fromIndex>>3), pixelSize>>3);
+}
+
+#ifdef PNG_READ_SUPPORTED
+/* Copy a complete row of pixels, taking into account potential partial
+ * bytes at the end.
+ */
+static void
+row_copy(png_bytep toBuffer, png_const_bytep fromBuffer, unsigned int bitWidth,
+      int littleendian)
+{
+   memcpy(toBuffer, fromBuffer, bitWidth >> 3);
+
+   if ((bitWidth & 7) != 0)
+   {
+      unsigned int mask;
+
+      toBuffer += bitWidth >> 3;
+      fromBuffer += bitWidth >> 3;
+      if (littleendian)
+         mask = 0xff << (bitWidth & 7);
+      else
+         mask = 0xff >> (bitWidth & 7);
+      *toBuffer = (png_byte)((*toBuffer & mask) | (*fromBuffer & ~mask));
+   }
+}
+
+/* Compare pixels - they are assumed to start at the first byte in the
+ * given buffers.
+ */
+static int
+pixel_cmp(png_const_bytep pa, png_const_bytep pb, png_uint_32 bit_width)
+{
+#if PNG_LIBPNG_VER < 10506
+   if (memcmp(pa, pb, bit_width>>3) == 0)
+   {
+      png_uint_32 p;
+
+      if ((bit_width & 7) == 0) return 0;
+
+      /* Ok, any differences? */
+      p = pa[bit_width >> 3];
+      p ^= pb[bit_width >> 3];
+
+      if (p == 0) return 0;
+
+      /* There are, but they may not be significant, remove the bits
+       * after the end (the low order bits in PNG.)
+       */
+      bit_width &= 7;
+      p >>= 8-bit_width;
+
+      if (p == 0) return 0;
+   }
+#else
+   /* From libpng-1.5.6 the overwrite should be fixed, so compare the trailing
+    * bits too:
+    */
+   if (memcmp(pa, pb, (bit_width+7)>>3) == 0)
+      return 0;
+#endif
+
+   /* Return the index of the changed byte. */
+   {
+      png_uint_32 where = 0;
+
+      while (pa[where] == pb[where]) ++where;
+      return 1+where;
+   }
+}
+#endif /* PNG_READ_SUPPORTED */
+
+/*************************** BASIC PNG FILE WRITING ***************************/
+/* A png_store takes data from the sequential writer or provides data
+ * to the sequential reader.  It can also store the result of a PNG
+ * write for later retrieval.
+ */
+#define STORE_BUFFER_SIZE 500 /* arbitrary */
+typedef struct png_store_buffer
+{
+   struct png_store_buffer*  prev;    /* NOTE: stored in reverse order */
+   png_byte                  buffer[STORE_BUFFER_SIZE];
+} png_store_buffer;
+
+#define FILE_NAME_SIZE 64
+
+typedef struct store_palette_entry /* record of a single palette entry */
+{
+   png_byte red;
+   png_byte green;
+   png_byte blue;
+   png_byte alpha;
+} store_palette_entry, store_palette[256];
+
+typedef struct png_store_file
+{
+   struct png_store_file*  next;      /* as many as you like... */
+   char                    name[FILE_NAME_SIZE];
+   png_uint_32             id;        /* must be correct (see FILEID) */
+   png_size_t              datacount; /* In this (the last) buffer */
+   png_store_buffer        data;      /* Last buffer in file */
+   int                     npalette;  /* Number of entries in palette */
+   store_palette_entry*    palette;   /* May be NULL */
+} png_store_file;
+
+/* The following is a pool of memory allocated by a single libpng read or write
+ * operation.
+ */
+typedef struct store_pool
+{
+   struct png_store    *store;   /* Back pointer */
+   struct store_memory *list;    /* List of allocated memory */
+   png_byte             mark[4]; /* Before and after data */
+
+   /* Statistics for this run. */
+   png_alloc_size_t     max;     /* Maximum single allocation */
+   png_alloc_size_t     current; /* Current allocation */
+   png_alloc_size_t     limit;   /* Highest current allocation */
+   png_alloc_size_t     total;   /* Total allocation */
+
+   /* Overall statistics (retained across successive runs). */
+   png_alloc_size_t     max_max;
+   png_alloc_size_t     max_limit;
+   png_alloc_size_t     max_total;
+} store_pool;
+
+typedef struct png_store
+{
+   /* For cexcept.h exception handling - simply store one of these;
+    * the context is a self pointer but it may point to a different
+    * png_store (in fact it never does in this program.)
+    */
+   struct exception_context
+                      exception_context;
+
+   unsigned int       verbose :1;
+   unsigned int       treat_warnings_as_errors :1;
+   unsigned int       expect_error :1;
+   unsigned int       expect_warning :1;
+   unsigned int       saw_warning :1;
+   unsigned int       speed :1;
+   unsigned int       progressive :1; /* use progressive read */
+   unsigned int       validated :1;   /* used as a temporary flag */
+   int                nerrors;
+   int                nwarnings;
+   int                noptions;       /* number of options below: */
+   struct {
+      unsigned char   option;         /* option number, 0..30 */
+      unsigned char   setting;        /* setting (unset,invalid,on,off) */
+   }                  options[16];
+   char               test[128];      /* Name of test */
+   char               error[256];
+
+   /* Read fields */
+   png_structp        pread;    /* Used to read a saved file */
+   png_infop          piread;
+   png_store_file*    current;  /* Set when reading */
+   png_store_buffer*  next;     /* Set when reading */
+   png_size_t         readpos;  /* Position in *next */
+   png_byte*          image;    /* Buffer for reading interlaced images */
+   png_size_t         cb_image; /* Size of this buffer */
+   png_size_t         cb_row;   /* Row size of the image(s) */
+   png_uint_32        image_h;  /* Number of rows in a single image */
+   store_pool         read_memory_pool;
+
+   /* Write fields */
+   png_store_file*    saved;
+   png_structp        pwrite;   /* Used when writing a new file */
+   png_infop          piwrite;
+   png_size_t         writepos; /* Position in .new */
+   char               wname[FILE_NAME_SIZE];
+   png_store_buffer   new;      /* The end of the new PNG file being written. */
+   store_pool         write_memory_pool;
+   store_palette_entry* palette;
+   int                  npalette;
+} png_store;
+
+/* Initialization and cleanup */
+static void
+store_pool_mark(png_bytep mark)
+{
+   static png_uint_32 store_seed[2] = { 0x12345678, 1};
+
+   make_four_random_bytes(store_seed, mark);
+}
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+/* Use this for random 32 bit values; this function makes sure the result is
+ * non-zero.
+ */
+static png_uint_32
+random_32(void)
+{
+
+   for (;;)
+   {
+      png_byte mark[4];
+      png_uint_32 result;
+
+      store_pool_mark(mark);
+      result = png_get_uint_32(mark);
+
+      if (result != 0)
+         return result;
+   }
+}
+#endif /* PNG_READ_SUPPORTED */
+
+static void
+store_pool_init(png_store *ps, store_pool *pool)
+{
+   memset(pool, 0, sizeof *pool);
+
+   pool->store = ps;
+   pool->list = NULL;
+   pool->max = pool->current = pool->limit = pool->total = 0;
+   pool->max_max = pool->max_limit = pool->max_total = 0;
+   store_pool_mark(pool->mark);
+}
+
+static void
+store_init(png_store* ps)
+{
+   memset(ps, 0, sizeof *ps);
+   init_exception_context(&ps->exception_context);
+   store_pool_init(ps, &ps->read_memory_pool);
+   store_pool_init(ps, &ps->write_memory_pool);
+   ps->verbose = 0;
+   ps->treat_warnings_as_errors = 0;
+   ps->expect_error = 0;
+   ps->expect_warning = 0;
+   ps->saw_warning = 0;
+   ps->speed = 0;
+   ps->progressive = 0;
+   ps->validated = 0;
+   ps->nerrors = ps->nwarnings = 0;
+   ps->pread = NULL;
+   ps->piread = NULL;
+   ps->saved = ps->current = NULL;
+   ps->next = NULL;
+   ps->readpos = 0;
+   ps->image = NULL;
+   ps->cb_image = 0;
+   ps->cb_row = 0;
+   ps->image_h = 0;
+   ps->pwrite = NULL;
+   ps->piwrite = NULL;
+   ps->writepos = 0;
+   ps->new.prev = NULL;
+   ps->palette = NULL;
+   ps->npalette = 0;
+   ps->noptions = 0;
+}
+
+static void
+store_freebuffer(png_store_buffer* psb)
+{
+   if (psb->prev)
+   {
+      store_freebuffer(psb->prev);
+      free(psb->prev);
+      psb->prev = NULL;
+   }
+}
+
+static void
+store_freenew(png_store *ps)
+{
+   store_freebuffer(&ps->new);
+   ps->writepos = 0;
+   if (ps->palette != NULL)
+   {
+      free(ps->palette);
+      ps->palette = NULL;
+      ps->npalette = 0;
+   }
+}
+
+static void
+store_storenew(png_store *ps)
+{
+   png_store_buffer *pb;
+
+   if (ps->writepos != STORE_BUFFER_SIZE)
+      png_error(ps->pwrite, "invalid store call");
+
+   pb = voidcast(png_store_buffer*, malloc(sizeof *pb));
+
+   if (pb == NULL)
+      png_error(ps->pwrite, "store new: OOM");
+
+   *pb = ps->new;
+   ps->new.prev = pb;
+   ps->writepos = 0;
+}
+
+static void
+store_freefile(png_store_file **ppf)
+{
+   if (*ppf != NULL)
+   {
+      store_freefile(&(*ppf)->next);
+
+      store_freebuffer(&(*ppf)->data);
+      (*ppf)->datacount = 0;
+      if ((*ppf)->palette != NULL)
+      {
+         free((*ppf)->palette);
+         (*ppf)->palette = NULL;
+         (*ppf)->npalette = 0;
+      }
+      free(*ppf);
+      *ppf = NULL;
+   }
+}
+
+/* Main interface to file storeage, after writing a new PNG file (see the API
+ * below) call store_storefile to store the result with the given name and id.
+ */
+static void
+store_storefile(png_store *ps, png_uint_32 id)
+{
+   png_store_file *pf = voidcast(png_store_file*, malloc(sizeof *pf));
+   if (pf == NULL)
+      png_error(ps->pwrite, "storefile: OOM");
+   safecat(pf->name, sizeof pf->name, 0, ps->wname);
+   pf->id = id;
+   pf->data = ps->new;
+   pf->datacount = ps->writepos;
+   ps->new.prev = NULL;
+   ps->writepos = 0;
+   pf->palette = ps->palette;
+   pf->npalette = ps->npalette;
+   ps->palette = 0;
+   ps->npalette = 0;
+
+   /* And save it. */
+   pf->next = ps->saved;
+   ps->saved = pf;
+}
+
+/* Generate an error message (in the given buffer) */
+static size_t
+store_message(png_store *ps, png_const_structp pp, char *buffer, size_t bufsize,
+   size_t pos, const char *msg)
+{
+   if (pp != NULL && pp == ps->pread)
+   {
+      /* Reading a file */
+      pos = safecat(buffer, bufsize, pos, "read: ");
+
+      if (ps->current != NULL)
+      {
+         pos = safecat(buffer, bufsize, pos, ps->current->name);
+         pos = safecat(buffer, bufsize, pos, sep);
+      }
+   }
+
+   else if (pp != NULL && pp == ps->pwrite)
+   {
+      /* Writing a file */
+      pos = safecat(buffer, bufsize, pos, "write: ");
+      pos = safecat(buffer, bufsize, pos, ps->wname);
+      pos = safecat(buffer, bufsize, pos, sep);
+   }
+
+   else
+   {
+      /* Neither reading nor writing (or a memory error in struct delete) */
+      pos = safecat(buffer, bufsize, pos, "pngvalid: ");
+   }
+
+   if (ps->test[0] != 0)
+   {
+      pos = safecat(buffer, bufsize, pos, ps->test);
+      pos = safecat(buffer, bufsize, pos, sep);
+   }
+   pos = safecat(buffer, bufsize, pos, msg);
+   return pos;
+}
+
+/* Verbose output to the error stream: */
+static void
+store_verbose(png_store *ps, png_const_structp pp, png_const_charp prefix,
+   png_const_charp message)
+{
+   char buffer[512];
+
+   if (prefix)
+      fputs(prefix, stderr);
+
+   (void)store_message(ps, pp, buffer, sizeof buffer, 0, message);
+   fputs(buffer, stderr);
+   fputc('\n', stderr);
+}
+
+/* Log an error or warning - the relevant count is always incremented. */
+static void
+store_log(png_store* ps, png_const_structp pp, png_const_charp message,
+   int is_error)
+{
+   /* The warning is copied to the error buffer if there are no errors and it is
+    * the first warning.  The error is copied to the error buffer if it is the
+    * first error (overwriting any prior warnings).
+    */
+   if (is_error ? (ps->nerrors)++ == 0 :
+       (ps->nwarnings)++ == 0 && ps->nerrors == 0)
+      store_message(ps, pp, ps->error, sizeof ps->error, 0, message);
+
+   if (ps->verbose)
+      store_verbose(ps, pp, is_error ? "error: " : "warning: ", message);
+}
+
+#ifdef PNG_READ_SUPPORTED
+/* Internal error function, called with a png_store but no libpng stuff. */
+static void
+internal_error(png_store *ps, png_const_charp message)
+{
+   store_log(ps, NULL, message, 1 /* error */);
+
+   /* And finally throw an exception. */
+   {
+      struct exception_context *the_exception_context = &ps->exception_context;
+      Throw ps;
+   }
+}
+#endif /* PNG_READ_SUPPORTED */
+
+/* Functions to use as PNG callbacks. */
+static void PNGCBAPI
+store_error(png_structp ppIn, png_const_charp message) /* PNG_NORETURN */
+{
+   png_const_structp pp = ppIn;
+   png_store *ps = voidcast(png_store*, png_get_error_ptr(pp));
+
+   if (!ps->expect_error)
+      store_log(ps, pp, message, 1 /* error */);
+
+   /* And finally throw an exception. */
+   {
+      struct exception_context *the_exception_context = &ps->exception_context;
+      Throw ps;
+   }
+}
+
+static void PNGCBAPI
+store_warning(png_structp ppIn, png_const_charp message)
+{
+   png_const_structp pp = ppIn;
+   png_store *ps = voidcast(png_store*, png_get_error_ptr(pp));
+
+   if (!ps->expect_warning)
+      store_log(ps, pp, message, 0 /* warning */);
+   else
+      ps->saw_warning = 1;
+}
+
+/* These somewhat odd functions are used when reading an image to ensure that
+ * the buffer is big enough, the png_structp is for errors.
+ */
+/* Return a single row from the correct image. */
+static png_bytep
+store_image_row(const png_store* ps, png_const_structp pp, int nImage,
+   png_uint_32 y)
+{
+   png_size_t coffset = (nImage * ps->image_h + y) * (ps->cb_row + 5) + 2;
+
+   if (ps->image == NULL)
+      png_error(pp, "no allocated image");
+
+   if (coffset + ps->cb_row + 3 > ps->cb_image)
+      png_error(pp, "image too small");
+
+   return ps->image + coffset;
+}
+
+static void
+store_image_free(png_store *ps, png_const_structp pp)
+{
+   if (ps->image != NULL)
+   {
+      png_bytep image = ps->image;
+
+      if (image[-1] != 0xed || image[ps->cb_image] != 0xfe)
+      {
+         if (pp != NULL)
+            png_error(pp, "png_store image overwrite (1)");
+         else
+            store_log(ps, NULL, "png_store image overwrite (2)", 1);
+      }
+
+      ps->image = NULL;
+      ps->cb_image = 0;
+      --image;
+      free(image);
+   }
+}
+
+static void
+store_ensure_image(png_store *ps, png_const_structp pp, int nImages,
+   png_size_t cbRow, png_uint_32 cRows)
+{
+   png_size_t cb = nImages * cRows * (cbRow + 5);
+
+   if (ps->cb_image < cb)
+   {
+      png_bytep image;
+
+      store_image_free(ps, pp);
+
+      /* The buffer is deliberately mis-aligned. */
+      image = voidcast(png_bytep, malloc(cb+2));
+      if (image == NULL)
+      {
+         /* Called from the startup - ignore the error for the moment. */
+         if (pp == NULL)
+            return;
+
+         png_error(pp, "OOM allocating image buffer");
+      }
+
+      /* These magic tags are used to detect overwrites above. */
+      ++image;
+      image[-1] = 0xed;
+      image[cb] = 0xfe;
+
+      ps->image = image;
+      ps->cb_image = cb;
+   }
+
+   /* We have an adequate sized image; lay out the rows.  There are 2 bytes at
+    * the start and three at the end of each (this ensures that the row
+    * alignment starts out odd - 2+1 and changes for larger images on each row.)
+    */
+   ps->cb_row = cbRow;
+   ps->image_h = cRows;
+
+   /* For error checking, the whole buffer is set to 10110010 (0xb2 - 178).
+    * This deliberately doesn't match the bits in the size test image which are
+    * outside the image; these are set to 0xff (all 1).  To make the row
+    * comparison work in the 'size' test case the size rows are pre-initialized
+    * to the same value prior to calling 'standard_row'.
+    */
+   memset(ps->image, 178, cb);
+
+   /* Then put in the marks. */
+   while (--nImages >= 0)
+   {
+      png_uint_32 y;
+
+      for (y=0; y<cRows; ++y)
+      {
+         png_bytep row = store_image_row(ps, pp, nImages, y);
+
+         /* The markers: */
+         row[-2] = 190;
+         row[-1] = 239;
+         row[cbRow] = 222;
+         row[cbRow+1] = 173;
+         row[cbRow+2] = 17;
+      }
+   }
+}
+
+#ifdef PNG_READ_SUPPORTED
+static void
+store_image_check(const png_store* ps, png_const_structp pp, int iImage)
+{
+   png_const_bytep image = ps->image;
+
+   if (image[-1] != 0xed || image[ps->cb_image] != 0xfe)
+      png_error(pp, "image overwrite");
+   else
+   {
+      png_size_t cbRow = ps->cb_row;
+      png_uint_32 rows = ps->image_h;
+
+      image += iImage * (cbRow+5) * ps->image_h;
+
+      image += 2; /* skip image first row markers */
+
+      while (rows-- > 0)
+      {
+         if (image[-2] != 190 || image[-1] != 239)
+            png_error(pp, "row start overwritten");
+
+         if (image[cbRow] != 222 || image[cbRow+1] != 173 ||
+            image[cbRow+2] != 17)
+            png_error(pp, "row end overwritten");
+
+         image += cbRow+5;
+      }
+   }
+}
+#endif /* PNG_READ_SUPPORTED */
+
+static void PNGCBAPI
+store_write(png_structp ppIn, png_bytep pb, png_size_t st)
+{
+   png_const_structp pp = ppIn;
+   png_store *ps = voidcast(png_store*, png_get_io_ptr(pp));
+
+   if (ps->pwrite != pp)
+      png_error(pp, "store state damaged");
+
+   while (st > 0)
+   {
+      size_t cb;
+
+      if (ps->writepos >= STORE_BUFFER_SIZE)
+         store_storenew(ps);
+
+      cb = st;
+
+      if (cb > STORE_BUFFER_SIZE - ps->writepos)
+         cb = STORE_BUFFER_SIZE - ps->writepos;
+
+      memcpy(ps->new.buffer + ps->writepos, pb, cb);
+      pb += cb;
+      st -= cb;
+      ps->writepos += cb;
+   }
+}
+
+static void PNGCBAPI
+store_flush(png_structp ppIn)
+{
+   UNUSED(ppIn) /*DOES NOTHING*/
+}
+
+#ifdef PNG_READ_SUPPORTED
+static size_t
+store_read_buffer_size(png_store *ps)
+{
+   /* Return the bytes available for read in the current buffer. */
+   if (ps->next != &ps->current->data)
+      return STORE_BUFFER_SIZE;
+
+   return ps->current->datacount;
+}
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+/* Return total bytes available for read. */
+static size_t
+store_read_buffer_avail(png_store *ps)
+{
+   if (ps->current != NULL && ps->next != NULL)
+   {
+      png_store_buffer *next = &ps->current->data;
+      size_t cbAvail = ps->current->datacount;
+
+      while (next != ps->next && next != NULL)
+      {
+         next = next->prev;
+         cbAvail += STORE_BUFFER_SIZE;
+      }
+
+      if (next != ps->next)
+         png_error(ps->pread, "buffer read error");
+
+      if (cbAvail > ps->readpos)
+         return cbAvail - ps->readpos;
+   }
+
+   return 0;
+}
+#endif
+
+static int
+store_read_buffer_next(png_store *ps)
+{
+   png_store_buffer *pbOld = ps->next;
+   png_store_buffer *pbNew = &ps->current->data;
+   if (pbOld != pbNew)
+   {
+      while (pbNew != NULL && pbNew->prev != pbOld)
+         pbNew = pbNew->prev;
+
+      if (pbNew != NULL)
+      {
+         ps->next = pbNew;
+         ps->readpos = 0;
+         return 1;
+      }
+
+      png_error(ps->pread, "buffer lost");
+   }
+
+   return 0; /* EOF or error */
+}
+
+/* Need separate implementation and callback to allow use of the same code
+ * during progressive read, where the io_ptr is set internally by libpng.
+ */
+static void
+store_read_imp(png_store *ps, png_bytep pb, png_size_t st)
+{
+   if (ps->current == NULL || ps->next == NULL)
+      png_error(ps->pread, "store state damaged");
+
+   while (st > 0)
+   {
+      size_t cbAvail = store_read_buffer_size(ps) - ps->readpos;
+
+      if (cbAvail > 0)
+      {
+         if (cbAvail > st) cbAvail = st;
+         memcpy(pb, ps->next->buffer + ps->readpos, cbAvail);
+         st -= cbAvail;
+         pb += cbAvail;
+         ps->readpos += cbAvail;
+      }
+
+      else if (!store_read_buffer_next(ps))
+         png_error(ps->pread, "read beyond end of file");
+   }
+}
+
+static void PNGCBAPI
+store_read(png_structp ppIn, png_bytep pb, png_size_t st)
+{
+   png_const_structp pp = ppIn;
+   png_store *ps = voidcast(png_store*, png_get_io_ptr(pp));
+
+   if (ps == NULL || ps->pread != pp)
+      png_error(pp, "bad store read call");
+
+   store_read_imp(ps, pb, st);
+}
+
+static void
+store_progressive_read(png_store *ps, png_structp pp, png_infop pi)
+{
+   /* Notice that a call to store_read will cause this function to fail because
+    * readpos will be set.
+    */
+   if (ps->pread != pp || ps->current == NULL || ps->next == NULL)
+      png_error(pp, "store state damaged (progressive)");
+
+   do
+   {
+      if (ps->readpos != 0)
+         png_error(pp, "store_read called during progressive read");
+
+      png_process_data(pp, pi, ps->next->buffer, store_read_buffer_size(ps));
+   }
+   while (store_read_buffer_next(ps));
+}
+#endif /* PNG_READ_SUPPORTED */
+
+/* The caller must fill this in: */
+static store_palette_entry *
+store_write_palette(png_store *ps, int npalette)
+{
+   if (ps->pwrite == NULL)
+      store_log(ps, NULL, "attempt to write palette without write stream", 1);
+
+   if (ps->palette != NULL)
+      png_error(ps->pwrite, "multiple store_write_palette calls");
+
+   /* This function can only return NULL if called with '0'! */
+   if (npalette > 0)
+   {
+      ps->palette = voidcast(store_palette_entry*, malloc(npalette *
+         sizeof *ps->palette));
+
+      if (ps->palette == NULL)
+         png_error(ps->pwrite, "store new palette: OOM");
+
+      ps->npalette = npalette;
+   }
+
+   return ps->palette;
+}
+
+#ifdef PNG_READ_SUPPORTED
+static store_palette_entry *
+store_current_palette(png_store *ps, int *npalette)
+{
+   /* This is an internal error (the call has been made outside a read
+    * operation.)
+    */
+   if (ps->current == NULL)
+   {
+      store_log(ps, ps->pread, "no current stream for palette", 1);
+      return NULL;
+   }
+
+   /* The result may be null if there is no palette. */
+   *npalette = ps->current->npalette;
+   return ps->current->palette;
+}
+#endif /* PNG_READ_SUPPORTED */
+
+/***************************** MEMORY MANAGEMENT*** ***************************/
+#ifdef PNG_USER_MEM_SUPPORTED
+/* A store_memory is simply the header for an allocated block of memory.  The
+ * pointer returned to libpng is just after the end of the header block, the
+ * allocated memory is followed by a second copy of the 'mark'.
+ */
+typedef struct store_memory
+{
+   store_pool          *pool;    /* Originating pool */
+   struct store_memory *next;    /* Singly linked list */
+   png_alloc_size_t     size;    /* Size of memory allocated */
+   png_byte             mark[4]; /* ID marker */
+} store_memory;
+
+/* Handle a fatal error in memory allocation.  This calls png_error if the
+ * libpng struct is non-NULL, else it outputs a message and returns.  This means
+ * that a memory problem while libpng is running will abort (png_error) the
+ * handling of particular file while one in cleanup (after the destroy of the
+ * struct has returned) will simply keep going and free (or attempt to free)
+ * all the memory.
+ */
+static void
+store_pool_error(png_store *ps, png_const_structp pp, const char *msg)
+{
+   if (pp != NULL)
+      png_error(pp, msg);
+
+   /* Else we have to do it ourselves.  png_error eventually calls store_log,
+    * above.  store_log accepts a NULL png_structp - it just changes what gets
+    * output by store_message.
+    */
+   store_log(ps, pp, msg, 1 /* error */);
+}
+
+static void
+store_memory_free(png_const_structp pp, store_pool *pool, store_memory *memory)
+{
+   /* Note that pp may be NULL (see store_pool_delete below), the caller has
+    * found 'memory' in pool->list *and* unlinked this entry, so this is a valid
+    * pointer (for sure), but the contents may have been trashed.
+    */
+   if (memory->pool != pool)
+      store_pool_error(pool->store, pp, "memory corrupted (pool)");
+
+   else if (memcmp(memory->mark, pool->mark, sizeof memory->mark) != 0)
+      store_pool_error(pool->store, pp, "memory corrupted (start)");
+
+   /* It should be safe to read the size field now. */
+   else
+   {
+      png_alloc_size_t cb = memory->size;
+
+      if (cb > pool->max)
+         store_pool_error(pool->store, pp, "memory corrupted (size)");
+
+      else if (memcmp((png_bytep)(memory+1)+cb, pool->mark, sizeof pool->mark)
+         != 0)
+         store_pool_error(pool->store, pp, "memory corrupted (end)");
+
+      /* Finally give the library a chance to find problems too: */
+      else
+         {
+         pool->current -= cb;
+         free(memory);
+         }
+   }
+}
+
+static void
+store_pool_delete(png_store *ps, store_pool *pool)
+{
+   if (pool->list != NULL)
+   {
+      fprintf(stderr, "%s: %s %s: memory lost (list follows):\n", ps->test,
+         pool == &ps->read_memory_pool ? "read" : "write",
+         pool == &ps->read_memory_pool ? (ps->current != NULL ?
+            ps->current->name : "unknown file") : ps->wname);
+      ++ps->nerrors;
+
+      do
+      {
+         store_memory *next = pool->list;
+         pool->list = next->next;
+         next->next = NULL;
+
+         fprintf(stderr, "\t%lu bytes @ %p\n",
+             (unsigned long)next->size, (const void*)(next+1));
+         /* The NULL means this will always return, even if the memory is
+          * corrupted.
+          */
+         store_memory_free(NULL, pool, next);
+      }
+      while (pool->list != NULL);
+   }
+
+   /* And reset the other fields too for the next time. */
+   if (pool->max > pool->max_max) pool->max_max = pool->max;
+   pool->max = 0;
+   if (pool->current != 0) /* unexpected internal error */
+      fprintf(stderr, "%s: %s %s: memory counter mismatch (internal error)\n",
+         ps->test, pool == &ps->read_memory_pool ? "read" : "write",
+         pool == &ps->read_memory_pool ? (ps->current != NULL ?
+            ps->current->name : "unknown file") : ps->wname);
+   pool->current = 0;
+
+   if (pool->limit > pool->max_limit)
+      pool->max_limit = pool->limit;
+
+   pool->limit = 0;
+
+   if (pool->total > pool->max_total)
+      pool->max_total = pool->total;
+
+   pool->total = 0;
+
+   /* Get a new mark too. */
+   store_pool_mark(pool->mark);
+}
+
+/* The memory callbacks: */
+static png_voidp PNGCBAPI
+store_malloc(png_structp ppIn, png_alloc_size_t cb)
+{
+   png_const_structp pp = ppIn;
+   store_pool *pool = voidcast(store_pool*, png_get_mem_ptr(pp));
+   store_memory *new = voidcast(store_memory*, malloc(cb + (sizeof *new) +
+      (sizeof pool->mark)));
+
+   if (new != NULL)
+   {
+      if (cb > pool->max)
+         pool->max = cb;
+
+      pool->current += cb;
+
+      if (pool->current > pool->limit)
+         pool->limit = pool->current;
+
+      pool->total += cb;
+
+      new->size = cb;
+      memcpy(new->mark, pool->mark, sizeof new->mark);
+      memcpy((png_byte*)(new+1) + cb, pool->mark, sizeof pool->mark);
+      new->pool = pool;
+      new->next = pool->list;
+      pool->list = new;
+      ++new;
+   }
+
+   else
+   {
+      /* NOTE: the PNG user malloc function cannot use the png_ptr it is passed
+       * other than to retrieve the allocation pointer!  libpng calls the
+       * store_malloc callback in two basic cases:
+       *
+       * 1) From png_malloc; png_malloc will do a png_error itself if NULL is
+       *    returned.
+       * 2) From png_struct or png_info structure creation; png_malloc is
+       *    to return so cleanup can be performed.
+       *
+       * To handle this store_malloc can log a message, but can't do anything
+       * else.
+       */
+      store_log(pool->store, pp, "out of memory", 1 /* is_error */);
+   }
+
+   return new;
+}
+
+static void PNGCBAPI
+store_free(png_structp ppIn, png_voidp memory)
+{
+   png_const_structp pp = ppIn;
+   store_pool *pool = voidcast(store_pool*, png_get_mem_ptr(pp));
+   store_memory *this = voidcast(store_memory*, memory), **test;
+
+   /* Because libpng calls store_free with a dummy png_struct when deleting
+    * png_struct or png_info via png_destroy_struct_2 it is necessary to check
+    * the passed in png_structp to ensure it is valid, and not pass it to
+    * png_error if it is not.
+    */
+   if (pp != pool->store->pread && pp != pool->store->pwrite)
+      pp = NULL;
+
+   /* First check that this 'memory' really is valid memory - it must be in the
+    * pool list.  If it is, use the shared memory_free function to free it.
+    */
+   --this;
+   for (test = &pool->list; *test != this; test = &(*test)->next)
+   {
+      if (*test == NULL)
+      {
+         store_pool_error(pool->store, pp, "bad pointer to free");
+         return;
+      }
+   }
+
+   /* Unlink this entry, *test == this. */
+   *test = this->next;
+   this->next = NULL;
+   store_memory_free(pp, pool, this);
+}
+#endif /* PNG_USER_MEM_SUPPORTED */
+
+/* Setup functions. */
+/* Cleanup when aborting a write or after storing the new file. */
+static void
+store_write_reset(png_store *ps)
+{
+   if (ps->pwrite != NULL)
+   {
+      anon_context(ps);
+
+      Try
+         png_destroy_write_struct(&ps->pwrite, &ps->piwrite);
+
+      Catch_anonymous
+      {
+         /* memory corruption: continue. */
+      }
+
+      ps->pwrite = NULL;
+      ps->piwrite = NULL;
+   }
+
+   /* And make sure that all the memory has been freed - this will output
+    * spurious errors in the case of memory corruption above, but this is safe.
+    */
+#  ifdef PNG_USER_MEM_SUPPORTED
+      store_pool_delete(ps, &ps->write_memory_pool);
+#  endif
+
+   store_freenew(ps);
+}
+
+/* The following is the main write function, it returns a png_struct and,
+ * optionally, a png_info suitable for writiing a new PNG file.  Use
+ * store_storefile above to record this file after it has been written.  The
+ * returned libpng structures as destroyed by store_write_reset above.
+ */
+static png_structp
+set_store_for_write(png_store *ps, png_infopp ppi, const char *name)
+{
+   anon_context(ps);
+
+   Try
+   {
+      if (ps->pwrite != NULL)
+         png_error(ps->pwrite, "write store already in use");
+
+      store_write_reset(ps);
+      safecat(ps->wname, sizeof ps->wname, 0, name);
+
+      /* Don't do the slow memory checks if doing a speed test, also if user
+       * memory is not supported we can't do it anyway.
+       */
+#     ifdef PNG_USER_MEM_SUPPORTED
+         if (!ps->speed)
+            ps->pwrite = png_create_write_struct_2(PNG_LIBPNG_VER_STRING,
+               ps, store_error, store_warning, &ps->write_memory_pool,
+               store_malloc, store_free);
+
+         else
+#     endif
+         ps->pwrite = png_create_write_struct(PNG_LIBPNG_VER_STRING,
+            ps, store_error, store_warning);
+
+      png_set_write_fn(ps->pwrite, ps, store_write, store_flush);
+
+#     ifdef PNG_SET_OPTION_SUPPORTED
+         {
+            int opt;
+            for (opt=0; opt<ps->noptions; ++opt)
+               if (png_set_option(ps->pwrite, ps->options[opt].option,
+                  ps->options[opt].setting) == PNG_OPTION_INVALID)
+                  png_error(ps->pwrite, "png option invalid");
+         }
+#     endif
+
+      if (ppi != NULL)
+         *ppi = ps->piwrite = png_create_info_struct(ps->pwrite);
+   }
+
+   Catch_anonymous
+      return NULL;
+
+   return ps->pwrite;
+}
+
+/* Cleanup when finished reading (either due to error or in the success case).
+ * This routine exists even when there is no read support to make the code
+ * tidier (avoid a mass of ifdefs) and so easier to maintain.
+ */
+static void
+store_read_reset(png_store *ps)
+{
+#  ifdef PNG_READ_SUPPORTED
+      if (ps->pread != NULL)
+      {
+         anon_context(ps);
+
+         Try
+            png_destroy_read_struct(&ps->pread, &ps->piread, NULL);
+
+         Catch_anonymous
+         {
+            /* error already output: continue */
+         }
+
+         ps->pread = NULL;
+         ps->piread = NULL;
+      }
+#  endif
+
+#  ifdef PNG_USER_MEM_SUPPORTED
+      /* Always do this to be safe. */
+      store_pool_delete(ps, &ps->read_memory_pool);
+#  endif
+
+   ps->current = NULL;
+   ps->next = NULL;
+   ps->readpos = 0;
+   ps->validated = 0;
+}
+
+#ifdef PNG_READ_SUPPORTED
+static void
+store_read_set(png_store *ps, png_uint_32 id)
+{
+   png_store_file *pf = ps->saved;
+
+   while (pf != NULL)
+   {
+      if (pf->id == id)
+      {
+         ps->current = pf;
+         ps->next = NULL;
+         store_read_buffer_next(ps);
+         return;
+      }
+
+      pf = pf->next;
+   }
+
+   {
+      size_t pos;
+      char msg[FILE_NAME_SIZE+64];
+
+      pos = standard_name_from_id(msg, sizeof msg, 0, id);
+      pos = safecat(msg, sizeof msg, pos, ": file not found");
+      png_error(ps->pread, msg);
+   }
+}
+
+/* The main interface for reading a saved file - pass the id number of the file
+ * to retrieve.  Ids must be unique or the earlier file will be hidden.  The API
+ * returns a png_struct and, optionally, a png_info.  Both of these will be
+ * destroyed by store_read_reset above.
+ */
+static png_structp
+set_store_for_read(png_store *ps, png_infopp ppi, png_uint_32 id,
+   const char *name)
+{
+   /* Set the name for png_error */
+   safecat(ps->test, sizeof ps->test, 0, name);
+
+   if (ps->pread != NULL)
+      png_error(ps->pread, "read store already in use");
+
+   store_read_reset(ps);
+
+   /* Both the create APIs can return NULL if used in their default mode
+    * (because there is no other way of handling an error because the jmp_buf
+    * by default is stored in png_struct and that has not been allocated!)
+    * However, given that store_error works correctly in these circumstances
+    * we don't ever expect NULL in this program.
+    */
+#  ifdef PNG_USER_MEM_SUPPORTED
+      if (!ps->speed)
+         ps->pread = png_create_read_struct_2(PNG_LIBPNG_VER_STRING, ps,
+             store_error, store_warning, &ps->read_memory_pool, store_malloc,
+             store_free);
+
+      else
+#  endif
+   ps->pread = png_create_read_struct(PNG_LIBPNG_VER_STRING, ps, store_error,
+      store_warning);
+
+   if (ps->pread == NULL)
+   {
+      struct exception_context *the_exception_context = &ps->exception_context;
+
+      store_log(ps, NULL, "png_create_read_struct returned NULL (unexpected)",
+         1 /*error*/);
+
+      Throw ps;
+   }
+
+#  ifdef PNG_SET_OPTION_SUPPORTED
+      {
+         int opt;
+         for (opt=0; opt<ps->noptions; ++opt)
+            if (png_set_option(ps->pread, ps->options[opt].option,
+               ps->options[opt].setting) == PNG_OPTION_INVALID)
+                  png_error(ps->pread, "png option invalid");
+      }
+#  endif
+
+   store_read_set(ps, id);
+
+   if (ppi != NULL)
+      *ppi = ps->piread = png_create_info_struct(ps->pread);
+
+   return ps->pread;
+}
+#endif /* PNG_READ_SUPPORTED */
+
+/* The overall cleanup of a store simply calls the above then removes all the
+ * saved files.  This does not delete the store itself.
+ */
+static void
+store_delete(png_store *ps)
+{
+   store_write_reset(ps);
+   store_read_reset(ps);
+   store_freefile(&ps->saved);
+   store_image_free(ps, NULL);
+}
+
+/*********************** PNG FILE MODIFICATION ON READ ************************/
+/* Files may be modified on read.  The following structure contains a complete
+ * png_store together with extra members to handle modification and a special
+ * read callback for libpng.  To use this the 'modifications' field must be set
+ * to a list of png_modification structures that actually perform the
+ * modification, otherwise a png_modifier is functionally equivalent to a
+ * png_store.  There is a special read function, set_modifier_for_read, which
+ * replaces set_store_for_read.
+ */
+typedef enum modifier_state
+{
+   modifier_start,                        /* Initial value */
+   modifier_signature,                    /* Have a signature */
+   modifier_IHDR                          /* Have an IHDR */
+} modifier_state;
+
+typedef struct CIE_color
+{
+   /* A single CIE tristimulus value, representing the unique response of a
+    * standard observer to a variety of light spectra.  The observer recognizes
+    * all spectra that produce this response as the same color, therefore this
+    * is effectively a description of a color.
+    */
+   double X, Y, Z;
+} CIE_color;
+
+typedef struct color_encoding
+{
+   /* A description of an (R,G,B) encoding of color (as defined above); this
+    * includes the actual colors of the (R,G,B) triples (1,0,0), (0,1,0) and
+    * (0,0,1) plus an encoding value that is used to encode the linear
+    * components R, G and B to give the actual values R^gamma, G^gamma and
+    * B^gamma that are stored.
+    */
+   double    gamma;            /* Encoding (file) gamma of space */
+   CIE_color red, green, blue; /* End points */
+} color_encoding;
+
+#ifdef PNG_READ_SUPPORTED
+#if defined PNG_READ_TRANSFORMS_SUPPORTED && defined PNG_READ_cHRM_SUPPORTED
+static double
+chromaticity_x(CIE_color c)
+{
+   return c.X / (c.X + c.Y + c.Z);
+}
+
+static double
+chromaticity_y(CIE_color c)
+{
+   return c.Y / (c.X + c.Y + c.Z);
+}
+
+static CIE_color
+white_point(const color_encoding *encoding)
+{
+   CIE_color white;
+
+   white.X = encoding->red.X + encoding->green.X + encoding->blue.X;
+   white.Y = encoding->red.Y + encoding->green.Y + encoding->blue.Y;
+   white.Z = encoding->red.Z + encoding->green.Z + encoding->blue.Z;
+
+   return white;
+}
+#endif /* READ_TRANSFORMS && READ_cHRM */
+
+#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
+static void
+normalize_color_encoding(color_encoding *encoding)
+{
+   const double whiteY = encoding->red.Y + encoding->green.Y +
+      encoding->blue.Y;
+
+   if (whiteY != 1)
+   {
+      encoding->red.X /= whiteY;
+      encoding->red.Y /= whiteY;
+      encoding->red.Z /= whiteY;
+      encoding->green.X /= whiteY;
+      encoding->green.Y /= whiteY;
+      encoding->green.Z /= whiteY;
+      encoding->blue.X /= whiteY;
+      encoding->blue.Y /= whiteY;
+      encoding->blue.Z /= whiteY;
+   }
+}
+#endif
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+static size_t
+safecat_color_encoding(char *buffer, size_t bufsize, size_t pos,
+   const color_encoding *e, double encoding_gamma)
+{
+   if (e != 0)
+   {
+      if (encoding_gamma != 0)
+         pos = safecat(buffer, bufsize, pos, "(");
+      pos = safecat(buffer, bufsize, pos, "R(");
+      pos = safecatd(buffer, bufsize, pos, e->red.X, 4);
+      pos = safecat(buffer, bufsize, pos, ",");
+      pos = safecatd(buffer, bufsize, pos, e->red.Y, 4);
+      pos = safecat(buffer, bufsize, pos, ",");
+      pos = safecatd(buffer, bufsize, pos, e->red.Z, 4);
+      pos = safecat(buffer, bufsize, pos, "),G(");
+      pos = safecatd(buffer, bufsize, pos, e->green.X, 4);
+      pos = safecat(buffer, bufsize, pos, ",");
+      pos = safecatd(buffer, bufsize, pos, e->green.Y, 4);
+      pos = safecat(buffer, bufsize, pos, ",");
+      pos = safecatd(buffer, bufsize, pos, e->green.Z, 4);
+      pos = safecat(buffer, bufsize, pos, "),B(");
+      pos = safecatd(buffer, bufsize, pos, e->blue.X, 4);
+      pos = safecat(buffer, bufsize, pos, ",");
+      pos = safecatd(buffer, bufsize, pos, e->blue.Y, 4);
+      pos = safecat(buffer, bufsize, pos, ",");
+      pos = safecatd(buffer, bufsize, pos, e->blue.Z, 4);
+      pos = safecat(buffer, bufsize, pos, ")");
+      if (encoding_gamma != 0)
+         pos = safecat(buffer, bufsize, pos, ")");
+   }
+
+   if (encoding_gamma != 0)
+   {
+      pos = safecat(buffer, bufsize, pos, "^");
+      pos = safecatd(buffer, bufsize, pos, encoding_gamma, 5);
+   }
+
+   return pos;
+}
+#endif /* READ_TRANSFORMS */
+#endif /* PNG_READ_SUPPORTED */
+
+typedef struct png_modifier
+{
+   png_store               this;             /* I am a png_store */
+   struct png_modification *modifications;   /* Changes to make */
+
+   modifier_state           state;           /* My state */
+
+   /* Information from IHDR: */
+   png_byte                 bit_depth;       /* From IHDR */
+   png_byte                 colour_type;     /* From IHDR */
+
+   /* While handling PLTE, IDAT and IEND these chunks may be pended to allow
+    * other chunks to be inserted.
+    */
+   png_uint_32              pending_len;
+   png_uint_32              pending_chunk;
+
+   /* Test values */
+   double                   *gammas;
+   unsigned int              ngammas;
+   unsigned int              ngamma_tests;     /* Number of gamma tests to run*/
+   double                    current_gamma;    /* 0 if not set */
+   const color_encoding *encodings;
+   unsigned int              nencodings;
+   const color_encoding *current_encoding; /* If an encoding has been set */
+   unsigned int              encoding_counter; /* For iteration */
+   int                       encoding_ignored; /* Something overwrote it */
+
+   /* Control variables used to iterate through possible encodings, the
+    * following must be set to 0 and tested by the function that uses the
+    * png_modifier because the modifier only sets it to 1 (true.)
+    */
+   unsigned int              repeat :1;   /* Repeat this transform test. */
+   unsigned int              test_uses_encoding :1;
+
+   /* Lowest sbit to test (pre-1.7 libpng fails for sbit < 8) */
+   png_byte                 sbitlow;
+
+   /* Error control - these are the limits on errors accepted by the gamma tests
+    * below.
+    */
+   double                   maxout8;  /* Maximum output value error */
+   double                   maxabs8;  /* Absolute sample error 0..1 */
+   double                   maxcalc8; /* Absolute sample error 0..1 */
+   double                   maxpc8;   /* Percentage sample error 0..100% */
+   double                   maxout16; /* Maximum output value error */
+   double                   maxabs16; /* Absolute sample error 0..1 */
+   double                   maxcalc16;/* Absolute sample error 0..1 */
+   double                   maxcalcG; /* Absolute sample error 0..1 */
+   double                   maxpc16;  /* Percentage sample error 0..100% */
+
+   /* This is set by transforms that need to allow a higher limit, it is an
+    * internal check on pngvalid to ensure that the calculated error limits are
+    * not ridiculous; without this it is too easy to make a mistake in pngvalid
+    * that allows any value through.
+    *
+    * NOTE: this is not checked in release builds.
+    */
+   double                   limit;    /* limit on error values, normally 4E-3 */
+
+   /* Log limits - values above this are logged, but not necessarily
+    * warned.
+    */
+   double                   log8;     /* Absolute error in 8 bits to log */
+   double                   log16;    /* Absolute error in 16 bits to log */
+
+   /* Logged 8 and 16 bit errors ('output' values): */
+   double                   error_gray_2;
+   double                   error_gray_4;
+   double                   error_gray_8;
+   double                   error_gray_16;
+   double                   error_color_8;
+   double                   error_color_16;
+   double                   error_indexed;
+
+   /* Flags: */
+   /* Whether to call png_read_update_info, not png_read_start_image, and how
+    * many times to call it.
+    */
+   int                      use_update_info;
+
+   /* Whether or not to interlace. */
+   int                      interlace_type :9; /* int, but must store '1' */
+
+   /* Run the standard tests? */
+   unsigned int             test_standard :1;
+
+   /* Run the odd-sized image and interlace read/write tests? */
+   unsigned int             test_size :1;
+
+   /* Run tests on reading with a combination of transforms, */
+   unsigned int             test_transform :1;
+   unsigned int             test_tRNS :1; /* Includes tRNS images */
+
+   /* When to use the use_input_precision option, this controls the gamma
+    * validation code checks.  If set any value that is within the transformed
+    * range input-.5 to input+.5 will be accepted, otherwise the value must be
+    * within the normal limits.  It should not be necessary to set this; the
+    * result should always be exact within the permitted error limits.
+    */
+   unsigned int             use_input_precision :1;
+   unsigned int             use_input_precision_sbit :1;
+   unsigned int             use_input_precision_16to8 :1;
+
+   /* If set assume that the calculation bit depth is set by the input
+    * precision, not the output precision.
+    */
+   unsigned int             calculations_use_input_precision :1;
+
+   /* If set assume that the calculations are done in 16 bits even if the sample
+    * depth is 8 bits.
+    */
+   unsigned int             assume_16_bit_calculations :1;
+
+   /* Which gamma tests to run: */
+   unsigned int             test_gamma_threshold :1;
+   unsigned int             test_gamma_transform :1; /* main tests */
+   unsigned int             test_gamma_sbit :1;
+   unsigned int             test_gamma_scale16 :1;
+   unsigned int             test_gamma_background :1;
+   unsigned int             test_gamma_alpha_mode :1;
+   unsigned int             test_gamma_expand16 :1;
+   unsigned int             test_exhaustive :1;
+
+   /* Whether or not to run the low-bit-depth grayscale tests.  This fails on
+    * gamma images in some cases because of gross inaccuracies in the grayscale
+    * gamma handling for low bit depth.
+    */
+   unsigned int             test_lbg :1;
+   unsigned int             test_lbg_gamma_threshold :1;
+   unsigned int             test_lbg_gamma_transform :1;
+   unsigned int             test_lbg_gamma_sbit :1;
+   unsigned int             test_lbg_gamma_composition :1;
+
+   unsigned int             log :1;   /* Log max error */
+
+   /* Buffer information, the buffer size limits the size of the chunks that can
+    * be modified - they must fit (including header and CRC) into the buffer!
+    */
+   size_t                   flush;           /* Count of bytes to flush */
+   size_t                   buffer_count;    /* Bytes in buffer */
+   size_t                   buffer_position; /* Position in buffer */
+   png_byte                 buffer[1024];
+} png_modifier;
+
+/* This returns true if the test should be stopped now because it has already
+ * failed and it is running silently.
+  */
+static int fail(png_modifier *pm)
+{
+   return !pm->log && !pm->this.verbose && (pm->this.nerrors > 0 ||
+       (pm->this.treat_warnings_as_errors && pm->this.nwarnings > 0));
+}
+
+static void
+modifier_init(png_modifier *pm)
+{
+   memset(pm, 0, sizeof *pm);
+   store_init(&pm->this);
+   pm->modifications = NULL;
+   pm->state = modifier_start;
+   pm->sbitlow = 1U;
+   pm->ngammas = 0;
+   pm->ngamma_tests = 0;
+   pm->gammas = 0;
+   pm->current_gamma = 0;
+   pm->encodings = 0;
+   pm->nencodings = 0;
+   pm->current_encoding = 0;
+   pm->encoding_counter = 0;
+   pm->encoding_ignored = 0;
+   pm->repeat = 0;
+   pm->test_uses_encoding = 0;
+   pm->maxout8 = pm->maxpc8 = pm->maxabs8 = pm->maxcalc8 = 0;
+   pm->maxout16 = pm->maxpc16 = pm->maxabs16 = pm->maxcalc16 = 0;
+   pm->maxcalcG = 0;
+   pm->limit = 4E-3;
+   pm->log8 = pm->log16 = 0; /* Means 'off' */
+   pm->error_gray_2 = pm->error_gray_4 = pm->error_gray_8 = 0;
+   pm->error_gray_16 = pm->error_color_8 = pm->error_color_16 = 0;
+   pm->error_indexed = 0;
+   pm->use_update_info = 0;
+   pm->interlace_type = PNG_INTERLACE_NONE;
+   pm->test_standard = 0;
+   pm->test_size = 0;
+   pm->test_transform = 0;
+#  ifdef PNG_WRITE_tRNS_SUPPORTED
+      pm->test_tRNS = 1;
+#  else
+      pm->test_tRNS = 0;
+#  endif
+   pm->use_input_precision = 0;
+   pm->use_input_precision_sbit = 0;
+   pm->use_input_precision_16to8 = 0;
+   pm->calculations_use_input_precision = 0;
+   pm->assume_16_bit_calculations = 0;
+   pm->test_gamma_threshold = 0;
+   pm->test_gamma_transform = 0;
+   pm->test_gamma_sbit = 0;
+   pm->test_gamma_scale16 = 0;
+   pm->test_gamma_background = 0;
+   pm->test_gamma_alpha_mode = 0;
+   pm->test_gamma_expand16 = 0;
+   pm->test_lbg = 1;
+   pm->test_lbg_gamma_threshold = 1;
+   pm->test_lbg_gamma_transform = 1;
+   pm->test_lbg_gamma_sbit = 1;
+   pm->test_lbg_gamma_composition = 1;
+   pm->test_exhaustive = 0;
+   pm->log = 0;
+
+   /* Rely on the memset for all the other fields - there are no pointers */
+}
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+
+/* This controls use of checks that explicitly know how libpng digitizes the
+ * samples in calculations; setting this circumvents simple error limit checking
+ * in the rgb_to_gray check, replacing it with an exact copy of the libpng 1.5
+ * algorithm.
+ */
+#define DIGITIZE PNG_LIBPNG_VER < 10700
+
+/* If pm->calculations_use_input_precision is set then operations will happen
+ * with the precision of the input, not the precision of the output depth.
+ *
+ * If pm->assume_16_bit_calculations is set then even 8 bit calculations use 16
+ * bit precision.  This only affects those of the following limits that pertain
+ * to a calculation - not a digitization operation - unless the following API is
+ * called directly.
+ */
+#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
+#if DIGITIZE
+static double digitize(double value, int depth, int do_round)
+{
+   /* 'value' is in the range 0 to 1, the result is the same value rounded to a
+    * multiple of the digitization factor - 8 or 16 bits depending on both the
+    * sample depth and the 'assume' setting.  Digitization is normally by
+    * rounding and 'do_round' should be 1, if it is 0 the digitized value will
+    * be truncated.
+    */
+   const unsigned int digitization_factor = (1U << depth) -1;
+
+   /* Limiting the range is done as a convenience to the caller - it's easier to
+    * do it once here than every time at the call site.
+    */
+   if (value <= 0)
+      value = 0;
+
+   else if (value >= 1)
+      value = 1;
+
+   value *= digitization_factor;
+   if (do_round) value += .5;
+   return floor(value)/digitization_factor;
+}
+#endif
+#endif /* RGB_TO_GRAY */
+
+#ifdef PNG_READ_GAMMA_SUPPORTED
+static double abserr(const png_modifier *pm, int in_depth, int out_depth)
+{
+   /* Absolute error permitted in linear values - affected by the bit depth of
+    * the calculations.
+    */
+   if (pm->assume_16_bit_calculations ||
+      (pm->calculations_use_input_precision ? in_depth : out_depth) == 16)
+      return pm->maxabs16;
+   else
+      return pm->maxabs8;
+}
+
+static double calcerr(const png_modifier *pm, int in_depth, int out_depth)
+{
+   /* Error in the linear composition arithmetic - only relevant when
+    * composition actually happens (0 < alpha < 1).
+    */
+   if ((pm->calculations_use_input_precision ? in_depth : out_depth) == 16)
+      return pm->maxcalc16;
+   else if (pm->assume_16_bit_calculations)
+      return pm->maxcalcG;
+   else
+      return pm->maxcalc8;
+}
+
+static double pcerr(const png_modifier *pm, int in_depth, int out_depth)
+{
+   /* Percentage error permitted in the linear values.  Note that the specified
+    * value is a percentage but this routine returns a simple number.
+    */
+   if (pm->assume_16_bit_calculations ||
+      (pm->calculations_use_input_precision ? in_depth : out_depth) == 16)
+      return pm->maxpc16 * .01;
+   else
+      return pm->maxpc8 * .01;
+}
+
+/* Output error - the error in the encoded value.  This is determined by the
+ * digitization of the output so can be +/-0.5 in the actual output value.  In
+ * the expand_16 case with the current code in libpng the expand happens after
+ * all the calculations are done in 8 bit arithmetic, so even though the output
+ * depth is 16 the output error is determined by the 8 bit calculation.
+ *
+ * This limit is not determined by the bit depth of internal calculations.
+ *
+ * The specified parameter does *not* include the base .5 digitization error but
+ * it is added here.
+ */
+static double outerr(const png_modifier *pm, int in_depth, int out_depth)
+{
+   /* There is a serious error in the 2 and 4 bit grayscale transform because
+    * the gamma table value (8 bits) is simply shifted, not rounded, so the
+    * error in 4 bit grayscale gamma is up to the value below.  This is a hack
+    * to allow pngvalid to succeed:
+    *
+    * TODO: fix this in libpng
+    */
+   if (out_depth == 2)
+      return .73182-.5;
+
+   if (out_depth == 4)
+      return .90644-.5;
+
+   if ((pm->calculations_use_input_precision ? in_depth : out_depth) == 16)
+      return pm->maxout16;
+
+   /* This is the case where the value was calculated at 8-bit precision then
+    * scaled to 16 bits.
+    */
+   else if (out_depth == 16)
+      return pm->maxout8 * 257;
+
+   else
+      return pm->maxout8;
+}
+
+/* This does the same thing as the above however it returns the value to log,
+ * rather than raising a warning.  This is useful for debugging to track down
+ * exactly what set of parameters cause high error values.
+ */
+static double outlog(const png_modifier *pm, int in_depth, int out_depth)
+{
+   /* The command line parameters are either 8 bit (0..255) or 16 bit (0..65535)
+    * and so must be adjusted for low bit depth grayscale:
+    */
+   if (out_depth <= 8)
+   {
+      if (pm->log8 == 0) /* switched off */
+         return 256;
+
+      if (out_depth < 8)
+         return pm->log8 / 255 * ((1<<out_depth)-1);
+
+      return pm->log8;
+   }
+
+   if ((pm->calculations_use_input_precision ? in_depth : out_depth) == 16)
+   {
+      if (pm->log16 == 0)
+         return 65536;
+
+      return pm->log16;
+   }
+
+   /* This is the case where the value was calculated at 8-bit precision then
+    * scaled to 16 bits.
+    */
+   if (pm->log8 == 0)
+      return 65536;
+
+   return pm->log8 * 257;
+}
+
+/* This complements the above by providing the appropriate quantization for the
+ * final value.  Normally this would just be quantization to an integral value,
+ * but in the 8 bit calculation case it's actually quantization to a multiple of
+ * 257!
+ */
+static int output_quantization_factor(const png_modifier *pm, int in_depth,
+   int out_depth)
+{
+   if (out_depth == 16 && in_depth != 16 &&
+      pm->calculations_use_input_precision)
+      return 257;
+   else
+      return 1;
+}
+#endif /* PNG_READ_GAMMA_SUPPORTED */
+
+/* One modification structure must be provided for each chunk to be modified (in
+ * fact more than one can be provided if multiple separate changes are desired
+ * for a single chunk.)  Modifications include adding a new chunk when a
+ * suitable chunk does not exist.
+ *
+ * The caller of modify_fn will reset the CRC of the chunk and record 'modified'
+ * or 'added' as appropriate if the modify_fn returns 1 (true).  If the
+ * modify_fn is NULL the chunk is simply removed.
+ */
+typedef struct png_modification
+{
+   struct png_modification *next;
+   png_uint_32              chunk;
+
+   /* If the following is NULL all matching chunks will be removed: */
+   int                    (*modify_fn)(struct png_modifier *pm,
+                               struct png_modification *me, int add);
+
+   /* If the following is set to PLTE, IDAT or IEND and the chunk has not been
+    * found and modified (and there is a modify_fn) the modify_fn will be called
+    * to add the chunk before the relevant chunk.
+    */
+   png_uint_32              add;
+   unsigned int             modified :1;     /* Chunk was modified */
+   unsigned int             added    :1;     /* Chunk was added */
+   unsigned int             removed  :1;     /* Chunk was removed */
+} png_modification;
+
+static void
+modification_reset(png_modification *pmm)
+{
+   if (pmm != NULL)
+   {
+      pmm->modified = 0;
+      pmm->added = 0;
+      pmm->removed = 0;
+      modification_reset(pmm->next);
+   }
+}
+
+static void
+modification_init(png_modification *pmm)
+{
+   memset(pmm, 0, sizeof *pmm);
+   pmm->next = NULL;
+   pmm->chunk = 0;
+   pmm->modify_fn = NULL;
+   pmm->add = 0;
+   modification_reset(pmm);
+}
+
+#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
+static void
+modifier_current_encoding(const png_modifier *pm, color_encoding *ce)
+{
+   if (pm->current_encoding != 0)
+      *ce = *pm->current_encoding;
+
+   else
+      memset(ce, 0, sizeof *ce);
+
+   ce->gamma = pm->current_gamma;
+}
+#endif
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+static size_t
+safecat_current_encoding(char *buffer, size_t bufsize, size_t pos,
+   const png_modifier *pm)
+{
+   pos = safecat_color_encoding(buffer, bufsize, pos, pm->current_encoding,
+      pm->current_gamma);
+
+   if (pm->encoding_ignored)
+      pos = safecat(buffer, bufsize, pos, "[overridden]");
+
+   return pos;
+}
+#endif
+
+/* Iterate through the usefully testable color encodings.  An encoding is one
+ * of:
+ *
+ * 1) Nothing (no color space, no gamma).
+ * 2) Just a gamma value from the gamma array (including 1.0)
+ * 3) A color space from the encodings array with the corresponding gamma.
+ * 4) The same, but with gamma 1.0 (only really useful with 16 bit calculations)
+ *
+ * The iterator selects these in turn, the randomizer selects one at random,
+ * which is used depends on the setting of the 'test_exhaustive' flag.  Notice
+ * that this function changes the colour space encoding so it must only be
+ * called on completion of the previous test.  This is what 'modifier_reset'
+ * does, below.
+ *
+ * After the function has been called the 'repeat' flag will still be set; the
+ * caller of modifier_reset must reset it at the start of each run of the test!
+ */
+static unsigned int
+modifier_total_encodings(const png_modifier *pm)
+{
+   return 1 +                 /* (1) nothing */
+      pm->ngammas +           /* (2) gamma values to test */
+      pm->nencodings +        /* (3) total number of encodings */
+      /* The following test only works after the first time through the
+       * png_modifier code because 'bit_depth' is set when the IHDR is read.
+       * modifier_reset, below, preserves the setting until after it has called
+       * the iterate function (also below.)
+       *
+       * For this reason do not rely on this function outside a call to
+       * modifier_reset.
+       */
+      ((pm->bit_depth == 16 || pm->assume_16_bit_calculations) ?
+         pm->nencodings : 0); /* (4) encodings with gamma == 1.0 */
+}
+
+static void
+modifier_encoding_iterate(png_modifier *pm)
+{
+   if (!pm->repeat && /* Else something needs the current encoding again. */
+      pm->test_uses_encoding) /* Some transform is encoding dependent */
+   {
+      if (pm->test_exhaustive)
+      {
+         if (++pm->encoding_counter >= modifier_total_encodings(pm))
+            pm->encoding_counter = 0; /* This will stop the repeat */
+      }
+
+      else
+      {
+         /* Not exhaustive - choose an encoding at random; generate a number in
+          * the range 1..(max-1), so the result is always non-zero:
+          */
+         if (pm->encoding_counter == 0)
+            pm->encoding_counter = random_mod(modifier_total_encodings(pm)-1)+1;
+         else
+            pm->encoding_counter = 0;
+      }
+
+      if (pm->encoding_counter > 0)
+         pm->repeat = 1;
+   }
+
+   else if (!pm->repeat)
+      pm->encoding_counter = 0;
+}
+
+static void
+modifier_reset(png_modifier *pm)
+{
+   store_read_reset(&pm->this);
+   pm->limit = 4E-3;
+   pm->pending_len = pm->pending_chunk = 0;
+   pm->flush = pm->buffer_count = pm->buffer_position = 0;
+   pm->modifications = NULL;
+   pm->state = modifier_start;
+   modifier_encoding_iterate(pm);
+   /* The following must be set in the next run.  In particular
+    * test_uses_encodings must be set in the _ini function of each transform
+    * that looks at the encodings.  (Not the 'add' function!)
+    */
+   pm->test_uses_encoding = 0;
+   pm->current_gamma = 0;
+   pm->current_encoding = 0;
+   pm->encoding_ignored = 0;
+   /* These only become value after IHDR is read: */
+   pm->bit_depth = pm->colour_type = 0;
+}
+
+/* The following must be called before anything else to get the encoding set up
+ * on the modifier.  In particular it must be called before the transform init
+ * functions are called.
+ */
+static void
+modifier_set_encoding(png_modifier *pm)
+{
+   /* Set the encoding to the one specified by the current encoding counter,
+    * first clear out all the settings - this corresponds to an encoding_counter
+    * of 0.
+    */
+   pm->current_gamma = 0;
+   pm->current_encoding = 0;
+   pm->encoding_ignored = 0; /* not ignored yet - happens in _ini functions. */
+
+   /* Now, if required, set the gamma and encoding fields. */
+   if (pm->encoding_counter > 0)
+   {
+      /* The gammas[] array is an array of screen gammas, not encoding gammas,
+       * so we need the inverse:
+       */
+      if (pm->encoding_counter <= pm->ngammas)
+         pm->current_gamma = 1/pm->gammas[pm->encoding_counter-1];
+
+      else
+      {
+         unsigned int i = pm->encoding_counter - pm->ngammas;
+
+         if (i >= pm->nencodings)
+         {
+            i %= pm->nencodings;
+            pm->current_gamma = 1; /* Linear, only in the 16 bit case */
+         }
+
+         else
+            pm->current_gamma = pm->encodings[i].gamma;
+
+         pm->current_encoding = pm->encodings + i;
+      }
+   }
+}
+
+/* Enquiry functions to find out what is set.  Notice that there is an implicit
+ * assumption below that the first encoding in the list is the one for sRGB.
+ */
+static int
+modifier_color_encoding_is_sRGB(const png_modifier *pm)
+{
+   return pm->current_encoding != 0 && pm->current_encoding == pm->encodings &&
+      pm->current_encoding->gamma == pm->current_gamma;
+}
+
+static int
+modifier_color_encoding_is_set(const png_modifier *pm)
+{
+   return pm->current_gamma != 0;
+}
+
+/* Convenience macros. */
+#define CHUNK(a,b,c,d) (((a)<<24)+((b)<<16)+((c)<<8)+(d))
+#define CHUNK_IHDR CHUNK(73,72,68,82)
+#define CHUNK_PLTE CHUNK(80,76,84,69)
+#define CHUNK_IDAT CHUNK(73,68,65,84)
+#define CHUNK_IEND CHUNK(73,69,78,68)
+#define CHUNK_cHRM CHUNK(99,72,82,77)
+#define CHUNK_gAMA CHUNK(103,65,77,65)
+#define CHUNK_sBIT CHUNK(115,66,73,84)
+#define CHUNK_sRGB CHUNK(115,82,71,66)
+
+/* The guts of modification are performed during a read. */
+static void
+modifier_crc(png_bytep buffer)
+{
+   /* Recalculate the chunk CRC - a complete chunk must be in
+    * the buffer, at the start.
+    */
+   uInt datalen = png_get_uint_32(buffer);
+   uLong crc = crc32(0, buffer+4, datalen+4);
+   /* The cast to png_uint_32 is safe because a crc32 is always a 32 bit value.
+    */
+   png_save_uint_32(buffer+datalen+8, (png_uint_32)crc);
+}
+
+static void
+modifier_setbuffer(png_modifier *pm)
+{
+   modifier_crc(pm->buffer);
+   pm->buffer_count = png_get_uint_32(pm->buffer)+12;
+   pm->buffer_position = 0;
+}
+
+/* Separate the callback into the actual implementation (which is passed the
+ * png_modifier explicitly) and the callback, which gets the modifier from the
+ * png_struct.
+ */
+static void
+modifier_read_imp(png_modifier *pm, png_bytep pb, png_size_t st)
+{
+   while (st > 0)
+   {
+      size_t cb;
+      png_uint_32 len, chunk;
+      png_modification *mod;
+
+      if (pm->buffer_position >= pm->buffer_count) switch (pm->state)
+      {
+         static png_byte sign[8] = { 137, 80, 78, 71, 13, 10, 26, 10 };
+         case modifier_start:
+            store_read_imp(&pm->this, pm->buffer, 8); /* size of signature. */
+            pm->buffer_count = 8;
+            pm->buffer_position = 0;
+
+            if (memcmp(pm->buffer, sign, 8) != 0)
+               png_error(pm->this.pread, "invalid PNG file signature");
+            pm->state = modifier_signature;
+            break;
+
+         case modifier_signature:
+            store_read_imp(&pm->this, pm->buffer, 13+12); /* size of IHDR */
+            pm->buffer_count = 13+12;
+            pm->buffer_position = 0;
+
+            if (png_get_uint_32(pm->buffer) != 13 ||
+                png_get_uint_32(pm->buffer+4) != CHUNK_IHDR)
+               png_error(pm->this.pread, "invalid IHDR");
+
+            /* Check the list of modifiers for modifications to the IHDR. */
+            mod = pm->modifications;
+            while (mod != NULL)
+            {
+               if (mod->chunk == CHUNK_IHDR && mod->modify_fn &&
+                   (*mod->modify_fn)(pm, mod, 0))
+                  {
+                  mod->modified = 1;
+                  modifier_setbuffer(pm);
+                  }
+
+               /* Ignore removal or add if IHDR! */
+               mod = mod->next;
+            }
+
+            /* Cache information from the IHDR (the modified one.) */
+            pm->bit_depth = pm->buffer[8+8];
+            pm->colour_type = pm->buffer[8+8+1];
+
+            pm->state = modifier_IHDR;
+            pm->flush = 0;
+            break;
+
+         case modifier_IHDR:
+         default:
+            /* Read a new chunk and process it until we see PLTE, IDAT or
+             * IEND.  'flush' indicates that there is still some data to
+             * output from the preceding chunk.
+             */
+            if ((cb = pm->flush) > 0)
+            {
+               if (cb > st) cb = st;
+               pm->flush -= cb;
+               store_read_imp(&pm->this, pb, cb);
+               pb += cb;
+               st -= cb;
+               if (st == 0) return;
+            }
+
+            /* No more bytes to flush, read a header, or handle a pending
+             * chunk.
+             */
+            if (pm->pending_chunk != 0)
+            {
+               png_save_uint_32(pm->buffer, pm->pending_len);
+               png_save_uint_32(pm->buffer+4, pm->pending_chunk);
+               pm->pending_len = 0;
+               pm->pending_chunk = 0;
+            }
+            else
+               store_read_imp(&pm->this, pm->buffer, 8);
+
+            pm->buffer_count = 8;
+            pm->buffer_position = 0;
+
+            /* Check for something to modify or a terminator chunk. */
+            len = png_get_uint_32(pm->buffer);
+            chunk = png_get_uint_32(pm->buffer+4);
+
+            /* Terminators first, they may have to be delayed for added
+             * chunks
+             */
+            if (chunk == CHUNK_PLTE || chunk == CHUNK_IDAT ||
+                chunk == CHUNK_IEND)
+            {
+               mod = pm->modifications;
+
+               while (mod != NULL)
+               {
+                  if ((mod->add == chunk ||
+                      (mod->add == CHUNK_PLTE && chunk == CHUNK_IDAT)) &&
+                      mod->modify_fn != NULL && !mod->modified && !mod->added)
+                  {
+                     /* Regardless of what the modify function does do not run
+                      * this again.
+                      */
+                     mod->added = 1;
+
+                     if ((*mod->modify_fn)(pm, mod, 1 /*add*/))
+                     {
+                        /* Reset the CRC on a new chunk */
+                        if (pm->buffer_count > 0)
+                           modifier_setbuffer(pm);
+
+                        else
+                           {
+                           pm->buffer_position = 0;
+                           mod->removed = 1;
+                           }
+
+                        /* The buffer has been filled with something (we assume)
+                         * so output this.  Pend the current chunk.
+                         */
+                        pm->pending_len = len;
+                        pm->pending_chunk = chunk;
+                        break; /* out of while */
+                     }
+                  }
+
+                  mod = mod->next;
+               }
+
+               /* Don't do any further processing if the buffer was modified -
+                * otherwise the code will end up modifying a chunk that was
+                * just added.
+                */
+               if (mod != NULL)
+                  break; /* out of switch */
+            }
+
+            /* If we get to here then this chunk may need to be modified.  To
+             * do this it must be less than 1024 bytes in total size, otherwise
+             * it just gets flushed.
+             */
+            if (len+12 <= sizeof pm->buffer)
+            {
+               store_read_imp(&pm->this, pm->buffer+pm->buffer_count,
+                   len+12-pm->buffer_count);
+               pm->buffer_count = len+12;
+
+               /* Check for a modification, else leave it be. */
+               mod = pm->modifications;
+               while (mod != NULL)
+               {
+                  if (mod->chunk == chunk)
+                  {
+                     if (mod->modify_fn == NULL)
+                     {
+                        /* Remove this chunk */
+                        pm->buffer_count = pm->buffer_position = 0;
+                        mod->removed = 1;
+                        break; /* Terminate the while loop */
+                     }
+
+                     else if ((*mod->modify_fn)(pm, mod, 0))
+                     {
+                        mod->modified = 1;
+                        /* The chunk may have been removed: */
+                        if (pm->buffer_count == 0)
+                        {
+                           pm->buffer_position = 0;
+                           break;
+                        }
+                        modifier_setbuffer(pm);
+                     }
+                  }
+
+                  mod = mod->next;
+               }
+            }
+
+            else
+               pm->flush = len+12 - pm->buffer_count; /* data + crc */
+
+            /* Take the data from the buffer (if there is any). */
+            break;
+      }
+
+      /* Here to read from the modifier buffer (not directly from
+       * the store, as in the flush case above.)
+       */
+      cb = pm->buffer_count - pm->buffer_position;
+
+      if (cb > st)
+         cb = st;
+
+      memcpy(pb, pm->buffer + pm->buffer_position, cb);
+      st -= cb;
+      pb += cb;
+      pm->buffer_position += cb;
+   }
+}
+
+/* The callback: */
+static void PNGCBAPI
+modifier_read(png_structp ppIn, png_bytep pb, png_size_t st)
+{
+   png_const_structp pp = ppIn;
+   png_modifier *pm = voidcast(png_modifier*, png_get_io_ptr(pp));
+
+   if (pm == NULL || pm->this.pread != pp)
+      png_error(pp, "bad modifier_read call");
+
+   modifier_read_imp(pm, pb, st);
+}
+
+/* Like store_progressive_read but the data is getting changed as we go so we
+ * need a local buffer.
+ */
+static void
+modifier_progressive_read(png_modifier *pm, png_structp pp, png_infop pi)
+{
+   if (pm->this.pread != pp || pm->this.current == NULL ||
+       pm->this.next == NULL)
+      png_error(pp, "store state damaged (progressive)");
+
+   /* This is another Horowitz and Hill random noise generator.  In this case
+    * the aim is to stress the progressive reader with truly horrible variable
+    * buffer sizes in the range 1..500, so a sequence of 9 bit random numbers
+    * is generated.  We could probably just count from 1 to 32767 and get as
+    * good a result.
+    */
+   for (;;)
+   {
+      static png_uint_32 noise = 1;
+      png_size_t cb, cbAvail;
+      png_byte buffer[512];
+
+      /* Generate 15 more bits of stuff: */
+      noise = (noise << 9) | ((noise ^ (noise >> (9-5))) & 0x1ff);
+      cb = noise & 0x1ff;
+
+      /* Check that this number of bytes are available (in the current buffer.)
+       * (This doesn't quite work - the modifier might delete a chunk; unlikely
+       * but possible, it doesn't happen at present because the modifier only
+       * adds chunks to standard images.)
+       */
+      cbAvail = store_read_buffer_avail(&pm->this);
+      if (pm->buffer_count > pm->buffer_position)
+         cbAvail += pm->buffer_count - pm->buffer_position;
+
+      if (cb > cbAvail)
+      {
+         /* Check for EOF: */
+         if (cbAvail == 0)
+            break;
+
+         cb = cbAvail;
+      }
+
+      modifier_read_imp(pm, buffer, cb);
+      png_process_data(pp, pi, buffer, cb);
+   }
+
+   /* Check the invariants at the end (if this fails it's a problem in this
+    * file!)
+    */
+   if (pm->buffer_count > pm->buffer_position ||
+       pm->this.next != &pm->this.current->data ||
+       pm->this.readpos < pm->this.current->datacount)
+      png_error(pp, "progressive read implementation error");
+}
+
+/* Set up a modifier. */
+static png_structp
+set_modifier_for_read(png_modifier *pm, png_infopp ppi, png_uint_32 id,
+    const char *name)
+{
+   /* Do this first so that the modifier fields are cleared even if an error
+    * happens allocating the png_struct.  No allocation is done here so no
+    * cleanup is required.
+    */
+   pm->state = modifier_start;
+   pm->bit_depth = 0;
+   pm->colour_type = 255;
+
+   pm->pending_len = 0;
+   pm->pending_chunk = 0;
+   pm->flush = 0;
+   pm->buffer_count = 0;
+   pm->buffer_position = 0;
+
+   return set_store_for_read(&pm->this, ppi, id, name);
+}
+
+
+/******************************** MODIFICATIONS *******************************/
+/* Standard modifications to add chunks.  These do not require the _SUPPORTED
+ * macros because the chunks can be there regardless of whether this specific
+ * libpng supports them.
+ */
+typedef struct gama_modification
+{
+   png_modification this;
+   png_fixed_point  gamma;
+} gama_modification;
+
+static int
+gama_modify(png_modifier *pm, png_modification *me, int add)
+{
+   UNUSED(add)
+   /* This simply dumps the given gamma value into the buffer. */
+   png_save_uint_32(pm->buffer, 4);
+   png_save_uint_32(pm->buffer+4, CHUNK_gAMA);
+   png_save_uint_32(pm->buffer+8, ((gama_modification*)me)->gamma);
+   return 1;
+}
+
+static void
+gama_modification_init(gama_modification *me, png_modifier *pm, double gammad)
+{
+   double g;
+
+   modification_init(&me->this);
+   me->this.chunk = CHUNK_gAMA;
+   me->this.modify_fn = gama_modify;
+   me->this.add = CHUNK_PLTE;
+   g = fix(gammad);
+   me->gamma = (png_fixed_point)g;
+   me->this.next = pm->modifications;
+   pm->modifications = &me->this;
+}
+
+typedef struct chrm_modification
+{
+   png_modification          this;
+   const color_encoding *encoding;
+   png_fixed_point           wx, wy, rx, ry, gx, gy, bx, by;
+} chrm_modification;
+
+static int
+chrm_modify(png_modifier *pm, png_modification *me, int add)
+{
+   UNUSED(add)
+   /* As with gAMA this just adds the required cHRM chunk to the buffer. */
+   png_save_uint_32(pm->buffer   , 32);
+   png_save_uint_32(pm->buffer+ 4, CHUNK_cHRM);
+   png_save_uint_32(pm->buffer+ 8, ((chrm_modification*)me)->wx);
+   png_save_uint_32(pm->buffer+12, ((chrm_modification*)me)->wy);
+   png_save_uint_32(pm->buffer+16, ((chrm_modification*)me)->rx);
+   png_save_uint_32(pm->buffer+20, ((chrm_modification*)me)->ry);
+   png_save_uint_32(pm->buffer+24, ((chrm_modification*)me)->gx);
+   png_save_uint_32(pm->buffer+28, ((chrm_modification*)me)->gy);
+   png_save_uint_32(pm->buffer+32, ((chrm_modification*)me)->bx);
+   png_save_uint_32(pm->buffer+36, ((chrm_modification*)me)->by);
+   return 1;
+}
+
+static void
+chrm_modification_init(chrm_modification *me, png_modifier *pm,
+   const color_encoding *encoding)
+{
+   CIE_color white = white_point(encoding);
+
+   /* Original end points: */
+   me->encoding = encoding;
+
+   /* Chromaticities (in fixed point): */
+   me->wx = fix(chromaticity_x(white));
+   me->wy = fix(chromaticity_y(white));
+
+   me->rx = fix(chromaticity_x(encoding->red));
+   me->ry = fix(chromaticity_y(encoding->red));
+   me->gx = fix(chromaticity_x(encoding->green));
+   me->gy = fix(chromaticity_y(encoding->green));
+   me->bx = fix(chromaticity_x(encoding->blue));
+   me->by = fix(chromaticity_y(encoding->blue));
+
+   modification_init(&me->this);
+   me->this.chunk = CHUNK_cHRM;
+   me->this.modify_fn = chrm_modify;
+   me->this.add = CHUNK_PLTE;
+   me->this.next = pm->modifications;
+   pm->modifications = &me->this;
+}
+
+typedef struct srgb_modification
+{
+   png_modification this;
+   png_byte         intent;
+} srgb_modification;
+
+static int
+srgb_modify(png_modifier *pm, png_modification *me, int add)
+{
+   UNUSED(add)
+   /* As above, ignore add and just make a new chunk */
+   png_save_uint_32(pm->buffer, 1);
+   png_save_uint_32(pm->buffer+4, CHUNK_sRGB);
+   pm->buffer[8] = ((srgb_modification*)me)->intent;
+   return 1;
+}
+
+static void
+srgb_modification_init(srgb_modification *me, png_modifier *pm, png_byte intent)
+{
+   modification_init(&me->this);
+   me->this.chunk = CHUNK_sBIT;
+
+   if (intent <= 3) /* if valid, else *delete* sRGB chunks */
+   {
+      me->this.modify_fn = srgb_modify;
+      me->this.add = CHUNK_PLTE;
+      me->intent = intent;
+   }
+
+   else
+   {
+      me->this.modify_fn = 0;
+      me->this.add = 0;
+      me->intent = 0;
+   }
+
+   me->this.next = pm->modifications;
+   pm->modifications = &me->this;
+}
+
+#ifdef PNG_READ_GAMMA_SUPPORTED
+typedef struct sbit_modification
+{
+   png_modification this;
+   png_byte         sbit;
+} sbit_modification;
+
+static int
+sbit_modify(png_modifier *pm, png_modification *me, int add)
+{
+   png_byte sbit = ((sbit_modification*)me)->sbit;
+   if (pm->bit_depth > sbit)
+   {
+      int cb = 0;
+      switch (pm->colour_type)
+      {
+         case 0:
+            cb = 1;
+            break;
+
+         case 2:
+         case 3:
+            cb = 3;
+            break;
+
+         case 4:
+            cb = 2;
+            break;
+
+         case 6:
+            cb = 4;
+            break;
+
+         default:
+            png_error(pm->this.pread,
+               "unexpected colour type in sBIT modification");
+      }
+
+      png_save_uint_32(pm->buffer, cb);
+      png_save_uint_32(pm->buffer+4, CHUNK_sBIT);
+
+      while (cb > 0)
+         (pm->buffer+8)[--cb] = sbit;
+
+      return 1;
+   }
+   else if (!add)
+   {
+      /* Remove the sBIT chunk */
+      pm->buffer_count = pm->buffer_position = 0;
+      return 1;
+   }
+   else
+      return 0; /* do nothing */
+}
+
+static void
+sbit_modification_init(sbit_modification *me, png_modifier *pm, png_byte sbit)
+{
+   modification_init(&me->this);
+   me->this.chunk = CHUNK_sBIT;
+   me->this.modify_fn = sbit_modify;
+   me->this.add = CHUNK_PLTE;
+   me->sbit = sbit;
+   me->this.next = pm->modifications;
+   pm->modifications = &me->this;
+}
+#endif /* PNG_READ_GAMMA_SUPPORTED */
+#endif /* PNG_READ_TRANSFORMS_SUPPORTED */
+
+/***************************** STANDARD PNG FILES *****************************/
+/* Standard files - write and save standard files. */
+/* There are two basic forms of standard images.  Those which attempt to have
+ * all the possible pixel values (not possible for 16bpp images, but a range of
+ * values are produced) and those which have a range of image sizes.  The former
+ * are used for testing transforms, in particular gamma correction and bit
+ * reduction and increase.  The latter are reserved for testing the behavior of
+ * libpng with respect to 'odd' image sizes - particularly small images where
+ * rows become 1 byte and interlace passes disappear.
+ *
+ * The first, most useful, set are the 'transform' images, the second set of
+ * small images are the 'size' images.
+ *
+ * The transform files are constructed with rows which fit into a 1024 byte row
+ * buffer.  This makes allocation easier below.  Further regardless of the file
+ * format every row has 128 pixels (giving 1024 bytes for 64bpp formats).
+ *
+ * Files are stored with no gAMA or sBIT chunks, with a PLTE only when needed
+ * and with an ID derived from the colour type, bit depth and interlace type
+ * as above (FILEID).  The width (128) and height (variable) are not stored in
+ * the FILEID - instead the fields are set to 0, indicating a transform file.
+ *
+ * The size files ar constructed with rows a maximum of 128 bytes wide, allowing
+ * a maximum width of 16 pixels (for the 64bpp case.)  They also have a maximum
+ * height of 16 rows.  The width and height are stored in the FILEID and, being
+ * non-zero, indicate a size file.
+ *
+ * Because the PNG filter code is typically the largest CPU consumer within
+ * libpng itself there is a tendency to attempt to optimize it.  This results in
+ * special case code which needs to be validated.  To cause this to happen the
+ * 'size' images are made to use each possible filter, in so far as this is
+ * possible for smaller images.
+ *
+ * For palette image (colour type 3) multiple transform images are stored with
+ * the same bit depth to allow testing of more colour combinations -
+ * particularly important for testing the gamma code because libpng uses a
+ * different code path for palette images.  For size images a single palette is
+ * used.
+ */
+
+/* Make a 'standard' palette.  Because there are only 256 entries in a palette
+ * (maximum) this actually makes a random palette in the hope that enough tests
+ * will catch enough errors.  (Note that the same palette isn't produced every
+ * time for the same test - it depends on what previous tests have been run -
+ * but a given set of arguments to pngvalid will always produce the same palette
+ * at the same test!  This is why pseudo-random number generators are useful for
+ * testing.)
+ *
+ * The store must be open for write when this is called, otherwise an internal
+ * error will occur.  This routine contains its own magic number seed, so the
+ * palettes generated don't change if there are intervening errors (changing the
+ * calls to the store_mark seed.)
+ */
+static store_palette_entry *
+make_standard_palette(png_store* ps, int npalette, int do_tRNS)
+{
+   static png_uint_32 palette_seed[2] = { 0x87654321, 9 };
+
+   int i = 0;
+   png_byte values[256][4];
+
+   /* Always put in black and white plus the six primary and secondary colors.
+    */
+   for (; i<8; ++i)
+   {
+      values[i][1] = (png_byte)((i&1) ? 255U : 0U);
+      values[i][2] = (png_byte)((i&2) ? 255U : 0U);
+      values[i][3] = (png_byte)((i&4) ? 255U : 0U);
+   }
+
+   /* Then add 62 grays (one quarter of the remaining 256 slots). */
+   {
+      int j = 0;
+      png_byte random_bytes[4];
+      png_byte need[256];
+
+      need[0] = 0; /*got black*/
+      memset(need+1, 1, (sizeof need)-2); /*need these*/
+      need[255] = 0; /*but not white*/
+
+      while (i<70)
+      {
+         png_byte b;
+
+         if (j==0)
+         {
+            make_four_random_bytes(palette_seed, random_bytes);
+            j = 4;
+         }
+
+         b = random_bytes[--j];
+         if (need[b])
+         {
+            values[i][1] = b;
+            values[i][2] = b;
+            values[i++][3] = b;
+         }
+      }
+   }
+
+   /* Finally add 192 colors at random - don't worry about matches to things we
+    * already have, chance is less than 1/65536.  Don't worry about grays,
+    * chance is the same, so we get a duplicate or extra gray less than 1 time
+    * in 170.
+    */
+   for (; i<256; ++i)
+      make_four_random_bytes(palette_seed, values[i]);
+
+   /* Fill in the alpha values in the first byte.  Just use all possible values
+    * (0..255) in an apparently random order:
+    */
+   {
+      store_palette_entry *palette;
+      png_byte selector[4];
+
+      make_four_random_bytes(palette_seed, selector);
+
+      if (do_tRNS)
+         for (i=0; i<256; ++i)
+            values[i][0] = (png_byte)(i ^ selector[0]);
+
+      else
+         for (i=0; i<256; ++i)
+            values[i][0] = 255; /* no transparency/tRNS chunk */
+
+      /* 'values' contains 256 ARGB values, but we only need 'npalette'.
+       * 'npalette' will always be a power of 2: 2, 4, 16 or 256.  In the low
+       * bit depth cases select colors at random, else it is difficult to have
+       * a set of low bit depth palette test with any chance of a reasonable
+       * range of colors.  Do this by randomly permuting values into the low
+       * 'npalette' entries using an XOR mask generated here.  This also
+       * permutes the npalette == 256 case in a potentially useful way (there is
+       * no relationship between palette index and the color value therein!)
+       */
+      palette = store_write_palette(ps, npalette);
+
+      for (i=0; i<npalette; ++i)
+      {
+         palette[i].alpha = values[i ^ selector[1]][0];
+         palette[i].red   = values[i ^ selector[1]][1];
+         palette[i].green = values[i ^ selector[1]][2];
+         palette[i].blue  = values[i ^ selector[1]][3];
+      }
+
+      return palette;
+   }
+}
+
+/* Initialize a standard palette on a write stream.  The 'do_tRNS' argument
+ * indicates whether or not to also set the tRNS chunk.
+ */
+/* TODO: the png_structp here can probably be 'const' in the future */
+static void
+init_standard_palette(png_store *ps, png_structp pp, png_infop pi, int npalette,
+   int do_tRNS)
+{
+   store_palette_entry *ppal = make_standard_palette(ps, npalette, do_tRNS);
+
+   {
+      int i;
+      png_color palette[256];
+
+      /* Set all entries to detect overread errors. */
+      for (i=0; i<npalette; ++i)
+      {
+         palette[i].red = ppal[i].red;
+         palette[i].green = ppal[i].green;
+         palette[i].blue = ppal[i].blue;
+      }
+
+      /* Just in case fill in the rest with detectable values: */
+      for (; i<256; ++i)
+         palette[i].red = palette[i].green = palette[i].blue = 42;
+
+      png_set_PLTE(pp, pi, palette, npalette);
+   }
+
+   if (do_tRNS)
+   {
+      int i, j;
+      png_byte tRNS[256];
+
+      /* Set all the entries, but skip trailing opaque entries */
+      for (i=j=0; i<npalette; ++i)
+         if ((tRNS[i] = ppal[i].alpha) < 255)
+            j = i+1;
+
+      /* Fill in the remainder with a detectable value: */
+      for (; i<256; ++i)
+         tRNS[i] = 24;
+
+#ifdef PNG_WRITE_tRNS_SUPPORTED
+      if (j > 0)
+         png_set_tRNS(pp, pi, tRNS, j, 0/*color*/);
+#endif
+   }
+}
+
+#ifdef PNG_WRITE_tRNS_SUPPORTED
+static void
+set_random_tRNS(png_structp pp, png_infop pi, const png_byte colour_type,
+   const int bit_depth)
+{
+   /* To make this useful the tRNS color needs to match at least one pixel.
+    * Random values are fine for gray, including the 16-bit case where we know
+    * that the test image contains all the gray values.  For RGB we need more
+    * method as only 65536 different RGB values are generated.
+    */
+   png_color_16 tRNS;
+   const png_uint_16 mask = (png_uint_16)((1U << bit_depth)-1);
+
+   R8(tRNS); /* makes unset fields random */
+
+   if (colour_type & 2/*RGB*/)
+   {
+      if (bit_depth == 8)
+      {
+         R16(tRNS.red);
+         R16(tRNS.green);
+         tRNS.blue = tRNS.red ^ tRNS.green;
+         tRNS.red &= mask;
+         tRNS.green &= mask;
+         tRNS.blue &= mask;
+      }
+
+      else /* bit_depth == 16 */
+      {
+         R16(tRNS.red);
+         tRNS.green = (png_uint_16)(tRNS.red * 257);
+         tRNS.blue = (png_uint_16)(tRNS.green * 17);
+      }
+   }
+
+   else
+   {
+      R16(tRNS.gray);
+      tRNS.gray &= mask;
+   }
+
+   png_set_tRNS(pp, pi, NULL, 0, &tRNS);
+}
+#endif
+
+/* The number of passes is related to the interlace type. There was no libpng
+ * API to determine this prior to 1.5, so we need an inquiry function:
+ */
+static int
+npasses_from_interlace_type(png_const_structp pp, int interlace_type)
+{
+   switch (interlace_type)
+   {
+   default:
+      png_error(pp, "invalid interlace type");
+
+   case PNG_INTERLACE_NONE:
+      return 1;
+
+   case PNG_INTERLACE_ADAM7:
+      return PNG_INTERLACE_ADAM7_PASSES;
+   }
+}
+
+static unsigned int
+bit_size(png_const_structp pp, png_byte colour_type, png_byte bit_depth)
+{
+   switch (colour_type)
+   {
+      default: png_error(pp, "invalid color type");
+
+      case 0:  return bit_depth;
+
+      case 2:  return 3*bit_depth;
+
+      case 3:  return bit_depth;
+
+      case 4:  return 2*bit_depth;
+
+      case 6:  return 4*bit_depth;
+   }
+}
+
+#define TRANSFORM_WIDTH  128U
+#define TRANSFORM_ROWMAX (TRANSFORM_WIDTH*8U)
+#define SIZE_ROWMAX (16*8U) /* 16 pixels, max 8 bytes each - 128 bytes */
+#define STANDARD_ROWMAX TRANSFORM_ROWMAX /* The larger of the two */
+#define SIZE_HEIGHTMAX 16 /* Maximum range of size images */
+
+static size_t
+transform_rowsize(png_const_structp pp, png_byte colour_type,
+   png_byte bit_depth)
+{
+   return (TRANSFORM_WIDTH * bit_size(pp, colour_type, bit_depth)) / 8;
+}
+
+/* transform_width(pp, colour_type, bit_depth) current returns the same number
+ * every time, so just use a macro:
+ */
+#define transform_width(pp, colour_type, bit_depth) TRANSFORM_WIDTH
+
+static png_uint_32
+transform_height(png_const_structp pp, png_byte colour_type, png_byte bit_depth)
+{
+   switch (bit_size(pp, colour_type, bit_depth))
+   {
+      case 1:
+      case 2:
+      case 4:
+         return 1;   /* Total of 128 pixels */
+
+      case 8:
+         return 2;   /* Total of 256 pixels/bytes */
+
+      case 16:
+         return 512; /* Total of 65536 pixels */
+
+      case 24:
+      case 32:
+         return 512; /* 65536 pixels */
+
+      case 48:
+      case 64:
+         return 2048;/* 4 x 65536 pixels. */
+#        define TRANSFORM_HEIGHTMAX 2048
+
+      default:
+         return 0;   /* Error, will be caught later */
+   }
+}
+
+#ifdef PNG_READ_SUPPORTED
+/* The following can only be defined here, now we have the definitions
+ * of the transform image sizes.
+ */
+static png_uint_32
+standard_width(png_const_structp pp, png_uint_32 id)
+{
+   png_uint_32 width = WIDTH_FROM_ID(id);
+   UNUSED(pp)
+
+   if (width == 0)
+      width = transform_width(pp, COL_FROM_ID(id), DEPTH_FROM_ID(id));
+
+   return width;
+}
+
+static png_uint_32
+standard_height(png_const_structp pp, png_uint_32 id)
+{
+   png_uint_32 height = HEIGHT_FROM_ID(id);
+
+   if (height == 0)
+      height = transform_height(pp, COL_FROM_ID(id), DEPTH_FROM_ID(id));
+
+   return height;
+}
+
+static png_uint_32
+standard_rowsize(png_const_structp pp, png_uint_32 id)
+{
+   png_uint_32 width = standard_width(pp, id);
+
+   /* This won't overflow: */
+   width *= bit_size(pp, COL_FROM_ID(id), DEPTH_FROM_ID(id));
+   return (width + 7) / 8;
+}
+#endif /* PNG_READ_SUPPORTED */
+
+static void
+transform_row(png_const_structp pp, png_byte buffer[TRANSFORM_ROWMAX],
+   png_byte colour_type, png_byte bit_depth, png_uint_32 y)
+{
+   png_uint_32 v = y << 7;
+   png_uint_32 i = 0;
+
+   switch (bit_size(pp, colour_type, bit_depth))
+   {
+      case 1:
+         while (i<128/8) buffer[i] = (png_byte)(v & 0xff), v += 17, ++i;
+         return;
+
+      case 2:
+         while (i<128/4) buffer[i] = (png_byte)(v & 0xff), v += 33, ++i;
+         return;
+
+      case 4:
+         while (i<128/2) buffer[i] = (png_byte)(v & 0xff), v += 65, ++i;
+         return;
+
+      case 8:
+         /* 256 bytes total, 128 bytes in each row set as follows: */
+         while (i<128) buffer[i] = (png_byte)(v & 0xff), ++v, ++i;
+         return;
+
+      case 16:
+         /* Generate all 65536 pixel values in order, which includes the 8 bit
+          * GA case as well as the 16 bit G case.
+          */
+         while (i<128)
+         {
+            buffer[2*i] = (png_byte)((v>>8) & 0xff);
+            buffer[2*i+1] = (png_byte)(v & 0xff);
+            ++v;
+            ++i;
+         }
+
+         return;
+
+      case 24:
+         /* 65535 pixels, but rotate the values. */
+         while (i<128)
+         {
+            /* Three bytes per pixel, r, g, b, make b by r^g */
+            buffer[3*i+0] = (png_byte)((v >> 8) & 0xff);
+            buffer[3*i+1] = (png_byte)(v & 0xff);
+            buffer[3*i+2] = (png_byte)(((v >> 8) ^ v) & 0xff);
+            ++v;
+            ++i;
+         }
+
+         return;
+
+      case 32:
+         /* 65535 pixels, r, g, b, a; just replicate */
+         while (i<128)
+         {
+            buffer[4*i+0] = (png_byte)((v >> 8) & 0xff);
+            buffer[4*i+1] = (png_byte)(v & 0xff);
+            buffer[4*i+2] = (png_byte)((v >> 8) & 0xff);
+            buffer[4*i+3] = (png_byte)(v & 0xff);
+            ++v;
+            ++i;
+         }
+
+         return;
+
+      case 48:
+         /* y is maximum 2047, giving 4x65536 pixels, make 'r' increase by 1 at
+          * each pixel, g increase by 257 (0x101) and 'b' by 0x1111:
+          */
+         while (i<128)
+         {
+            png_uint_32 t = v++;
+            buffer[6*i+0] = (png_byte)((t >> 8) & 0xff);
+            buffer[6*i+1] = (png_byte)(t & 0xff);
+            t *= 257;
+            buffer[6*i+2] = (png_byte)((t >> 8) & 0xff);
+            buffer[6*i+3] = (png_byte)(t & 0xff);
+            t *= 17;
+            buffer[6*i+4] = (png_byte)((t >> 8) & 0xff);
+            buffer[6*i+5] = (png_byte)(t & 0xff);
+            ++i;
+         }
+
+         return;
+
+      case 64:
+         /* As above in the 32 bit case. */
+         while (i<128)
+         {
+            png_uint_32 t = v++;
+            buffer[8*i+0] = (png_byte)((t >> 8) & 0xff);
+            buffer[8*i+1] = (png_byte)(t & 0xff);
+            buffer[8*i+4] = (png_byte)((t >> 8) & 0xff);
+            buffer[8*i+5] = (png_byte)(t & 0xff);
+            t *= 257;
+            buffer[8*i+2] = (png_byte)((t >> 8) & 0xff);
+            buffer[8*i+3] = (png_byte)(t & 0xff);
+            buffer[8*i+6] = (png_byte)((t >> 8) & 0xff);
+            buffer[8*i+7] = (png_byte)(t & 0xff);
+            ++i;
+         }
+         return;
+
+      default:
+         break;
+   }
+
+   png_error(pp, "internal error");
+}
+
+/* This is just to do the right cast - could be changed to a function to check
+ * 'bd' but there isn't much point.
+ */
+#define DEPTH(bd) ((png_byte)(1U << (bd)))
+
+/* This is just a helper for compiling on minimal systems with no write
+ * interlacing support.  If there is no write interlacing we can't generate test
+ * cases with interlace:
+ */
+#ifdef PNG_WRITE_INTERLACING_SUPPORTED
+#  define INTERLACE_LAST PNG_INTERLACE_LAST
+#  define check_interlace_type(type) ((void)(type))
+#  define set_write_interlace_handling(pp,type) png_set_interlace_handling(pp)
+#  define do_own_interlace 0
+#elif PNG_LIBPNG_VER < 10700
+#  define set_write_interlace_handling(pp,type) (1)
+static void
+check_interlace_type(int const interlace_type)
+{
+   /* Prior to 1.7.0 libpng does not support the write of an interlaced image
+    * unless PNG_WRITE_INTERLACING_SUPPORTED, even with do_interlace so the
+    * code here does the pixel interlace itself, so:
+    */
+   if (interlace_type != PNG_INTERLACE_NONE)
+   {
+      /* This is an internal error - --interlace tests should be skipped, not
+       * attempted.
+       */
+      fprintf(stderr, "pngvalid: no interlace support\n");
+      exit(99);
+   }
+}
+#  define INTERLACE_LAST (PNG_INTERLACE_NONE+1)
+#  define do_own_interlace 0
+#else /* libpng 1.7+ */
+#  define set_write_interlace_handling(pp,type)\
+      npasses_from_interlace_type(pp,type)
+#  define check_interlace_type(type) ((void)(type))
+#  define INTERLACE_LAST PNG_INTERLACE_LAST
+#  define do_own_interlace 1
+#endif /* WRITE_INTERLACING tests */
+
+#define CAN_WRITE_INTERLACE\
+   PNG_LIBPNG_VER >= 10700 || defined PNG_WRITE_INTERLACING_SUPPORTED
+
+/* Do the same thing for read interlacing; this controls whether read tests do
+ * their own de-interlace or use libpng.
+ */
+#ifdef PNG_READ_INTERLACING_SUPPORTED
+#  define do_read_interlace 0
+#else /* no libpng read interlace support */
+#  define do_read_interlace 1
+#endif
+/* The following two routines use the PNG interlace support macros from
+ * png.h to interlace or deinterlace rows.
+ */
+static void
+interlace_row(png_bytep buffer, png_const_bytep imageRow,
+   unsigned int pixel_size, png_uint_32 w, int pass, int littleendian)
+{
+   png_uint_32 xin, xout, xstep;
+
+   /* Note that this can, trivially, be optimized to a memcpy on pass 7, the
+    * code is presented this way to make it easier to understand.  In practice
+    * consult the code in the libpng source to see other ways of doing this.
+    *
+    * It is OK for buffer and imageRow to be identical, because 'xin' moves
+    * faster than 'xout' and we copy up.
+    */
+   xin = PNG_PASS_START_COL(pass);
+   xstep = 1U<<PNG_PASS_COL_SHIFT(pass);
+
+   for (xout=0; xin<w; xin+=xstep)
+   {
+      pixel_copy(buffer, xout, imageRow, xin, pixel_size, littleendian);
+      ++xout;
+   }
+}
+
+#ifdef PNG_READ_SUPPORTED
+static void
+deinterlace_row(png_bytep buffer, png_const_bytep row,
+   unsigned int pixel_size, png_uint_32 w, int pass, int littleendian)
+{
+   /* The inverse of the above, 'row' is part of row 'y' of the output image,
+    * in 'buffer'.  The image is 'w' wide and this is pass 'pass', distribute
+    * the pixels of row into buffer and return the number written (to allow
+    * this to be checked).
+    */
+   png_uint_32 xin, xout, xstep;
+
+   xout = PNG_PASS_START_COL(pass);
+   xstep = 1U<<PNG_PASS_COL_SHIFT(pass);
+
+   for (xin=0; xout<w; xout+=xstep)
+   {
+      pixel_copy(buffer, xout, row, xin, pixel_size, littleendian);
+      ++xin;
+   }
+}
+#endif /* PNG_READ_SUPPORTED */
+
+/* Make a standardized image given an image colour type, bit depth and
+ * interlace type.  The standard images have a very restricted range of
+ * rows and heights and are used for testing transforms rather than image
+ * layout details.  See make_size_images below for a way to make images
+ * that test odd sizes along with the libpng interlace handling.
+ */
+#ifdef PNG_WRITE_FILTER_SUPPORTED
+static void
+choose_random_filter(png_structp pp, int start)
+{
+   /* Choose filters randomly except that on the very first row ensure that
+    * there is at least one previous row filter.
+    */
+   int filters = PNG_ALL_FILTERS & random_mod(256U);
+
+   /* There may be no filters; skip the setting. */
+   if (filters != 0)
+   {
+      if (start && filters < PNG_FILTER_UP)
+         filters |= PNG_FILTER_UP;
+
+      png_set_filter(pp, 0/*method*/, filters);
+   }
+}
+#else /* !WRITE_FILTER */
+#  define choose_random_filter(pp, start) ((void)0)
+#endif /* !WRITE_FILTER */
+
+static void
+make_transform_image(png_store* const ps, png_byte const colour_type,
+    png_byte const bit_depth, unsigned int palette_number,
+    int interlace_type, png_const_charp name)
+{
+   context(ps, fault);
+
+   check_interlace_type(interlace_type);
+
+   Try
+   {
+      png_infop pi;
+      png_structp pp = set_store_for_write(ps, &pi, name);
+      png_uint_32 h, w;
+
+      /* In the event of a problem return control to the Catch statement below
+       * to do the clean up - it is not possible to 'return' directly from a Try
+       * block.
+       */
+      if (pp == NULL)
+         Throw ps;
+
+      w = transform_width(pp, colour_type, bit_depth);
+      h = transform_height(pp, colour_type, bit_depth);
+
+      png_set_IHDR(pp, pi, w, h, bit_depth, colour_type, interlace_type,
+         PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
+
+#ifdef PNG_TEXT_SUPPORTED
+#  if defined(PNG_READ_zTXt_SUPPORTED) && defined(PNG_WRITE_zTXt_SUPPORTED)
+#     define TEXT_COMPRESSION PNG_TEXT_COMPRESSION_zTXt
+#  else
+#     define TEXT_COMPRESSION PNG_TEXT_COMPRESSION_NONE
+#  endif
+      {
+         static char key[] = "image name"; /* must be writeable */
+         size_t pos;
+         png_text text;
+         char copy[FILE_NAME_SIZE];
+
+         /* Use a compressed text string to test the correct interaction of text
+          * compression and IDAT compression.
+          */
+         text.compression = TEXT_COMPRESSION;
+         text.key = key;
+         /* Yuck: the text must be writable! */
+         pos = safecat(copy, sizeof copy, 0, ps->wname);
+         text.text = copy;
+         text.text_length = pos;
+         text.itxt_length = 0;
+         text.lang = 0;
+         text.lang_key = 0;
+
+         png_set_text(pp, pi, &text, 1);
+      }
+#endif
+
+      if (colour_type == 3) /* palette */
+         init_standard_palette(ps, pp, pi, 1U << bit_depth, 1/*do tRNS*/);
+
+#     ifdef PNG_WRITE_tRNS_SUPPORTED
+         else if (palette_number)
+            set_random_tRNS(pp, pi, colour_type, bit_depth);
+#     endif
+
+      png_write_info(pp, pi);
+
+      if (png_get_rowbytes(pp, pi) !=
+          transform_rowsize(pp, colour_type, bit_depth))
+         png_error(pp, "transform row size incorrect");
+
+      else
+      {
+         /* Somewhat confusingly this must be called *after* png_write_info
+          * because if it is called before, the information in *pp has not been
+          * updated to reflect the interlaced image.
+          */
+         int npasses = set_write_interlace_handling(pp, interlace_type);
+         int pass;
+
+         if (npasses != npasses_from_interlace_type(pp, interlace_type))
+            png_error(pp, "write: png_set_interlace_handling failed");
+
+         for (pass=0; pass<npasses; ++pass)
+         {
+            png_uint_32 y;
+
+            /* do_own_interlace is a pre-defined boolean (a #define) which is
+             * set if we have to work out the interlaced rows here.
+             */
+            for (y=0; y<h; ++y)
+            {
+               png_byte buffer[TRANSFORM_ROWMAX];
+
+               transform_row(pp, buffer, colour_type, bit_depth, y);
+
+#              if do_own_interlace
+                  /* If do_own_interlace *and* the image is interlaced we need a
+                   * reduced interlace row; this may be reduced to empty.
+                   */
+                  if (interlace_type == PNG_INTERLACE_ADAM7)
+                  {
+                     /* The row must not be written if it doesn't exist, notice
+                      * that there are two conditions here, either the row isn't
+                      * ever in the pass or the row would be but isn't wide
+                      * enough to contribute any pixels.  In fact the wPass test
+                      * can be used to skip the whole y loop in this case.
+                      */
+                     if (PNG_ROW_IN_INTERLACE_PASS(y, pass) &&
+                         PNG_PASS_COLS(w, pass) > 0)
+                        interlace_row(buffer, buffer,
+                              bit_size(pp, colour_type, bit_depth), w, pass,
+                              0/*data always bigendian*/);
+                     else
+                        continue;
+                  }
+#              endif /* do_own_interlace */
+
+               choose_random_filter(pp, pass == 0 && y == 0);
+               png_write_row(pp, buffer);
+            }
+         }
+      }
+
+#ifdef PNG_TEXT_SUPPORTED
+      {
+         static char key[] = "end marker";
+         static char comment[] = "end";
+         png_text text;
+
+         /* Use a compressed text string to test the correct interaction of text
+          * compression and IDAT compression.
+          */
+         text.compression = TEXT_COMPRESSION;
+         text.key = key;
+         text.text = comment;
+         text.text_length = (sizeof comment)-1;
+         text.itxt_length = 0;
+         text.lang = 0;
+         text.lang_key = 0;
+
+         png_set_text(pp, pi, &text, 1);
+      }
+#endif
+
+      png_write_end(pp, pi);
+
+      /* And store this under the appropriate id, then clean up. */
+      store_storefile(ps, FILEID(colour_type, bit_depth, palette_number,
+         interlace_type, 0, 0, 0));
+
+      store_write_reset(ps);
+   }
+
+   Catch(fault)
+   {
+      /* Use the png_store returned by the exception. This may help the compiler
+       * because 'ps' is not used in this branch of the setjmp.  Note that fault
+       * and ps will always be the same value.
+       */
+      store_write_reset(fault);
+   }
+}
+
+static void
+make_transform_images(png_modifier *pm)
+{
+   png_byte colour_type = 0;
+   png_byte bit_depth = 0;
+   unsigned int palette_number = 0;
+
+   /* This is in case of errors. */
+   safecat(pm->this.test, sizeof pm->this.test, 0, "make standard images");
+
+   /* Use next_format to enumerate all the combinations we test, including
+    * generating multiple low bit depth palette images. Non-A images (palette
+    * and direct) are created with and without tRNS chunks.
+    */
+   while (next_format(&colour_type, &bit_depth, &palette_number, 1, 1))
+   {
+      int interlace_type;
+
+      for (interlace_type = PNG_INTERLACE_NONE;
+           interlace_type < INTERLACE_LAST; ++interlace_type)
+      {
+         char name[FILE_NAME_SIZE];
+
+         standard_name(name, sizeof name, 0, colour_type, bit_depth,
+            palette_number, interlace_type, 0, 0, do_own_interlace);
+         make_transform_image(&pm->this, colour_type, bit_depth, palette_number,
+            interlace_type, name);
+      }
+   }
+}
+
+/* Build a single row for the 'size' test images; this fills in only the
+ * first bit_width bits of the sample row.
+ */
+static void
+size_row(png_byte buffer[SIZE_ROWMAX], png_uint_32 bit_width, png_uint_32 y)
+{
+   /* height is in the range 1 to 16, so: */
+   y = ((y & 1) << 7) + ((y & 2) << 6) + ((y & 4) << 5) + ((y & 8) << 4);
+   /* the following ensures bits are set in small images: */
+   y ^= 0xA5;
+
+   while (bit_width >= 8)
+      *buffer++ = (png_byte)y++, bit_width -= 8;
+
+   /* There may be up to 7 remaining bits, these go in the most significant
+    * bits of the byte.
+    */
+   if (bit_width > 0)
+   {
+      png_uint_32 mask = (1U<<(8-bit_width))-1;
+      *buffer = (png_byte)((*buffer & mask) | (y & ~mask));
+   }
+}
+
+static void
+make_size_image(png_store* const ps, png_byte const colour_type,
+    png_byte const bit_depth, int const interlace_type,
+    png_uint_32 const w, png_uint_32 const h,
+    int const do_interlace)
+{
+   context(ps, fault);
+
+   check_interlace_type(interlace_type);
+
+   Try
+   {
+      png_infop pi;
+      png_structp pp;
+      unsigned int pixel_size;
+
+      /* Make a name and get an appropriate id for the store: */
+      char name[FILE_NAME_SIZE];
+      const png_uint_32 id = FILEID(colour_type, bit_depth, 0/*palette*/,
+         interlace_type, w, h, do_interlace);
+
+      standard_name_from_id(name, sizeof name, 0, id);
+      pp = set_store_for_write(ps, &pi, name);
+
+      /* In the event of a problem return control to the Catch statement below
+       * to do the clean up - it is not possible to 'return' directly from a Try
+       * block.
+       */
+      if (pp == NULL)
+         Throw ps;
+
+      png_set_IHDR(pp, pi, w, h, bit_depth, colour_type, interlace_type,
+         PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
+
+#ifdef PNG_TEXT_SUPPORTED
+      {
+         static char key[] = "image name"; /* must be writeable */
+         size_t pos;
+         png_text text;
+         char copy[FILE_NAME_SIZE];
+
+         /* Use a compressed text string to test the correct interaction of text
+          * compression and IDAT compression.
+          */
+         text.compression = TEXT_COMPRESSION;
+         text.key = key;
+         /* Yuck: the text must be writable! */
+         pos = safecat(copy, sizeof copy, 0, ps->wname);
+         text.text = copy;
+         text.text_length = pos;
+         text.itxt_length = 0;
+         text.lang = 0;
+         text.lang_key = 0;
+
+         png_set_text(pp, pi, &text, 1);
+      }
+#endif
+
+      if (colour_type == 3) /* palette */
+         init_standard_palette(ps, pp, pi, 1U << bit_depth, 0/*do tRNS*/);
+
+      png_write_info(pp, pi);
+
+      /* Calculate the bit size, divide by 8 to get the byte size - this won't
+       * overflow because we know the w values are all small enough even for
+       * a system where 'unsigned int' is only 16 bits.
+       */
+      pixel_size = bit_size(pp, colour_type, bit_depth);
+      if (png_get_rowbytes(pp, pi) != ((w * pixel_size) + 7) / 8)
+         png_error(pp, "size row size incorrect");
+
+      else
+      {
+         int npasses = npasses_from_interlace_type(pp, interlace_type);
+         png_uint_32 y;
+         int pass;
+         png_byte image[16][SIZE_ROWMAX];
+
+         /* To help consistent error detection make the parts of this buffer
+          * that aren't set below all '1':
+          */
+         memset(image, 0xff, sizeof image);
+
+         if (!do_interlace &&
+             npasses != set_write_interlace_handling(pp, interlace_type))
+            png_error(pp, "write: png_set_interlace_handling failed");
+
+         /* Prepare the whole image first to avoid making it 7 times: */
+         for (y=0; y<h; ++y)
+            size_row(image[y], w * pixel_size, y);
+
+         for (pass=0; pass<npasses; ++pass)
+         {
+            /* The following two are for checking the macros: */
+            const png_uint_32 wPass = PNG_PASS_COLS(w, pass);
+
+            /* If do_interlace is set we don't call png_write_row for every
+             * row because some of them are empty.  In fact, for a 1x1 image,
+             * most of them are empty!
+             */
+            for (y=0; y<h; ++y)
+            {
+               png_const_bytep row = image[y];
+               png_byte tempRow[SIZE_ROWMAX];
+
+               /* If do_interlace *and* the image is interlaced we
+                * need a reduced interlace row; this may be reduced
+                * to empty.
+                */
+               if (do_interlace && interlace_type == PNG_INTERLACE_ADAM7)
+               {
+                  /* The row must not be written if it doesn't exist, notice
+                   * that there are two conditions here, either the row isn't
+                   * ever in the pass or the row would be but isn't wide
+                   * enough to contribute any pixels.  In fact the wPass test
+                   * can be used to skip the whole y loop in this case.
+                   */
+                  if (PNG_ROW_IN_INTERLACE_PASS(y, pass) && wPass > 0)
+                  {
+                     /* Set to all 1's for error detection (libpng tends to
+                      * set unset things to 0).
+                      */
+                     memset(tempRow, 0xff, sizeof tempRow);
+                     interlace_row(tempRow, row, pixel_size, w, pass,
+                           0/*data always bigendian*/);
+                     row = tempRow;
+                  }
+                  else
+                     continue;
+               }
+
+#           ifdef PNG_WRITE_FILTER_SUPPORTED
+               /* Only get to here if the row has some pixels in it, set the
+                * filters to 'all' for the very first row and thereafter to a
+                * single filter.  It isn't well documented, but png_set_filter
+                * does accept a filter number (per the spec) as well as a bit
+                * mask.
+                *
+                * The code now uses filters at random, except that on the first
+                * row of an image it ensures that a previous row filter is in
+                * the set so that libpng allocates the row buffer.
+                */
+               {
+                  int filters = 8 << random_mod(PNG_FILTER_VALUE_LAST);
+
+                  if (pass == 0 && y == 0 &&
+                      (filters < PNG_FILTER_UP || w == 1U))
+                     filters |= PNG_FILTER_UP;
+
+                  png_set_filter(pp, 0/*method*/, filters);
+               }
+#           endif
+
+               png_write_row(pp, row);
+            }
+         }
+      }
+
+#ifdef PNG_TEXT_SUPPORTED
+      {
+         static char key[] = "end marker";
+         static char comment[] = "end";
+         png_text text;
+
+         /* Use a compressed text string to test the correct interaction of text
+          * compression and IDAT compression.
+          */
+         text.compression = TEXT_COMPRESSION;
+         text.key = key;
+         text.text = comment;
+         text.text_length = (sizeof comment)-1;
+         text.itxt_length = 0;
+         text.lang = 0;
+         text.lang_key = 0;
+
+         png_set_text(pp, pi, &text, 1);
+      }
+#endif
+
+      png_write_end(pp, pi);
+
+      /* And store this under the appropriate id, then clean up. */
+      store_storefile(ps, id);
+
+      store_write_reset(ps);
+   }
+
+   Catch(fault)
+   {
+      /* Use the png_store returned by the exception. This may help the compiler
+       * because 'ps' is not used in this branch of the setjmp.  Note that fault
+       * and ps will always be the same value.
+       */
+      store_write_reset(fault);
+   }
+}
+
+static void
+make_size(png_store* const ps, png_byte const colour_type, int bdlo,
+    int const bdhi)
+{
+   for (; bdlo <= bdhi; ++bdlo)
+   {
+      png_uint_32 width;
+
+      for (width = 1; width <= 16; ++width)
+      {
+         png_uint_32 height;
+
+         for (height = 1; height <= 16; ++height)
+         {
+            /* The four combinations of DIY interlace and interlace or not -
+             * no interlace + DIY should be identical to no interlace with
+             * libpng doing it.
+             */
+            make_size_image(ps, colour_type, DEPTH(bdlo), PNG_INTERLACE_NONE,
+               width, height, 0);
+            make_size_image(ps, colour_type, DEPTH(bdlo), PNG_INTERLACE_NONE,
+               width, height, 1);
+#        ifdef PNG_WRITE_INTERLACING_SUPPORTED
+            make_size_image(ps, colour_type, DEPTH(bdlo), PNG_INTERLACE_ADAM7,
+               width, height, 0);
+#        endif
+#        if CAN_WRITE_INTERLACE
+            /* 1.7.0 removes the hack that prevented app write of an interlaced
+             * image if WRITE_INTERLACE was not supported
+             */
+            make_size_image(ps, colour_type, DEPTH(bdlo), PNG_INTERLACE_ADAM7,
+               width, height, 1);
+#        endif
+         }
+      }
+   }
+}
+
+static void
+make_size_images(png_store *ps)
+{
+   /* This is in case of errors. */
+   safecat(ps->test, sizeof ps->test, 0, "make size images");
+
+   /* Arguments are colour_type, low bit depth, high bit depth
+    */
+   make_size(ps, 0, 0, WRITE_BDHI);
+   make_size(ps, 2, 3, WRITE_BDHI);
+   make_size(ps, 3, 0, 3 /*palette: max 8 bits*/);
+   make_size(ps, 4, 3, WRITE_BDHI);
+   make_size(ps, 6, 3, WRITE_BDHI);
+}
+
+#ifdef PNG_READ_SUPPORTED
+/* Return a row based on image id and 'y' for checking: */
+static void
+standard_row(png_const_structp pp, png_byte std[STANDARD_ROWMAX],
+   png_uint_32 id, png_uint_32 y)
+{
+   if (WIDTH_FROM_ID(id) == 0)
+      transform_row(pp, std, COL_FROM_ID(id), DEPTH_FROM_ID(id), y);
+   else
+      size_row(std, WIDTH_FROM_ID(id) * bit_size(pp, COL_FROM_ID(id),
+         DEPTH_FROM_ID(id)), y);
+}
+#endif /* PNG_READ_SUPPORTED */
+
+/* Tests - individual test cases */
+/* Like 'make_standard' but errors are deliberately introduced into the calls
+ * to ensure that they get detected - it should not be possible to write an
+ * invalid image with libpng!
+ */
+/* TODO: the 'set' functions can probably all be made to take a
+ * png_const_structp rather than a modifiable one.
+ */
+#ifdef PNG_WARNINGS_SUPPORTED
+static void
+sBIT0_error_fn(png_structp pp, png_infop pi)
+{
+   /* 0 is invalid... */
+   png_color_8 bad;
+   bad.red = bad.green = bad.blue = bad.gray = bad.alpha = 0;
+   png_set_sBIT(pp, pi, &bad);
+}
+
+static void
+sBIT_error_fn(png_structp pp, png_infop pi)
+{
+   png_byte bit_depth;
+   png_color_8 bad;
+
+   if (png_get_color_type(pp, pi) == PNG_COLOR_TYPE_PALETTE)
+      bit_depth = 8;
+
+   else
+      bit_depth = png_get_bit_depth(pp, pi);
+
+   /* Now we know the bit depth we can easily generate an invalid sBIT entry */
+   bad.red = bad.green = bad.blue = bad.gray = bad.alpha =
+      (png_byte)(bit_depth+1);
+   png_set_sBIT(pp, pi, &bad);
+}
+
+static const struct
+{
+   void          (*fn)(png_structp, png_infop);
+   const char *msg;
+   unsigned int    warning :1; /* the error is a warning... */
+} error_test[] =
+    {
+       /* no warnings makes these errors undetectable prior to 1.7.0 */
+       { sBIT0_error_fn, "sBIT(0): failed to detect error",
+         PNG_LIBPNG_VER < 10700 },
+
+       { sBIT_error_fn, "sBIT(too big): failed to detect error",
+         PNG_LIBPNG_VER < 10700 },
+    };
+
+static void
+make_error(png_store* const ps, png_byte const colour_type,
+    png_byte bit_depth, int interlace_type, int test, png_const_charp name)
+{
+   context(ps, fault);
+
+   check_interlace_type(interlace_type);
+
+   Try
+   {
+      png_infop pi;
+      const png_structp pp = set_store_for_write(ps, &pi, name);
+      png_uint_32 w, h;
+      gnu_volatile(pp)
+
+      if (pp == NULL)
+         Throw ps;
+
+      w = transform_width(pp, colour_type, bit_depth);
+      gnu_volatile(w)
+      h = transform_height(pp, colour_type, bit_depth);
+      gnu_volatile(h)
+      png_set_IHDR(pp, pi, w, h, bit_depth, colour_type, interlace_type,
+            PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
+
+      if (colour_type == 3) /* palette */
+         init_standard_palette(ps, pp, pi, 1U << bit_depth, 0/*do tRNS*/);
+
+      /* Time for a few errors; these are in various optional chunks, the
+       * standard tests test the standard chunks pretty well.
+       */
+#     define exception__prev exception_prev_1
+#     define exception__env exception_env_1
+      Try
+      {
+         gnu_volatile(exception__prev)
+
+         /* Expect this to throw: */
+         ps->expect_error = !error_test[test].warning;
+         ps->expect_warning = error_test[test].warning;
+         ps->saw_warning = 0;
+         error_test[test].fn(pp, pi);
+
+         /* Normally the error is only detected here: */
+         png_write_info(pp, pi);
+
+         /* And handle the case where it was only a warning: */
+         if (ps->expect_warning && ps->saw_warning)
+            Throw ps;
+
+         /* If we get here there is a problem, we have success - no error or
+          * no warning - when we shouldn't have success.  Log an error.
+          */
+         store_log(ps, pp, error_test[test].msg, 1 /*error*/);
+      }
+
+      Catch (fault)
+      { /* expected exit */
+      }
+#undef exception__prev
+#undef exception__env
+
+      /* And clear these flags */
+      ps->expect_warning = 0;
+
+      if (ps->expect_error)
+         ps->expect_error = 0;
+
+      else
+      {
+         /* Now write the whole image, just to make sure that the detected, or
+          * undetected, errro has not created problems inside libpng.  This
+          * doesn't work if there was a png_error in png_write_info because that
+          * can abort before PLTE was written.
+          */
+         if (png_get_rowbytes(pp, pi) !=
+             transform_rowsize(pp, colour_type, bit_depth))
+            png_error(pp, "row size incorrect");
+
+         else
+         {
+            int npasses = set_write_interlace_handling(pp, interlace_type);
+            int pass;
+
+            if (npasses != npasses_from_interlace_type(pp, interlace_type))
+               png_error(pp, "write: png_set_interlace_handling failed");
+
+            for (pass=0; pass<npasses; ++pass)
+            {
+               png_uint_32 y;
+
+               for (y=0; y<h; ++y)
+               {
+                  png_byte buffer[TRANSFORM_ROWMAX];
+
+                  transform_row(pp, buffer, colour_type, bit_depth, y);
+
+#                 if do_own_interlace
+                     /* If do_own_interlace *and* the image is interlaced we
+                      * need a reduced interlace row; this may be reduced to
+                      * empty.
+                      */
+                     if (interlace_type == PNG_INTERLACE_ADAM7)
+                     {
+                        /* The row must not be written if it doesn't exist,
+                         * notice that there are two conditions here, either the
+                         * row isn't ever in the pass or the row would be but
+                         * isn't wide enough to contribute any pixels.  In fact
+                         * the wPass test can be used to skip the whole y loop
+                         * in this case.
+                         */
+                        if (PNG_ROW_IN_INTERLACE_PASS(y, pass) &&
+                            PNG_PASS_COLS(w, pass) > 0)
+                           interlace_row(buffer, buffer,
+                                 bit_size(pp, colour_type, bit_depth), w, pass,
+                                 0/*data always bigendian*/);
+                        else
+                           continue;
+                     }
+#                 endif /* do_own_interlace */
+
+                  png_write_row(pp, buffer);
+               }
+            }
+         } /* image writing */
+
+         png_write_end(pp, pi);
+      }
+
+      /* The following deletes the file that was just written. */
+      store_write_reset(ps);
+   }
+
+   Catch(fault)
+   {
+      store_write_reset(fault);
+   }
+}
+
+static int
+make_errors(png_modifier* const pm, png_byte const colour_type,
+    int bdlo, int const bdhi)
+{
+   for (; bdlo <= bdhi; ++bdlo)
+   {
+      int interlace_type;
+
+      for (interlace_type = PNG_INTERLACE_NONE;
+           interlace_type < INTERLACE_LAST; ++interlace_type)
+      {
+         unsigned int test;
+         char name[FILE_NAME_SIZE];
+
+         standard_name(name, sizeof name, 0, colour_type, 1<<bdlo, 0,
+            interlace_type, 0, 0, do_own_interlace);
+
+         for (test=0; test<ARRAY_SIZE(error_test); ++test)
+         {
+            make_error(&pm->this, colour_type, DEPTH(bdlo), interlace_type,
+               test, name);
+
+            if (fail(pm))
+               return 0;
+         }
+      }
+   }
+
+   return 1; /* keep going */
+}
+#endif /* PNG_WARNINGS_SUPPORTED */
+
+static void
+perform_error_test(png_modifier *pm)
+{
+#ifdef PNG_WARNINGS_SUPPORTED /* else there are no cases that work! */
+   /* Need to do this here because we just write in this test. */
+   safecat(pm->this.test, sizeof pm->this.test, 0, "error test");
+
+   if (!make_errors(pm, 0, 0, WRITE_BDHI))
+      return;
+
+   if (!make_errors(pm, 2, 3, WRITE_BDHI))
+      return;
+
+   if (!make_errors(pm, 3, 0, 3))
+      return;
+
+   if (!make_errors(pm, 4, 3, WRITE_BDHI))
+      return;
+
+   if (!make_errors(pm, 6, 3, WRITE_BDHI))
+      return;
+#else
+   UNUSED(pm)
+#endif
+}
+
+/* This is just to validate the internal PNG formatting code - if this fails
+ * then the warning messages the library outputs will probably be garbage.
+ */
+static void
+perform_formatting_test(png_store *ps)
+{
+#ifdef PNG_TIME_RFC1123_SUPPORTED
+   /* The handle into the formatting code is the RFC1123 support; this test does
+    * nothing if that is compiled out.
+    */
+   context(ps, fault);
+
+   Try
+   {
+      png_const_charp correct = "29 Aug 2079 13:53:60 +0000";
+      png_const_charp result;
+#     if PNG_LIBPNG_VER >= 10600
+         char timestring[29];
+#     endif
+      png_structp pp;
+      png_time pt;
+
+      pp = set_store_for_write(ps, NULL, "libpng formatting test");
+
+      if (pp == NULL)
+         Throw ps;
+
+
+      /* Arbitrary settings: */
+      pt.year = 2079;
+      pt.month = 8;
+      pt.day = 29;
+      pt.hour = 13;
+      pt.minute = 53;
+      pt.second = 60; /* a leap second */
+
+#     if PNG_LIBPNG_VER < 10600
+         result = png_convert_to_rfc1123(pp, &pt);
+#     else
+         if (png_convert_to_rfc1123_buffer(timestring, &pt))
+            result = timestring;
+
+         else
+            result = NULL;
+#     endif
+
+      if (result == NULL)
+         png_error(pp, "png_convert_to_rfc1123 failed");
+
+      if (strcmp(result, correct) != 0)
+      {
+         size_t pos = 0;
+         char msg[128];
+
+         pos = safecat(msg, sizeof msg, pos, "png_convert_to_rfc1123(");
+         pos = safecat(msg, sizeof msg, pos, correct);
+         pos = safecat(msg, sizeof msg, pos, ") returned: '");
+         pos = safecat(msg, sizeof msg, pos, result);
+         pos = safecat(msg, sizeof msg, pos, "'");
+
+         png_error(pp, msg);
+      }
+
+      store_write_reset(ps);
+   }
+
+   Catch(fault)
+   {
+      store_write_reset(fault);
+   }
+#else
+   UNUSED(ps)
+#endif
+}
+
+#ifdef PNG_READ_SUPPORTED
+/* Because we want to use the same code in both the progressive reader and the
+ * sequential reader it is necessary to deal with the fact that the progressive
+ * reader callbacks only have one parameter (png_get_progressive_ptr()), so this
+ * must contain all the test parameters and all the local variables directly
+ * accessible to the sequential reader implementation.
+ *
+ * The technique adopted is to reinvent part of what Dijkstra termed a
+ * 'display'; an array of pointers to the stack frames of enclosing functions so
+ * that a nested function definition can access the local (C auto) variables of
+ * the functions that contain its definition.  In fact C provides the first
+ * pointer (the local variables - the stack frame pointer) and the last (the
+ * global variables - the BCPL global vector typically implemented as global
+ * addresses), this code requires one more pointer to make the display - the
+ * local variables (and function call parameters) of the function that actually
+ * invokes either the progressive or sequential reader.
+ *
+ * Perhaps confusingly this technique is confounded with classes - the
+ * 'standard_display' defined here is sub-classed as the 'gamma_display' below.
+ * A gamma_display is a standard_display, taking advantage of the ANSI-C
+ * requirement that the pointer to the first member of a structure must be the
+ * same as the pointer to the structure.  This allows us to reuse standard_
+ * functions in the gamma test code; something that could not be done with
+ * nested functions!
+ */
+typedef struct standard_display
+{
+   png_store*  ps;             /* Test parameters (passed to the function) */
+   png_byte    colour_type;
+   png_byte    bit_depth;
+   png_byte    red_sBIT;       /* Input data sBIT values. */
+   png_byte    green_sBIT;
+   png_byte    blue_sBIT;
+   png_byte    alpha_sBIT;
+   png_byte    interlace_type;
+   png_byte    filler;         /* Output has a filler */
+   png_uint_32 id;             /* Calculated file ID */
+   png_uint_32 w;              /* Width of image */
+   png_uint_32 h;              /* Height of image */
+   int         npasses;        /* Number of interlaced passes */
+   png_uint_32 pixel_size;     /* Width of one pixel in bits */
+   png_uint_32 bit_width;      /* Width of output row in bits */
+   size_t      cbRow;          /* Bytes in a row of the output image */
+   int         do_interlace;   /* Do interlacing internally */
+   int         littleendian;   /* App (row) data is little endian */
+   int         is_transparent; /* Transparency information was present. */
+   int         has_tRNS;       /* color type GRAY or RGB with a tRNS chunk. */
+   int         speed;          /* Doing a speed test */
+   int         use_update_info;/* Call update_info, not start_image */
+   struct
+   {
+      png_uint_16 red;
+      png_uint_16 green;
+      png_uint_16 blue;
+   }           transparent;    /* The transparent color, if set. */
+   int         npalette;       /* Number of entries in the palette. */
+   store_palette
+               palette;
+} standard_display;
+
+static void
+standard_display_init(standard_display *dp, png_store* ps, png_uint_32 id,
+   int do_interlace, int use_update_info)
+{
+   memset(dp, 0, sizeof *dp);
+
+   dp->ps = ps;
+   dp->colour_type = COL_FROM_ID(id);
+   dp->bit_depth = DEPTH_FROM_ID(id);
+   if (dp->bit_depth < 1 || dp->bit_depth > 16)
+      internal_error(ps, "internal: bad bit depth");
+   if (dp->colour_type == 3)
+      dp->red_sBIT = dp->blue_sBIT = dp->green_sBIT = dp->alpha_sBIT = 8;
+   else
+      dp->red_sBIT = dp->blue_sBIT = dp->green_sBIT = dp->alpha_sBIT =
+         dp->bit_depth;
+   dp->interlace_type = INTERLACE_FROM_ID(id);
+   check_interlace_type(dp->interlace_type);
+   dp->id = id;
+   /* All the rest are filled in after the read_info: */
+   dp->w = 0;
+   dp->h = 0;
+   dp->npasses = 0;
+   dp->pixel_size = 0;
+   dp->bit_width = 0;
+   dp->cbRow = 0;
+   dp->do_interlace = do_interlace;
+   dp->littleendian = 0;
+   dp->is_transparent = 0;
+   dp->speed = ps->speed;
+   dp->use_update_info = use_update_info;
+   dp->npalette = 0;
+   /* Preset the transparent color to black: */
+   memset(&dp->transparent, 0, sizeof dp->transparent);
+   /* Preset the palette to full intensity/opaque througout: */
+   memset(dp->palette, 0xff, sizeof dp->palette);
+}
+
+/* Initialize the palette fields - this must be done later because the palette
+ * comes from the particular png_store_file that is selected.
+ */
+static void
+standard_palette_init(standard_display *dp)
+{
+   store_palette_entry *palette = store_current_palette(dp->ps, &dp->npalette);
+
+   /* The remaining entries remain white/opaque. */
+   if (dp->npalette > 0)
+   {
+      int i = dp->npalette;
+      memcpy(dp->palette, palette, i * sizeof *palette);
+
+      /* Check for a non-opaque palette entry: */
+      while (--i >= 0)
+         if (palette[i].alpha < 255)
+            break;
+
+#     ifdef __GNUC__
+         /* GCC can't handle the more obviously optimizable version. */
+         if (i >= 0)
+            dp->is_transparent = 1;
+         else
+            dp->is_transparent = 0;
+#     else
+         dp->is_transparent = (i >= 0);
+#     endif
+   }
+}
+
+/* Utility to read the palette from the PNG file and convert it into
+ * store_palette format.  This returns 1 if there is any transparency in the
+ * palette (it does not check for a transparent colour in the non-palette case.)
+ */
+static int
+read_palette(store_palette palette, int *npalette, png_const_structp pp,
+   png_infop pi)
+{
+   png_colorp pal;
+   png_bytep trans_alpha;
+   int num;
+
+   pal = 0;
+   *npalette = -1;
+
+   if (png_get_PLTE(pp, pi, &pal, npalette) & PNG_INFO_PLTE)
+   {
+      int i = *npalette;
+
+      if (i <= 0 || i > 256)
+         png_error(pp, "validate: invalid PLTE count");
+
+      while (--i >= 0)
+      {
+         palette[i].red = pal[i].red;
+         palette[i].green = pal[i].green;
+         palette[i].blue = pal[i].blue;
+      }
+
+      /* Mark the remainder of the entries with a flag value (other than
+       * white/opaque which is the flag value stored above.)
+       */
+      memset(palette + *npalette, 126, (256-*npalette) * sizeof *palette);
+   }
+
+   else /* !png_get_PLTE */
+   {
+      if (*npalette != (-1))
+         png_error(pp, "validate: invalid PLTE result");
+      /* But there is no palette, so record this: */
+      *npalette = 0;
+      memset(palette, 113, sizeof (store_palette));
+   }
+
+   trans_alpha = 0;
+   num = 2; /* force error below */
+   if ((png_get_tRNS(pp, pi, &trans_alpha, &num, 0) & PNG_INFO_tRNS) != 0 &&
+      (trans_alpha != NULL || num != 1/*returns 1 for a transparent color*/) &&
+      /* Oops, if a palette tRNS gets expanded png_read_update_info (at least so
+       * far as 1.5.4) does not remove the trans_alpha pointer, only num_trans,
+       * so in the above call we get a success, we get a pointer (who knows what
+       * to) and we get num_trans == 0:
+       */
+      !(trans_alpha != NULL && num == 0)) /* TODO: fix this in libpng. */
+   {
+      int i;
+
+      /* Any of these are crash-worthy - given the implementation of
+       * png_get_tRNS up to 1.5 an app won't crash if it just checks the
+       * result above and fails to check that the variables it passed have
+       * actually been filled in!  Note that if the app were to pass the
+       * last, png_color_16p, variable too it couldn't rely on this.
+       */
+      if (trans_alpha == NULL || num <= 0 || num > 256 || num > *npalette)
+         png_error(pp, "validate: unexpected png_get_tRNS (palette) result");
+
+      for (i=0; i<num; ++i)
+         palette[i].alpha = trans_alpha[i];
+
+      for (num=*npalette; i<num; ++i)
+         palette[i].alpha = 255;
+
+      for (; i<256; ++i)
+         palette[i].alpha = 33; /* flag value */
+
+      return 1; /* transparency */
+   }
+
+   else
+   {
+      /* No palette transparency - just set the alpha channel to opaque. */
+      int i;
+
+      for (i=0, num=*npalette; i<num; ++i)
+         palette[i].alpha = 255;
+
+      for (; i<256; ++i)
+         palette[i].alpha = 55; /* flag value */
+
+      return 0; /* no transparency */
+   }
+}
+
+/* Utility to validate the palette if it should not have changed (the
+ * non-transform case).
+ */
+static void
+standard_palette_validate(standard_display *dp, png_const_structp pp,
+   png_infop pi)
+{
+   int npalette;
+   store_palette palette;
+
+   if (read_palette(palette, &npalette, pp, pi) != dp->is_transparent)
+      png_error(pp, "validate: palette transparency changed");
+
+   if (npalette != dp->npalette)
+   {
+      size_t pos = 0;
+      char msg[64];
+
+      pos = safecat(msg, sizeof msg, pos, "validate: palette size changed: ");
+      pos = safecatn(msg, sizeof msg, pos, dp->npalette);
+      pos = safecat(msg, sizeof msg, pos, " -> ");
+      pos = safecatn(msg, sizeof msg, pos, npalette);
+      png_error(pp, msg);
+   }
+
+   {
+      int i = npalette; /* npalette is aliased */
+
+      while (--i >= 0)
+         if (palette[i].red != dp->palette[i].red ||
+            palette[i].green != dp->palette[i].green ||
+            palette[i].blue != dp->palette[i].blue ||
+            palette[i].alpha != dp->palette[i].alpha)
+            png_error(pp, "validate: PLTE or tRNS chunk changed");
+   }
+}
+
+/* By passing a 'standard_display' the progressive callbacks can be used
+ * directly by the sequential code, the functions suffixed "_imp" are the
+ * implementations, the functions without the suffix are the callbacks.
+ *
+ * The code for the info callback is split into two because this callback calls
+ * png_read_update_info or png_start_read_image and what gets called depends on
+ * whether the info needs updating (we want to test both calls in pngvalid.)
+ */
+static void
+standard_info_part1(standard_display *dp, png_structp pp, png_infop pi)
+{
+   if (png_get_bit_depth(pp, pi) != dp->bit_depth)
+      png_error(pp, "validate: bit depth changed");
+
+   if (png_get_color_type(pp, pi) != dp->colour_type)
+      png_error(pp, "validate: color type changed");
+
+   if (png_get_filter_type(pp, pi) != PNG_FILTER_TYPE_BASE)
+      png_error(pp, "validate: filter type changed");
+
+   if (png_get_interlace_type(pp, pi) != dp->interlace_type)
+      png_error(pp, "validate: interlacing changed");
+
+   if (png_get_compression_type(pp, pi) != PNG_COMPRESSION_TYPE_BASE)
+      png_error(pp, "validate: compression type changed");
+
+   dp->w = png_get_image_width(pp, pi);
+
+   if (dp->w != standard_width(pp, dp->id))
+      png_error(pp, "validate: image width changed");
+
+   dp->h = png_get_image_height(pp, pi);
+
+   if (dp->h != standard_height(pp, dp->id))
+      png_error(pp, "validate: image height changed");
+
+   /* Record (but don't check at present) the input sBIT according to the colour
+    * type information.
+    */
+   {
+      png_color_8p sBIT = 0;
+
+      if (png_get_sBIT(pp, pi, &sBIT) & PNG_INFO_sBIT)
+      {
+         int sBIT_invalid = 0;
+
+         if (sBIT == 0)
+            png_error(pp, "validate: unexpected png_get_sBIT result");
+
+         if (dp->colour_type & PNG_COLOR_MASK_COLOR)
+         {
+            if (sBIT->red == 0 || sBIT->red > dp->bit_depth)
+               sBIT_invalid = 1;
+            else
+               dp->red_sBIT = sBIT->red;
+
+            if (sBIT->green == 0 || sBIT->green > dp->bit_depth)
+               sBIT_invalid = 1;
+            else
+               dp->green_sBIT = sBIT->green;
+
+            if (sBIT->blue == 0 || sBIT->blue > dp->bit_depth)
+               sBIT_invalid = 1;
+            else
+               dp->blue_sBIT = sBIT->blue;
+         }
+
+         else /* !COLOR */
+         {
+            if (sBIT->gray == 0 || sBIT->gray > dp->bit_depth)
+               sBIT_invalid = 1;
+            else
+               dp->blue_sBIT = dp->green_sBIT = dp->red_sBIT = sBIT->gray;
+         }
+
+         /* All 8 bits in tRNS for a palette image are significant - see the
+          * spec.
+          */
+         if (dp->colour_type & PNG_COLOR_MASK_ALPHA)
+         {
+            if (sBIT->alpha == 0 || sBIT->alpha > dp->bit_depth)
+               sBIT_invalid = 1;
+            else
+               dp->alpha_sBIT = sBIT->alpha;
+         }
+
+         if (sBIT_invalid)
+            png_error(pp, "validate: sBIT value out of range");
+      }
+   }
+
+   /* Important: this is validating the value *before* any transforms have been
+    * put in place.  It doesn't matter for the standard tests, where there are
+    * no transforms, but it does for other tests where rowbytes may change after
+    * png_read_update_info.
+    */
+   if (png_get_rowbytes(pp, pi) != standard_rowsize(pp, dp->id))
+      png_error(pp, "validate: row size changed");
+
+   /* Validate the colour type 3 palette (this can be present on other color
+    * types.)
+    */
+   standard_palette_validate(dp, pp, pi);
+
+   /* In any case always check for a tranparent color (notice that the
+    * colour type 3 case must not give a successful return on the get_tRNS call
+    * with these arguments!)
+    */
+   {
+      png_color_16p trans_color = 0;
+
+      if (png_get_tRNS(pp, pi, 0, 0, &trans_color) & PNG_INFO_tRNS)
+      {
+         if (trans_color == 0)
+            png_error(pp, "validate: unexpected png_get_tRNS (color) result");
+
+         switch (dp->colour_type)
+         {
+         case 0:
+            dp->transparent.red = dp->transparent.green = dp->transparent.blue =
+               trans_color->gray;
+            dp->has_tRNS = 1;
+            break;
+
+         case 2:
+            dp->transparent.red = trans_color->red;
+            dp->transparent.green = trans_color->green;
+            dp->transparent.blue = trans_color->blue;
+            dp->has_tRNS = 1;
+            break;
+
+         case 3:
+            /* Not expected because it should result in the array case
+             * above.
+             */
+            png_error(pp, "validate: unexpected png_get_tRNS result");
+            break;
+
+         default:
+            png_error(pp, "validate: invalid tRNS chunk with alpha image");
+         }
+      }
+   }
+
+   /* Read the number of passes - expected to match the value used when
+    * creating the image (interlaced or not).  This has the side effect of
+    * turning on interlace handling (if do_interlace is not set.)
+    */
+   dp->npasses = npasses_from_interlace_type(pp, dp->interlace_type);
+   if (!dp->do_interlace)
+   {
+#     ifdef PNG_READ_INTERLACING_SUPPORTED
+         if (dp->npasses != png_set_interlace_handling(pp))
+            png_error(pp, "validate: file changed interlace type");
+#     else /* !READ_INTERLACING */
+         /* This should never happen: the relevant tests (!do_interlace) should
+          * not be run.
+          */
+         if (dp->npasses > 1)
+            png_error(pp, "validate: no libpng interlace support");
+#     endif /* !READ_INTERLACING */
+   }
+
+   /* Caller calls png_read_update_info or png_start_read_image now, then calls
+    * part2.
+    */
+}
+
+/* This must be called *after* the png_read_update_info call to get the correct
+ * 'rowbytes' value, otherwise png_get_rowbytes will refer to the untransformed
+ * image.
+ */
+static void
+standard_info_part2(standard_display *dp, png_const_structp pp,
+    png_const_infop pi, int nImages)
+{
+   /* Record cbRow now that it can be found. */
+   {
+      png_byte ct = png_get_color_type(pp, pi);
+      png_byte bd = png_get_bit_depth(pp, pi);
+
+      if (bd >= 8 && (ct == PNG_COLOR_TYPE_RGB || ct == PNG_COLOR_TYPE_GRAY) &&
+          dp->filler)
+          ct |= 4; /* handle filler as faked alpha channel */
+
+      dp->pixel_size = bit_size(pp, ct, bd);
+   }
+   dp->bit_width = png_get_image_width(pp, pi) * dp->pixel_size;
+   dp->cbRow = png_get_rowbytes(pp, pi);
+
+   /* Validate the rowbytes here again. */
+   if (dp->cbRow != (dp->bit_width+7)/8)
+      png_error(pp, "bad png_get_rowbytes calculation");
+
+   /* Then ensure there is enough space for the output image(s). */
+   store_ensure_image(dp->ps, pp, nImages, dp->cbRow, dp->h);
+}
+
+static void
+standard_info_imp(standard_display *dp, png_structp pp, png_infop pi,
+    int nImages)
+{
+   /* Note that the validation routine has the side effect of turning on
+    * interlace handling in the subsequent code.
+    */
+   standard_info_part1(dp, pp, pi);
+
+   /* And the info callback has to call this (or png_read_update_info - see
+    * below in the png_modifier code for that variant.
+    */
+   if (dp->use_update_info)
+   {
+      /* For debugging the effect of multiple calls: */
+      int i = dp->use_update_info;
+      while (i-- > 0)
+         png_read_update_info(pp, pi);
+   }
+
+   else
+      png_start_read_image(pp);
+
+   /* Validate the height, width and rowbytes plus ensure that sufficient buffer
+    * exists for decoding the image.
+    */
+   standard_info_part2(dp, pp, pi, nImages);
+}
+
+static void PNGCBAPI
+standard_info(png_structp pp, png_infop pi)
+{
+   standard_display *dp = voidcast(standard_display*,
+      png_get_progressive_ptr(pp));
+
+   /* Call with nImages==1 because the progressive reader can only produce one
+    * image.
+    */
+   standard_info_imp(dp, pp, pi, 1 /*only one image*/);
+}
+
+static void PNGCBAPI
+progressive_row(png_structp ppIn, png_bytep new_row, png_uint_32 y, int pass)
+{
+   png_const_structp pp = ppIn;
+   const standard_display *dp = voidcast(standard_display*,
+      png_get_progressive_ptr(pp));
+
+   /* When handling interlacing some rows will be absent in each pass, the
+    * callback still gets called, but with a NULL pointer.  This is checked
+    * in the 'else' clause below.  We need our own 'cbRow', but we can't call
+    * png_get_rowbytes because we got no info structure.
+    */
+   if (new_row != NULL)
+   {
+      png_bytep row;
+
+      /* In the case where the reader doesn't do the interlace it gives
+       * us the y in the sub-image:
+       */
+      if (dp->do_interlace && dp->interlace_type == PNG_INTERLACE_ADAM7)
+      {
+#ifdef PNG_USER_TRANSFORM_INFO_SUPPORTED
+         /* Use this opportunity to validate the png 'current' APIs: */
+         if (y != png_get_current_row_number(pp))
+            png_error(pp, "png_get_current_row_number is broken");
+
+         if (pass != png_get_current_pass_number(pp))
+            png_error(pp, "png_get_current_pass_number is broken");
+#endif /* USER_TRANSFORM_INFO */
+
+         y = PNG_ROW_FROM_PASS_ROW(y, pass);
+      }
+
+      /* Validate this just in case. */
+      if (y >= dp->h)
+         png_error(pp, "invalid y to progressive row callback");
+
+      row = store_image_row(dp->ps, pp, 0, y);
+
+      /* Combine the new row into the old: */
+#ifdef PNG_READ_INTERLACING_SUPPORTED
+      if (dp->do_interlace)
+#endif /* READ_INTERLACING */
+      {
+         if (dp->interlace_type == PNG_INTERLACE_ADAM7)
+            deinterlace_row(row, new_row, dp->pixel_size, dp->w, pass,
+                  dp->littleendian);
+         else
+            row_copy(row, new_row, dp->pixel_size * dp->w, dp->littleendian);
+      }
+#ifdef PNG_READ_INTERLACING_SUPPORTED
+      else
+         png_progressive_combine_row(pp, row, new_row);
+#endif /* PNG_READ_INTERLACING_SUPPORTED */
+   }
+
+   else if (dp->interlace_type == PNG_INTERLACE_ADAM7 &&
+       PNG_ROW_IN_INTERLACE_PASS(y, pass) &&
+       PNG_PASS_COLS(dp->w, pass) > 0)
+      png_error(pp, "missing row in progressive de-interlacing");
+}
+
+static void
+sequential_row(standard_display *dp, png_structp pp, png_infop pi,
+    const int iImage, const int iDisplay)
+{
+   const int         npasses = dp->npasses;
+   const int         do_interlace = dp->do_interlace &&
+      dp->interlace_type == PNG_INTERLACE_ADAM7;
+   const png_uint_32 height = standard_height(pp, dp->id);
+   const png_uint_32 width = standard_width(pp, dp->id);
+   const png_store*  ps = dp->ps;
+   int pass;
+
+   for (pass=0; pass<npasses; ++pass)
+   {
+      png_uint_32 y;
+      png_uint_32 wPass = PNG_PASS_COLS(width, pass);
+
+      for (y=0; y<height; ++y)
+      {
+         if (do_interlace)
+         {
+            /* wPass may be zero or this row may not be in this pass.
+             * png_read_row must not be called in either case.
+             */
+            if (wPass > 0 && PNG_ROW_IN_INTERLACE_PASS(y, pass))
+            {
+               /* Read the row into a pair of temporary buffers, then do the
+                * merge here into the output rows.
+                */
+               png_byte row[STANDARD_ROWMAX], display[STANDARD_ROWMAX];
+
+               /* The following aids (to some extent) error detection - we can
+                * see where png_read_row wrote.  Use opposite values in row and
+                * display to make this easier.  Don't use 0xff (which is used in
+                * the image write code to fill unused bits) or 0 (which is a
+                * likely value to overwrite unused bits with).
+                */
+               memset(row, 0xc5, sizeof row);
+               memset(display, 0x5c, sizeof display);
+
+               png_read_row(pp, row, display);
+
+               if (iImage >= 0)
+                  deinterlace_row(store_image_row(ps, pp, iImage, y), row,
+                     dp->pixel_size, dp->w, pass, dp->littleendian);
+
+               if (iDisplay >= 0)
+                  deinterlace_row(store_image_row(ps, pp, iDisplay, y), display,
+                     dp->pixel_size, dp->w, pass, dp->littleendian);
+            }
+         }
+         else
+            png_read_row(pp,
+               iImage >= 0 ? store_image_row(ps, pp, iImage, y) : NULL,
+               iDisplay >= 0 ? store_image_row(ps, pp, iDisplay, y) : NULL);
+      }
+   }
+
+   /* And finish the read operation (only really necessary if the caller wants
+    * to find additional data in png_info from chunks after the last IDAT.)
+    */
+   png_read_end(pp, pi);
+}
+
+#ifdef PNG_TEXT_SUPPORTED
+static void
+standard_check_text(png_const_structp pp, png_const_textp tp,
+   png_const_charp keyword, png_const_charp text)
+{
+   char msg[1024];
+   size_t pos = safecat(msg, sizeof msg, 0, "text: ");
+   size_t ok;
+
+   pos = safecat(msg, sizeof msg, pos, keyword);
+   pos = safecat(msg, sizeof msg, pos, ": ");
+   ok = pos;
+
+   if (tp->compression != TEXT_COMPRESSION)
+   {
+      char buf[64];
+
+      sprintf(buf, "compression [%d->%d], ", TEXT_COMPRESSION,
+         tp->compression);
+      pos = safecat(msg, sizeof msg, pos, buf);
+   }
+
+   if (tp->key == NULL || strcmp(tp->key, keyword) != 0)
+   {
+      pos = safecat(msg, sizeof msg, pos, "keyword \"");
+      if (tp->key != NULL)
+      {
+         pos = safecat(msg, sizeof msg, pos, tp->key);
+         pos = safecat(msg, sizeof msg, pos, "\", ");
+      }
+
+      else
+         pos = safecat(msg, sizeof msg, pos, "null, ");
+   }
+
+   if (tp->text == NULL)
+      pos = safecat(msg, sizeof msg, pos, "text lost, ");
+
+   else
+   {
+      if (tp->text_length != strlen(text))
+      {
+         char buf[64];
+         sprintf(buf, "text length changed[%lu->%lu], ",
+            (unsigned long)strlen(text), (unsigned long)tp->text_length);
+         pos = safecat(msg, sizeof msg, pos, buf);
+      }
+
+      if (strcmp(tp->text, text) != 0)
+      {
+         pos = safecat(msg, sizeof msg, pos, "text becomes \"");
+         pos = safecat(msg, sizeof msg, pos, tp->text);
+         pos = safecat(msg, sizeof msg, pos, "\" (was \"");
+         pos = safecat(msg, sizeof msg, pos, text);
+         pos = safecat(msg, sizeof msg, pos, "\"), ");
+      }
+   }
+
+   if (tp->itxt_length != 0)
+      pos = safecat(msg, sizeof msg, pos, "iTXt length set, ");
+
+   if (tp->lang != NULL)
+   {
+      pos = safecat(msg, sizeof msg, pos, "iTXt language \"");
+      pos = safecat(msg, sizeof msg, pos, tp->lang);
+      pos = safecat(msg, sizeof msg, pos, "\", ");
+   }
+
+   if (tp->lang_key != NULL)
+   {
+      pos = safecat(msg, sizeof msg, pos, "iTXt keyword \"");
+      pos = safecat(msg, sizeof msg, pos, tp->lang_key);
+      pos = safecat(msg, sizeof msg, pos, "\", ");
+   }
+
+   if (pos > ok)
+   {
+      msg[pos-2] = '\0'; /* Remove the ", " at the end */
+      png_error(pp, msg);
+   }
+}
+
+static void
+standard_text_validate(standard_display *dp, png_const_structp pp,
+   png_infop pi, int check_end)
+{
+   png_textp tp = NULL;
+   png_uint_32 num_text = png_get_text(pp, pi, &tp, NULL);
+
+   if (num_text == 2 && tp != NULL)
+   {
+      standard_check_text(pp, tp, "image name", dp->ps->current->name);
+
+      /* This exists because prior to 1.5.18 the progressive reader left the
+       * png_struct z_stream unreset at the end of the image, so subsequent
+       * attempts to use it simply returns Z_STREAM_END.
+       */
+      if (check_end)
+         standard_check_text(pp, tp+1, "end marker", "end");
+   }
+
+   else
+   {
+      char msg[64];
+
+      sprintf(msg, "expected two text items, got %lu",
+         (unsigned long)num_text);
+      png_error(pp, msg);
+   }
+}
+#else
+#  define standard_text_validate(dp,pp,pi,check_end) ((void)0)
+#endif
+
+static void
+standard_row_validate(standard_display *dp, png_const_structp pp,
+   int iImage, int iDisplay, png_uint_32 y)
+{
+   int where;
+   png_byte std[STANDARD_ROWMAX];
+
+   /* The row must be pre-initialized to the magic number here for the size
+    * tests to pass:
+    */
+   memset(std, 178, sizeof std);
+   standard_row(pp, std, dp->id, y);
+
+   /* At the end both the 'row' and 'display' arrays should end up identical.
+    * In earlier passes 'row' will be partially filled in, with only the pixels
+    * that have been read so far, but 'display' will have those pixels
+    * replicated to fill the unread pixels while reading an interlaced image.
+    */
+   if (iImage >= 0 &&
+      (where = pixel_cmp(std, store_image_row(dp->ps, pp, iImage, y),
+            dp->bit_width)) != 0)
+   {
+      char msg[64];
+      sprintf(msg, "PNG image row[%lu][%d] changed from %.2x to %.2x",
+         (unsigned long)y, where-1, std[where-1],
+         store_image_row(dp->ps, pp, iImage, y)[where-1]);
+      png_error(pp, msg);
+   }
+
+   if (iDisplay >= 0 &&
+      (where = pixel_cmp(std, store_image_row(dp->ps, pp, iDisplay, y),
+         dp->bit_width)) != 0)
+   {
+      char msg[64];
+      sprintf(msg, "display row[%lu][%d] changed from %.2x to %.2x",
+         (unsigned long)y, where-1, std[where-1],
+         store_image_row(dp->ps, pp, iDisplay, y)[where-1]);
+      png_error(pp, msg);
+   }
+}
+
+static void
+standard_image_validate(standard_display *dp, png_const_structp pp, int iImage,
+    int iDisplay)
+{
+   png_uint_32 y;
+
+   if (iImage >= 0)
+      store_image_check(dp->ps, pp, iImage);
+
+   if (iDisplay >= 0)
+      store_image_check(dp->ps, pp, iDisplay);
+
+   for (y=0; y<dp->h; ++y)
+      standard_row_validate(dp, pp, iImage, iDisplay, y);
+
+   /* This avoids false positives if the validation code is never called! */
+   dp->ps->validated = 1;
+}
+
+static void PNGCBAPI
+standard_end(png_structp ppIn, png_infop pi)
+{
+   png_const_structp pp = ppIn;
+   standard_display *dp = voidcast(standard_display*,
+      png_get_progressive_ptr(pp));
+
+   UNUSED(pi)
+
+   /* Validate the image - progressive reading only produces one variant for
+    * interlaced images.
+    */
+   standard_text_validate(dp, pp, pi,
+      PNG_LIBPNG_VER >= 10518/*check_end: see comments above*/);
+   standard_image_validate(dp, pp, 0, -1);
+}
+
+/* A single test run checking the standard image to ensure it is not damaged. */
+static void
+standard_test(png_store* const psIn, png_uint_32 const id,
+   int do_interlace, int use_update_info)
+{
+   standard_display d;
+   context(psIn, fault);
+
+   /* Set up the display (stack frame) variables from the arguments to the
+    * function and initialize the locals that are filled in later.
+    */
+   standard_display_init(&d, psIn, id, do_interlace, use_update_info);
+
+   /* Everything is protected by a Try/Catch.  The functions called also
+    * typically have local Try/Catch blocks.
+    */
+   Try
+   {
+      png_structp pp;
+      png_infop pi;
+
+      /* Get a png_struct for reading the image. This will throw an error if it
+       * fails, so we don't need to check the result.
+       */
+      pp = set_store_for_read(d.ps, &pi, d.id,
+         d.do_interlace ?  (d.ps->progressive ?
+            "pngvalid progressive deinterlacer" :
+            "pngvalid sequential deinterlacer") : (d.ps->progressive ?
+               "progressive reader" : "sequential reader"));
+
+      /* Initialize the palette correctly from the png_store_file. */
+      standard_palette_init(&d);
+
+      /* Introduce the correct read function. */
+      if (d.ps->progressive)
+      {
+         png_set_progressive_read_fn(pp, &d, standard_info, progressive_row,
+            standard_end);
+
+         /* Now feed data into the reader until we reach the end: */
+         store_progressive_read(d.ps, pp, pi);
+      }
+      else
+      {
+         /* Note that this takes the store, not the display. */
+         png_set_read_fn(pp, d.ps, store_read);
+
+         /* Check the header values: */
+         png_read_info(pp, pi);
+
+         /* The code tests both versions of the images that the sequential
+          * reader can produce.
+          */
+         standard_info_imp(&d, pp, pi, 2 /*images*/);
+
+         /* Need the total bytes in the image below; we can't get to this point
+          * unless the PNG file values have been checked against the expected
+          * values.
+          */
+         {
+            sequential_row(&d, pp, pi, 0, 1);
+
+            /* After the last pass loop over the rows again to check that the
+             * image is correct.
+             */
+            if (!d.speed)
+            {
+               standard_text_validate(&d, pp, pi, 1/*check_end*/);
+               standard_image_validate(&d, pp, 0, 1);
+            }
+            else
+               d.ps->validated = 1;
+         }
+      }
+
+      /* Check for validation. */
+      if (!d.ps->validated)
+         png_error(pp, "image read failed silently");
+
+      /* Successful completion. */
+   }
+
+   Catch(fault)
+      d.ps = fault; /* make sure this hasn't been clobbered. */
+
+   /* In either case clean up the store. */
+   store_read_reset(d.ps);
+}
+
+static int
+test_standard(png_modifier* const pm, png_byte const colour_type,
+    int bdlo, int const bdhi)
+{
+   for (; bdlo <= bdhi; ++bdlo)
+   {
+      int interlace_type;
+
+      for (interlace_type = PNG_INTERLACE_NONE;
+           interlace_type < INTERLACE_LAST; ++interlace_type)
+      {
+         standard_test(&pm->this, FILEID(colour_type, DEPTH(bdlo), 0/*palette*/,
+            interlace_type, 0, 0, 0), do_read_interlace, pm->use_update_info);
+
+         if (fail(pm))
+            return 0;
+      }
+   }
+
+   return 1; /* keep going */
+}
+
+static void
+perform_standard_test(png_modifier *pm)
+{
+   /* Test each colour type over the valid range of bit depths (expressed as
+    * log2(bit_depth) in turn, stop as soon as any error is detected.
+    */
+   if (!test_standard(pm, 0, 0, READ_BDHI))
+      return;
+
+   if (!test_standard(pm, 2, 3, READ_BDHI))
+      return;
+
+   if (!test_standard(pm, 3, 0, 3))
+      return;
+
+   if (!test_standard(pm, 4, 3, READ_BDHI))
+      return;
+
+   if (!test_standard(pm, 6, 3, READ_BDHI))
+      return;
+}
+
+
+/********************************** SIZE TESTS ********************************/
+static int
+test_size(png_modifier* const pm, png_byte const colour_type,
+    int bdlo, int const bdhi)
+{
+   /* Run the tests on each combination.
+    *
+    * NOTE: on my 32 bit x86 each of the following blocks takes
+    * a total of 3.5 seconds if done across every combo of bit depth
+    * width and height.  This is a waste of time in practice, hence the
+    * hinc and winc stuff:
+    */
+   static const png_byte hinc[] = {1, 3, 11, 1, 5};
+   static const png_byte winc[] = {1, 9, 5, 7, 1};
+   const int save_bdlo = bdlo;
+
+   for (; bdlo <= bdhi; ++bdlo)
+   {
+      png_uint_32 h, w;
+
+      for (h=1; h<=16; h+=hinc[bdlo]) for (w=1; w<=16; w+=winc[bdlo])
+      {
+         /* First test all the 'size' images against the sequential
+          * reader using libpng to deinterlace (where required.)  This
+          * validates the write side of libpng.  There are four possibilities
+          * to validate.
+          */
+         standard_test(&pm->this, FILEID(colour_type, DEPTH(bdlo), 0/*palette*/,
+            PNG_INTERLACE_NONE, w, h, 0), 0/*do_interlace*/,
+            pm->use_update_info);
+
+         if (fail(pm))
+            return 0;
+
+         standard_test(&pm->this, FILEID(colour_type, DEPTH(bdlo), 0/*palette*/,
+            PNG_INTERLACE_NONE, w, h, 1), 0/*do_interlace*/,
+            pm->use_update_info);
+
+         if (fail(pm))
+            return 0;
+
+         /* Now validate the interlaced read side - do_interlace true,
+          * in the progressive case this does actually make a difference
+          * to the code used in the non-interlaced case too.
+          */
+         standard_test(&pm->this, FILEID(colour_type, DEPTH(bdlo), 0/*palette*/,
+            PNG_INTERLACE_NONE, w, h, 0), 1/*do_interlace*/,
+            pm->use_update_info);
+
+         if (fail(pm))
+            return 0;
+
+#     if CAN_WRITE_INTERLACE
+         /* Validate the pngvalid code itself: */
+         standard_test(&pm->this, FILEID(colour_type, DEPTH(bdlo), 0/*palette*/,
+            PNG_INTERLACE_ADAM7, w, h, 1), 1/*do_interlace*/,
+            pm->use_update_info);
+
+         if (fail(pm))
+            return 0;
+#     endif
+      }
+   }
+
+   /* Now do the tests of libpng interlace handling, after we have made sure
+    * that the pngvalid version works:
+    */
+   for (bdlo = save_bdlo; bdlo <= bdhi; ++bdlo)
+   {
+      png_uint_32 h, w;
+
+      for (h=1; h<=16; h+=hinc[bdlo]) for (w=1; w<=16; w+=winc[bdlo])
+      {
+#     ifdef PNG_READ_INTERLACING_SUPPORTED
+         /* Test with pngvalid generated interlaced images first; we have
+          * already verify these are ok (unless pngvalid has self-consistent
+          * read/write errors, which is unlikely), so this detects errors in the
+          * read side first:
+          */
+#     if CAN_WRITE_INTERLACE
+         standard_test(&pm->this, FILEID(colour_type, DEPTH(bdlo), 0/*palette*/,
+            PNG_INTERLACE_ADAM7, w, h, 1), 0/*do_interlace*/,
+            pm->use_update_info);
+
+         if (fail(pm))
+            return 0;
+#     endif
+#     endif /* READ_INTERLACING */
+
+#     ifdef PNG_WRITE_INTERLACING_SUPPORTED
+         /* Test the libpng write side against the pngvalid read side: */
+         standard_test(&pm->this, FILEID(colour_type, DEPTH(bdlo), 0/*palette*/,
+            PNG_INTERLACE_ADAM7, w, h, 0), 1/*do_interlace*/,
+            pm->use_update_info);
+
+         if (fail(pm))
+            return 0;
+#     endif
+
+#     ifdef PNG_READ_INTERLACING_SUPPORTED
+#     ifdef PNG_WRITE_INTERLACING_SUPPORTED
+         /* Test both together: */
+         standard_test(&pm->this, FILEID(colour_type, DEPTH(bdlo), 0/*palette*/,
+            PNG_INTERLACE_ADAM7, w, h, 0), 0/*do_interlace*/,
+            pm->use_update_info);
+
+         if (fail(pm))
+            return 0;
+#     endif
+#     endif /* READ_INTERLACING */
+      }
+   }
+
+   return 1; /* keep going */
+}
+
+static void
+perform_size_test(png_modifier *pm)
+{
+   /* Test each colour type over the valid range of bit depths (expressed as
+    * log2(bit_depth) in turn, stop as soon as any error is detected.
+    */
+   if (!test_size(pm, 0, 0, READ_BDHI))
+      return;
+
+   if (!test_size(pm, 2, 3, READ_BDHI))
+      return;
+
+   /* For the moment don't do the palette test - it's a waste of time when
+    * compared to the grayscale test.
+    */
+#if 0
+   if (!test_size(pm, 3, 0, 3))
+      return;
+#endif
+
+   if (!test_size(pm, 4, 3, READ_BDHI))
+      return;
+
+   if (!test_size(pm, 6, 3, READ_BDHI))
+      return;
+}
+
+
+/******************************* TRANSFORM TESTS ******************************/
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+/* A set of tests to validate libpng image transforms.  The possibilities here
+ * are legion because the transforms can be combined in a combinatorial
+ * fashion.  To deal with this some measure of restraint is required, otherwise
+ * the tests would take forever.
+ */
+typedef struct image_pixel
+{
+   /* A local (pngvalid) representation of a PNG pixel, in all its
+    * various forms.
+    */
+   unsigned int red, green, blue, alpha; /* For non-palette images. */
+   unsigned int palette_index;           /* For a palette image. */
+   png_byte     colour_type;             /* As in the spec. */
+   png_byte     bit_depth;               /* Defines bit size in row */
+   png_byte     sample_depth;            /* Scale of samples */
+   unsigned int have_tRNS :1;            /* tRNS chunk may need processing */
+   unsigned int swap_rgb :1;             /* RGB swapped to BGR */
+   unsigned int alpha_first :1;          /* Alpha at start, not end */
+   unsigned int alpha_inverted :1;       /* Alpha channel inverted */
+   unsigned int mono_inverted :1;        /* Gray channel inverted */
+   unsigned int swap16 :1;               /* Byte swap 16-bit components */
+   unsigned int littleendian :1;         /* High bits on right */
+   unsigned int sig_bits :1;             /* Pixel shifted (sig bits only) */
+
+   /* For checking the code calculates double precision floating point values
+    * along with an error value, accumulated from the transforms.  Because an
+    * sBIT setting allows larger error bounds (indeed, by the spec, apparently
+    * up to just less than +/-1 in the scaled value) the *lowest* sBIT for each
+    * channel is stored.  This sBIT value is folded in to the stored error value
+    * at the end of the application of the transforms to the pixel.
+    *
+    * If sig_bits is set above the red, green, blue and alpha values have been
+    * scaled so they only contain the significant bits of the component values.
+    */
+   double   redf, greenf, bluef, alphaf;
+   double   rede, greene, bluee, alphae;
+   png_byte red_sBIT, green_sBIT, blue_sBIT, alpha_sBIT;
+} image_pixel;
+
+/* Shared utility function, see below. */
+static void
+image_pixel_setf(image_pixel *this, unsigned int rMax, unsigned int gMax,
+        unsigned int bMax, unsigned int aMax)
+{
+   this->redf = this->red / (double)rMax;
+   this->greenf = this->green / (double)gMax;
+   this->bluef = this->blue / (double)bMax;
+   this->alphaf = this->alpha / (double)aMax;
+
+   if (this->red < rMax)
+      this->rede = this->redf * DBL_EPSILON;
+   else
+      this->rede = 0;
+   if (this->green < gMax)
+      this->greene = this->greenf * DBL_EPSILON;
+   else
+      this->greene = 0;
+   if (this->blue < bMax)
+      this->bluee = this->bluef * DBL_EPSILON;
+   else
+      this->bluee = 0;
+   if (this->alpha < aMax)
+      this->alphae = this->alphaf * DBL_EPSILON;
+   else
+      this->alphae = 0;
+}
+
+/* Initialize the structure for the next pixel - call this before doing any
+ * transforms and call it for each pixel since all the fields may need to be
+ * reset.
+ */
+static void
+image_pixel_init(image_pixel *this, png_const_bytep row, png_byte colour_type,
+    png_byte bit_depth, png_uint_32 x, store_palette palette,
+    const image_pixel *format /*from pngvalid transform of input*/)
+{
+   const png_byte sample_depth = (png_byte)(colour_type ==
+      PNG_COLOR_TYPE_PALETTE ? 8 : bit_depth);
+   const unsigned int max = (1U<<sample_depth)-1;
+   const int swap16 = (format != 0 && format->swap16);
+   const int littleendian = (format != 0 && format->littleendian);
+   const int sig_bits = (format != 0 && format->sig_bits);
+
+   /* Initially just set everything to the same number and the alpha to opaque.
+    * Note that this currently assumes a simple palette where entry x has colour
+    * rgb(x,x,x)!
+    */
+   this->palette_index = this->red = this->green = this->blue =
+      sample(row, colour_type, bit_depth, x, 0, swap16, littleendian);
+   this->alpha = max;
+   this->red_sBIT = this->green_sBIT = this->blue_sBIT = this->alpha_sBIT =
+      sample_depth;
+
+   /* Then override as appropriate: */
+   if (colour_type == 3) /* palette */
+   {
+      /* This permits the caller to default to the sample value. */
+      if (palette != 0)
+      {
+         const unsigned int i = this->palette_index;
+
+         this->red = palette[i].red;
+         this->green = palette[i].green;
+         this->blue = palette[i].blue;
+         this->alpha = palette[i].alpha;
+      }
+   }
+
+   else /* not palette */
+   {
+      unsigned int i = 0;
+
+      if ((colour_type & 4) != 0 && format != 0 && format->alpha_first)
+      {
+         this->alpha = this->red;
+         /* This handles the gray case for 'AG' pixels */
+         this->palette_index = this->red = this->green = this->blue =
+            sample(row, colour_type, bit_depth, x, 1, swap16, littleendian);
+         i = 1;
+      }
+
+      if (colour_type & 2)
+      {
+         /* Green is second for both BGR and RGB: */
+         this->green = sample(row, colour_type, bit_depth, x, ++i, swap16,
+                 littleendian);
+
+         if (format != 0 && format->swap_rgb) /* BGR */
+             this->red = sample(row, colour_type, bit_depth, x, ++i, swap16,
+                     littleendian);
+         else
+             this->blue = sample(row, colour_type, bit_depth, x, ++i, swap16,
+                     littleendian);
+      }
+
+      else /* grayscale */ if (format != 0 && format->mono_inverted)
+         this->red = this->green = this->blue = this->red ^ max;
+
+      if ((colour_type & 4) != 0) /* alpha */
+      {
+         if (format == 0 || !format->alpha_first)
+             this->alpha = sample(row, colour_type, bit_depth, x, ++i, swap16,
+                     littleendian);
+
+         if (format != 0 && format->alpha_inverted)
+            this->alpha ^= max;
+      }
+   }
+
+   /* Calculate the scaled values, these are simply the values divided by
+    * 'max' and the error is initialized to the double precision epsilon value
+    * from the header file.
+    */
+   image_pixel_setf(this,
+      sig_bits ? (1U << format->red_sBIT)-1 : max,
+      sig_bits ? (1U << format->green_sBIT)-1 : max,
+      sig_bits ? (1U << format->blue_sBIT)-1 : max,
+      sig_bits ? (1U << format->alpha_sBIT)-1 : max);
+
+   /* Store the input information for use in the transforms - these will
+    * modify the information.
+    */
+   this->colour_type = colour_type;
+   this->bit_depth = bit_depth;
+   this->sample_depth = sample_depth;
+   this->have_tRNS = 0;
+   this->swap_rgb = 0;
+   this->alpha_first = 0;
+   this->alpha_inverted = 0;
+   this->mono_inverted = 0;
+   this->swap16 = 0;
+   this->littleendian = 0;
+   this->sig_bits = 0;
+}
+
+#if defined PNG_READ_EXPAND_SUPPORTED || defined PNG_READ_GRAY_TO_RGB_SUPPORTED\
+   || defined PNG_READ_EXPAND_SUPPORTED || defined PNG_READ_EXPAND_16_SUPPORTED\
+   || defined PNG_READ_BACKGROUND_SUPPORTED
+/* Convert a palette image to an rgb image.  This necessarily converts the tRNS
+ * chunk at the same time, because the tRNS will be in palette form.  The way
+ * palette validation works means that the original palette is never updated,
+ * instead the image_pixel value from the row contains the RGB of the
+ * corresponding palette entry and *this* is updated.  Consequently this routine
+ * only needs to change the colour type information.
+ */
+static void
+image_pixel_convert_PLTE(image_pixel *this)
+{
+   if (this->colour_type == PNG_COLOR_TYPE_PALETTE)
+   {
+      if (this->have_tRNS)
+      {
+         this->colour_type = PNG_COLOR_TYPE_RGB_ALPHA;
+         this->have_tRNS = 0;
+      }
+      else
+         this->colour_type = PNG_COLOR_TYPE_RGB;
+
+      /* The bit depth of the row changes at this point too (notice that this is
+       * the row format, not the sample depth, which is separate.)
+       */
+      this->bit_depth = 8;
+   }
+}
+
+/* Add an alpha channel; this will import the tRNS information because tRNS is
+ * not valid in an alpha image.  The bit depth will invariably be set to at
+ * least 8 prior to 1.7.0.  Palette images will be converted to alpha (using
+ * the above API).  With png_set_background the alpha channel is never expanded
+ * but this routine is used by pngvalid to simplify code; 'for_background'
+ * records this.
+ */
+static void
+image_pixel_add_alpha(image_pixel *this, const standard_display *display,
+   int for_background)
+{
+   if (this->colour_type == PNG_COLOR_TYPE_PALETTE)
+      image_pixel_convert_PLTE(this);
+
+   if ((this->colour_type & PNG_COLOR_MASK_ALPHA) == 0)
+   {
+      if (this->colour_type == PNG_COLOR_TYPE_GRAY)
+      {
+#        if PNG_LIBPNG_VER < 10700
+            if (!for_background && this->bit_depth < 8)
+               this->bit_depth = this->sample_depth = 8;
+#        endif
+
+         if (this->have_tRNS)
+         {
+            /* After 1.7 the expansion of bit depth only happens if there is a
+             * tRNS chunk to expand at this point.
+             */
+#           if PNG_LIBPNG_VER >= 10700
+               if (!for_background && this->bit_depth < 8)
+                  this->bit_depth = this->sample_depth = 8;
+#           endif
+
+            this->have_tRNS = 0;
+
+            /* Check the input, original, channel value here against the
+             * original tRNS gray chunk valie.
+             */
+            if (this->red == display->transparent.red)
+               this->alphaf = 0;
+            else
+               this->alphaf = 1;
+         }
+         else
+            this->alphaf = 1;
+
+         this->colour_type = PNG_COLOR_TYPE_GRAY_ALPHA;
+      }
+
+      else if (this->colour_type == PNG_COLOR_TYPE_RGB)
+      {
+         if (this->have_tRNS)
+         {
+            this->have_tRNS = 0;
+
+            /* Again, check the exact input values, not the current transformed
+             * value!
+             */
+            if (this->red == display->transparent.red &&
+               this->green == display->transparent.green &&
+               this->blue == display->transparent.blue)
+               this->alphaf = 0;
+            else
+               this->alphaf = 1;
+         }
+         else
+            this->alphaf = 1;
+
+         this->colour_type = PNG_COLOR_TYPE_RGB_ALPHA;
+      }
+
+      /* The error in the alpha is zero and the sBIT value comes from the
+       * original sBIT data (actually it will always be the original bit depth).
+       */
+      this->alphae = 0;
+      this->alpha_sBIT = display->alpha_sBIT;
+   }
+}
+#endif /* transforms that need image_pixel_add_alpha */
+
+struct transform_display;
+typedef struct image_transform
+{
+   /* The name of this transform: a string. */
+   const char *name;
+
+   /* Each transform can be disabled from the command line: */
+   int enable;
+
+   /* The global list of transforms; read only. */
+   struct image_transform *const list;
+
+   /* The global count of the number of times this transform has been set on an
+    * image.
+    */
+   unsigned int global_use;
+
+   /* The local count of the number of times this transform has been set. */
+   unsigned int local_use;
+
+   /* The next transform in the list, each transform must call its own next
+    * transform after it has processed the pixel successfully.
+    */
+   const struct image_transform *next;
+
+   /* A single transform for the image, expressed as a series of function
+    * callbacks and some space for values.
+    *
+    * First a callback to add any required modifications to the png_modifier;
+    * this gets called just before the modifier is set up for read.
+    */
+   void (*ini)(const struct image_transform *this,
+      struct transform_display *that);
+
+   /* And a callback to set the transform on the current png_read_struct:
+    */
+   void (*set)(const struct image_transform *this,
+      struct transform_display *that, png_structp pp, png_infop pi);
+
+   /* Then a transform that takes an input pixel in one PNG format or another
+    * and modifies it by a pngvalid implementation of the transform (thus
+    * duplicating the libpng intent without, we hope, duplicating the bugs
+    * in the libpng implementation!)  The png_structp is solely to allow error
+    * reporting via png_error and png_warning.
+    */
+   void (*mod)(const struct image_transform *this, image_pixel *that,
+      png_const_structp pp, const struct transform_display *display);
+
+   /* Add this transform to the list and return true if the transform is
+    * meaningful for this colour type and bit depth - if false then the
+    * transform should have no effect on the image so there's not a lot of
+    * point running it.
+    */
+   int (*add)(struct image_transform *this,
+      const struct image_transform **that, png_byte colour_type,
+      png_byte bit_depth);
+} image_transform;
+
+typedef struct transform_display
+{
+   standard_display this;
+
+   /* Parameters */
+   png_modifier*              pm;
+   const image_transform* transform_list;
+   unsigned int max_gamma_8;
+
+   /* Local variables */
+   png_byte output_colour_type;
+   png_byte output_bit_depth;
+   png_byte unpacked;
+
+   /* Modifications (not necessarily used.) */
+   gama_modification gama_mod;
+   chrm_modification chrm_mod;
+   srgb_modification srgb_mod;
+} transform_display;
+
+/* Set sRGB, cHRM and gAMA transforms as required by the current encoding. */
+static void
+transform_set_encoding(transform_display *this)
+{
+   /* Set up the png_modifier '_current' fields then use these to determine how
+    * to add appropriate chunks.
+    */
+   png_modifier *pm = this->pm;
+
+   modifier_set_encoding(pm);
+
+   if (modifier_color_encoding_is_set(pm))
+   {
+      if (modifier_color_encoding_is_sRGB(pm))
+         srgb_modification_init(&this->srgb_mod, pm, PNG_sRGB_INTENT_ABSOLUTE);
+
+      else
+      {
+         /* Set gAMA and cHRM separately. */
+         gama_modification_init(&this->gama_mod, pm, pm->current_gamma);
+
+         if (pm->current_encoding != 0)
+            chrm_modification_init(&this->chrm_mod, pm, pm->current_encoding);
+      }
+   }
+}
+
+/* Three functions to end the list: */
+static void
+image_transform_ini_end(const image_transform *this,
+   transform_display *that)
+{
+   UNUSED(this)
+   UNUSED(that)
+}
+
+static void
+image_transform_set_end(const image_transform *this,
+   transform_display *that, png_structp pp, png_infop pi)
+{
+   UNUSED(this)
+   UNUSED(that)
+   UNUSED(pp)
+   UNUSED(pi)
+}
+
+/* At the end of the list recalculate the output image pixel value from the
+ * double precision values set up by the preceding 'mod' calls:
+ */
+static unsigned int
+sample_scale(double sample_value, unsigned int scale)
+{
+   sample_value = floor(sample_value * scale + .5);
+
+   /* Return NaN as 0: */
+   if (!(sample_value > 0))
+      sample_value = 0;
+   else if (sample_value > scale)
+      sample_value = scale;
+
+   return (unsigned int)sample_value;
+}
+
+static void
+image_transform_mod_end(const image_transform *this, image_pixel *that,
+    png_const_structp pp, const transform_display *display)
+{
+   const unsigned int scale = (1U<<that->sample_depth)-1;
+   const int sig_bits = that->sig_bits;
+
+   UNUSED(this)
+   UNUSED(pp)
+   UNUSED(display)
+
+   /* At the end recalculate the digitized red green and blue values according
+    * to the current sample_depth of the pixel.
+    *
+    * The sample value is simply scaled to the maximum, checking for over
+    * and underflow (which can both happen for some image transforms,
+    * including simple size scaling, though libpng doesn't do that at present.
+    */
+   that->red = sample_scale(that->redf, scale);
+
+   /* This is a bit bogus; really the above calculation should use the red_sBIT
+    * value, not sample_depth, but because libpng does png_set_shift by just
+    * shifting the bits we get errors if we don't do it the same way.
+    */
+   if (sig_bits && that->red_sBIT < that->sample_depth)
+      that->red >>= that->sample_depth - that->red_sBIT;
+
+   /* The error value is increased, at the end, according to the lowest sBIT
+    * value seen.  Common sense tells us that the intermediate integer
+    * representations are no more accurate than +/- 0.5 in the integral values,
+    * the sBIT allows the implementation to be worse than this.  In addition the
+    * PNG specification actually permits any error within the range (-1..+1),
+    * but that is ignored here.  Instead the final digitized value is compared,
+    * below to the digitized value of the error limits - this has the net effect
+    * of allowing (almost) +/-1 in the output value.  It's difficult to see how
+    * any algorithm that digitizes intermediate results can be more accurate.
+    */
+   that->rede += 1./(2*((1U<<that->red_sBIT)-1));
+
+   if (that->colour_type & PNG_COLOR_MASK_COLOR)
+   {
+      that->green = sample_scale(that->greenf, scale);
+      if (sig_bits && that->green_sBIT < that->sample_depth)
+         that->green >>= that->sample_depth - that->green_sBIT;
+
+      that->blue = sample_scale(that->bluef, scale);
+      if (sig_bits && that->blue_sBIT < that->sample_depth)
+         that->blue >>= that->sample_depth - that->blue_sBIT;
+
+      that->greene += 1./(2*((1U<<that->green_sBIT)-1));
+      that->bluee += 1./(2*((1U<<that->blue_sBIT)-1));
+   }
+   else
+   {
+      that->blue = that->green = that->red;
+      that->bluef = that->greenf = that->redf;
+      that->bluee = that->greene = that->rede;
+   }
+
+   if ((that->colour_type & PNG_COLOR_MASK_ALPHA) ||
+      that->colour_type == PNG_COLOR_TYPE_PALETTE)
+   {
+      that->alpha = sample_scale(that->alphaf, scale);
+      that->alphae += 1./(2*((1U<<that->alpha_sBIT)-1));
+   }
+   else
+   {
+      that->alpha = scale; /* opaque */
+      that->alphaf = 1;    /* Override this. */
+      that->alphae = 0;    /* It's exact ;-) */
+   }
+
+   if (sig_bits && that->alpha_sBIT < that->sample_depth)
+      that->alpha >>= that->sample_depth - that->alpha_sBIT;
+}
+
+/* Static 'end' structure: */
+static image_transform image_transform_end =
+{
+   "(end)", /* name */
+   1, /* enable */
+   0, /* list */
+   0, /* global_use */
+   0, /* local_use */
+   0, /* next */
+   image_transform_ini_end,
+   image_transform_set_end,
+   image_transform_mod_end,
+   0 /* never called, I want it to crash if it is! */
+};
+
+/* Reader callbacks and implementations, where they differ from the standard
+ * ones.
+ */
+static void
+transform_display_init(transform_display *dp, png_modifier *pm, png_uint_32 id,
+    const image_transform *transform_list)
+{
+   memset(dp, 0, sizeof *dp);
+
+   /* Standard fields */
+   standard_display_init(&dp->this, &pm->this, id, do_read_interlace,
+      pm->use_update_info);
+
+   /* Parameter fields */
+   dp->pm = pm;
+   dp->transform_list = transform_list;
+   dp->max_gamma_8 = 16;
+
+   /* Local variable fields */
+   dp->output_colour_type = 255; /* invalid */
+   dp->output_bit_depth = 255;  /* invalid */
+   dp->unpacked = 0; /* not unpacked */
+}
+
+static void
+transform_info_imp(transform_display *dp, png_structp pp, png_infop pi)
+{
+   /* Reuse the standard stuff as appropriate. */
+   standard_info_part1(&dp->this, pp, pi);
+
+   /* Now set the list of transforms. */
+   dp->transform_list->set(dp->transform_list, dp, pp, pi);
+
+   /* Update the info structure for these transforms: */
+   {
+      int i = dp->this.use_update_info;
+      /* Always do one call, even if use_update_info is 0. */
+      do
+         png_read_update_info(pp, pi);
+      while (--i > 0);
+   }
+
+   /* And get the output information into the standard_display */
+   standard_info_part2(&dp->this, pp, pi, 1/*images*/);
+
+   /* Plus the extra stuff we need for the transform tests: */
+   dp->output_colour_type = png_get_color_type(pp, pi);
+   dp->output_bit_depth = png_get_bit_depth(pp, pi);
+
+   /* If png_set_filler is in action then fake the output color type to include
+    * an alpha channel where appropriate.
+    */
+   if (dp->output_bit_depth >= 8 &&
+       (dp->output_colour_type == PNG_COLOR_TYPE_RGB ||
+        dp->output_colour_type == PNG_COLOR_TYPE_GRAY) && dp->this.filler)
+       dp->output_colour_type |= 4;
+
+   /* Validate the combination of colour type and bit depth that we are getting
+    * out of libpng; the semantics of something not in the PNG spec are, at
+    * best, unclear.
+    */
+   switch (dp->output_colour_type)
+   {
+   case PNG_COLOR_TYPE_PALETTE:
+      if (dp->output_bit_depth > 8) goto error;
+      /*FALL THROUGH*/
+   case PNG_COLOR_TYPE_GRAY:
+      if (dp->output_bit_depth == 1 || dp->output_bit_depth == 2 ||
+         dp->output_bit_depth == 4)
+         break;
+      /*FALL THROUGH*/
+   default:
+      if (dp->output_bit_depth == 8 || dp->output_bit_depth == 16)
+         break;
+      /*FALL THROUGH*/
+   error:
+      {
+         char message[128];
+         size_t pos;
+
+         pos = safecat(message, sizeof message, 0,
+            "invalid final bit depth: colour type(");
+         pos = safecatn(message, sizeof message, pos, dp->output_colour_type);
+         pos = safecat(message, sizeof message, pos, ") with bit depth: ");
+         pos = safecatn(message, sizeof message, pos, dp->output_bit_depth);
+
+         png_error(pp, message);
+      }
+   }
+
+   /* Use a test pixel to check that the output agrees with what we expect -
+    * this avoids running the whole test if the output is unexpected.  This also
+    * checks for internal errors.
+    */
+   {
+      image_pixel test_pixel;
+
+      memset(&test_pixel, 0, sizeof test_pixel);
+      test_pixel.colour_type = dp->this.colour_type; /* input */
+      test_pixel.bit_depth = dp->this.bit_depth;
+      if (test_pixel.colour_type == PNG_COLOR_TYPE_PALETTE)
+         test_pixel.sample_depth = 8;
+      else
+         test_pixel.sample_depth = test_pixel.bit_depth;
+      /* Don't need sBIT here, but it must be set to non-zero to avoid
+       * arithmetic overflows.
+       */
+      test_pixel.have_tRNS = dp->this.is_transparent != 0;
+      test_pixel.red_sBIT = test_pixel.green_sBIT = test_pixel.blue_sBIT =
+         test_pixel.alpha_sBIT = test_pixel.sample_depth;
+
+      dp->transform_list->mod(dp->transform_list, &test_pixel, pp, dp);
+
+      if (test_pixel.colour_type != dp->output_colour_type)
+      {
+         char message[128];
+         size_t pos = safecat(message, sizeof message, 0, "colour type ");
+
+         pos = safecatn(message, sizeof message, pos, dp->output_colour_type);
+         pos = safecat(message, sizeof message, pos, " expected ");
+         pos = safecatn(message, sizeof message, pos, test_pixel.colour_type);
+
+         png_error(pp, message);
+      }
+
+      if (test_pixel.bit_depth != dp->output_bit_depth)
+      {
+         char message[128];
+         size_t pos = safecat(message, sizeof message, 0, "bit depth ");
+
+         pos = safecatn(message, sizeof message, pos, dp->output_bit_depth);
+         pos = safecat(message, sizeof message, pos, " expected ");
+         pos = safecatn(message, sizeof message, pos, test_pixel.bit_depth);
+
+         png_error(pp, message);
+      }
+
+      /* If both bit depth and colour type are correct check the sample depth.
+       */
+      if (test_pixel.colour_type == PNG_COLOR_TYPE_PALETTE &&
+          test_pixel.sample_depth != 8) /* oops - internal error! */
+         png_error(pp, "pngvalid: internal: palette sample depth not 8");
+      else if (dp->unpacked && test_pixel.bit_depth != 8)
+         png_error(pp, "pngvalid: internal: bad unpacked pixel depth");
+      else if (!dp->unpacked && test_pixel.colour_type != PNG_COLOR_TYPE_PALETTE
+              && test_pixel.bit_depth != test_pixel.sample_depth)
+      {
+         char message[128];
+         size_t pos = safecat(message, sizeof message, 0,
+            "internal: sample depth ");
+
+         /* Because unless something has set 'unpacked' or the image is palette
+          * mapped we expect the transform to keep sample depth and bit depth
+          * the same.
+          */
+         pos = safecatn(message, sizeof message, pos, test_pixel.sample_depth);
+         pos = safecat(message, sizeof message, pos, " expected ");
+         pos = safecatn(message, sizeof message, pos, test_pixel.bit_depth);
+
+         png_error(pp, message);
+      }
+      else if (test_pixel.bit_depth != dp->output_bit_depth)
+      {
+         /* This could be a libpng error too; libpng has not produced what we
+          * expect for the output bit depth.
+          */
+         char message[128];
+         size_t pos = safecat(message, sizeof message, 0,
+            "internal: bit depth ");
+
+         pos = safecatn(message, sizeof message, pos, dp->output_bit_depth);
+         pos = safecat(message, sizeof message, pos, " expected ");
+         pos = safecatn(message, sizeof message, pos, test_pixel.bit_depth);
+
+         png_error(pp, message);
+      }
+   }
+}
+
+static void PNGCBAPI
+transform_info(png_structp pp, png_infop pi)
+{
+   transform_info_imp(voidcast(transform_display*, png_get_progressive_ptr(pp)),
+      pp, pi);
+}
+
+static void
+transform_range_check(png_const_structp pp, unsigned int r, unsigned int g,
+   unsigned int b, unsigned int a, unsigned int in_digitized, double in,
+   unsigned int out, png_byte sample_depth, double err, double limit,
+   const char *name, double digitization_error)
+{
+   /* Compare the scaled, digitzed, values of our local calculation (in+-err)
+    * with the digitized values libpng produced;  'sample_depth' is the actual
+    * digitization depth of the libpng output colors (the bit depth except for
+    * palette images where it is always 8.)  The check on 'err' is to detect
+    * internal errors in pngvalid itself.
+    */
+   unsigned int max = (1U<<sample_depth)-1;
+   double in_min = ceil((in-err)*max - digitization_error);
+   double in_max = floor((in+err)*max + digitization_error);
+   if (debugonly(err > limit ||) !(out >= in_min && out <= in_max))
+   {
+      char message[256];
+      size_t pos;
+
+      pos = safecat(message, sizeof message, 0, name);
+      pos = safecat(message, sizeof message, pos, " output value error: rgba(");
+      pos = safecatn(message, sizeof message, pos, r);
+      pos = safecat(message, sizeof message, pos, ",");
+      pos = safecatn(message, sizeof message, pos, g);
+      pos = safecat(message, sizeof message, pos, ",");
+      pos = safecatn(message, sizeof message, pos, b);
+      pos = safecat(message, sizeof message, pos, ",");
+      pos = safecatn(message, sizeof message, pos, a);
+      pos = safecat(message, sizeof message, pos, "): ");
+      pos = safecatn(message, sizeof message, pos, out);
+      pos = safecat(message, sizeof message, pos, " expected: ");
+      pos = safecatn(message, sizeof message, pos, in_digitized);
+      pos = safecat(message, sizeof message, pos, " (");
+      pos = safecatd(message, sizeof message, pos, (in-err)*max, 3);
+      pos = safecat(message, sizeof message, pos, "..");
+      pos = safecatd(message, sizeof message, pos, (in+err)*max, 3);
+      pos = safecat(message, sizeof message, pos, ")");
+
+      png_error(pp, message);
+   }
+
+   UNUSED(limit)
+}
+
+static void
+transform_image_validate(transform_display *dp, png_const_structp pp,
+   png_infop pi)
+{
+   /* Constants for the loop below: */
+   const png_store* const ps = dp->this.ps;
+   const png_byte in_ct = dp->this.colour_type;
+   const png_byte in_bd = dp->this.bit_depth;
+   const png_uint_32 w = dp->this.w;
+   const png_uint_32 h = dp->this.h;
+   const png_byte out_ct = dp->output_colour_type;
+   const png_byte out_bd = dp->output_bit_depth;
+   const png_byte sample_depth = (png_byte)(out_ct ==
+      PNG_COLOR_TYPE_PALETTE ? 8 : out_bd);
+   const png_byte red_sBIT = dp->this.red_sBIT;
+   const png_byte green_sBIT = dp->this.green_sBIT;
+   const png_byte blue_sBIT = dp->this.blue_sBIT;
+   const png_byte alpha_sBIT = dp->this.alpha_sBIT;
+   const int have_tRNS = dp->this.is_transparent;
+   double digitization_error;
+
+   store_palette out_palette;
+   png_uint_32 y;
+
+   UNUSED(pi)
+
+   /* Check for row overwrite errors */
+   store_image_check(dp->this.ps, pp, 0);
+
+   /* Read the palette corresponding to the output if the output colour type
+    * indicates a palette, othewise set out_palette to garbage.
+    */
+   if (out_ct == PNG_COLOR_TYPE_PALETTE)
+   {
+      /* Validate that the palette count itself has not changed - this is not
+       * expected.
+       */
+      int npalette = (-1);
+
+      (void)read_palette(out_palette, &npalette, pp, pi);
+      if (npalette != dp->this.npalette)
+         png_error(pp, "unexpected change in palette size");
+
+      digitization_error = .5;
+   }
+   else
+   {
+      png_byte in_sample_depth;
+
+      memset(out_palette, 0x5e, sizeof out_palette);
+
+      /* use-input-precision means assume that if the input has 8 bit (or less)
+       * samples and the output has 16 bit samples the calculations will be done
+       * with 8 bit precision, not 16.
+       */
+      if (in_ct == PNG_COLOR_TYPE_PALETTE || in_bd < 16)
+         in_sample_depth = 8;
+      else
+         in_sample_depth = in_bd;
+
+      if (sample_depth != 16 || in_sample_depth > 8 ||
+         !dp->pm->calculations_use_input_precision)
+         digitization_error = .5;
+
+      /* Else calculations are at 8 bit precision, and the output actually
+       * consists of scaled 8-bit values, so scale .5 in 8 bits to the 16 bits:
+       */
+      else
+         digitization_error = .5 * 257;
+   }
+
+   for (y=0; y<h; ++y)
+   {
+      png_const_bytep const pRow = store_image_row(ps, pp, 0, y);
+      png_uint_32 x;
+
+      /* The original, standard, row pre-transforms. */
+      png_byte std[STANDARD_ROWMAX];
+
+      transform_row(pp, std, in_ct, in_bd, y);
+
+      /* Go through each original pixel transforming it and comparing with what
+       * libpng did to the same pixel.
+       */
+      for (x=0; x<w; ++x)
+      {
+         image_pixel in_pixel, out_pixel;
+         unsigned int r, g, b, a;
+
+         /* Find out what we think the pixel should be: */
+         image_pixel_init(&in_pixel, std, in_ct, in_bd, x, dp->this.palette,
+                 NULL);
+
+         in_pixel.red_sBIT = red_sBIT;
+         in_pixel.green_sBIT = green_sBIT;
+         in_pixel.blue_sBIT = blue_sBIT;
+         in_pixel.alpha_sBIT = alpha_sBIT;
+         in_pixel.have_tRNS = have_tRNS != 0;
+
+         /* For error detection, below. */
+         r = in_pixel.red;
+         g = in_pixel.green;
+         b = in_pixel.blue;
+         a = in_pixel.alpha;
+
+         /* This applies the transforms to the input data, including output
+          * format operations which must be used when reading the output
+          * pixel that libpng produces.
+          */
+         dp->transform_list->mod(dp->transform_list, &in_pixel, pp, dp);
+
+         /* Read the output pixel and compare it to what we got, we don't
+          * use the error field here, so no need to update sBIT.  in_pixel
+          * says whether we expect libpng to change the output format.
+          */
+         image_pixel_init(&out_pixel, pRow, out_ct, out_bd, x, out_palette,
+                 &in_pixel);
+
+         /* We don't expect changes to the index here even if the bit depth is
+          * changed.
+          */
+         if (in_ct == PNG_COLOR_TYPE_PALETTE &&
+            out_ct == PNG_COLOR_TYPE_PALETTE)
+         {
+            if (in_pixel.palette_index != out_pixel.palette_index)
+               png_error(pp, "unexpected transformed palette index");
+         }
+
+         /* Check the colours for palette images too - in fact the palette could
+          * be separately verified itself in most cases.
+          */
+         if (in_pixel.red != out_pixel.red)
+            transform_range_check(pp, r, g, b, a, in_pixel.red, in_pixel.redf,
+               out_pixel.red, sample_depth, in_pixel.rede,
+               dp->pm->limit + 1./(2*((1U<<in_pixel.red_sBIT)-1)), "red/gray",
+               digitization_error);
+
+         if ((out_ct & PNG_COLOR_MASK_COLOR) != 0 &&
+            in_pixel.green != out_pixel.green)
+            transform_range_check(pp, r, g, b, a, in_pixel.green,
+               in_pixel.greenf, out_pixel.green, sample_depth, in_pixel.greene,
+               dp->pm->limit + 1./(2*((1U<<in_pixel.green_sBIT)-1)), "green",
+               digitization_error);
+
+         if ((out_ct & PNG_COLOR_MASK_COLOR) != 0 &&
+            in_pixel.blue != out_pixel.blue)
+            transform_range_check(pp, r, g, b, a, in_pixel.blue, in_pixel.bluef,
+               out_pixel.blue, sample_depth, in_pixel.bluee,
+               dp->pm->limit + 1./(2*((1U<<in_pixel.blue_sBIT)-1)), "blue",
+               digitization_error);
+
+         if ((out_ct & PNG_COLOR_MASK_ALPHA) != 0 &&
+            in_pixel.alpha != out_pixel.alpha)
+            transform_range_check(pp, r, g, b, a, in_pixel.alpha,
+               in_pixel.alphaf, out_pixel.alpha, sample_depth, in_pixel.alphae,
+               dp->pm->limit + 1./(2*((1U<<in_pixel.alpha_sBIT)-1)), "alpha",
+               digitization_error);
+      } /* pixel (x) loop */
+   } /* row (y) loop */
+
+   /* Record that something was actually checked to avoid a false positive. */
+   dp->this.ps->validated = 1;
+}
+
+static void PNGCBAPI
+transform_end(png_structp ppIn, png_infop pi)
+{
+   png_const_structp pp = ppIn;
+   transform_display *dp = voidcast(transform_display*,
+      png_get_progressive_ptr(pp));
+
+   if (!dp->this.speed)
+      transform_image_validate(dp, pp, pi);
+   else
+      dp->this.ps->validated = 1;
+}
+
+/* A single test run. */
+static void
+transform_test(png_modifier *pmIn, const png_uint_32 idIn,
+    const image_transform* transform_listIn, const char * const name)
+{
+   transform_display d;
+   context(&pmIn->this, fault);
+
+   transform_display_init(&d, pmIn, idIn, transform_listIn);
+
+   Try
+   {
+      size_t pos = 0;
+      png_structp pp;
+      png_infop pi;
+      char full_name[256];
+
+      /* Make sure the encoding fields are correct and enter the required
+       * modifications.
+       */
+      transform_set_encoding(&d);
+
+      /* Add any modifications required by the transform list. */
+      d.transform_list->ini(d.transform_list, &d);
+
+      /* Add the color space information, if any, to the name. */
+      pos = safecat(full_name, sizeof full_name, pos, name);
+      pos = safecat_current_encoding(full_name, sizeof full_name, pos, d.pm);
+
+      /* Get a png_struct for reading the image. */
+      pp = set_modifier_for_read(d.pm, &pi, d.this.id, full_name);
+      standard_palette_init(&d.this);
+
+#     if 0
+         /* Logging (debugging only) */
+         {
+            char buffer[256];
+
+            (void)store_message(&d.pm->this, pp, buffer, sizeof buffer, 0,
+               "running test");
+
+            fprintf(stderr, "%s\n", buffer);
+         }
+#     endif
+
+      /* Introduce the correct read function. */
+      if (d.pm->this.progressive)
+      {
+         /* Share the row function with the standard implementation. */
+         png_set_progressive_read_fn(pp, &d, transform_info, progressive_row,
+            transform_end);
+
+         /* Now feed data into the reader until we reach the end: */
+         modifier_progressive_read(d.pm, pp, pi);
+      }
+      else
+      {
+         /* modifier_read expects a png_modifier* */
+         png_set_read_fn(pp, d.pm, modifier_read);
+
+         /* Check the header values: */
+         png_read_info(pp, pi);
+
+         /* Process the 'info' requirements. Only one image is generated */
+         transform_info_imp(&d, pp, pi);
+
+         sequential_row(&d.this, pp, pi, -1, 0);
+
+         if (!d.this.speed)
+            transform_image_validate(&d, pp, pi);
+         else
+            d.this.ps->validated = 1;
+      }
+
+      modifier_reset(d.pm);
+   }
+
+   Catch(fault)
+   {
+      modifier_reset(voidcast(png_modifier*,(void*)fault));
+   }
+}
+
+/* The transforms: */
+#define ITSTRUCT(name) image_transform_##name
+#define ITDATA(name) image_transform_data_##name
+#define image_transform_ini image_transform_default_ini
+#define IT(name)\
+static image_transform ITSTRUCT(name) =\
+{\
+   #name,\
+   1, /*enable*/\
+   &PT, /*list*/\
+   0, /*global_use*/\
+   0, /*local_use*/\
+   0, /*next*/\
+   image_transform_ini,\
+   image_transform_png_set_##name##_set,\
+   image_transform_png_set_##name##_mod,\
+   image_transform_png_set_##name##_add\
+}
+#define PT ITSTRUCT(end) /* stores the previous transform */
+
+/* To save code: */
+extern void image_transform_default_ini(const image_transform *this,
+   transform_display *that); /* silence GCC warnings */
+
+void /* private, but almost always needed */
+image_transform_default_ini(const image_transform *this,
+    transform_display *that)
+{
+   this->next->ini(this->next, that);
+}
+
+#ifdef PNG_READ_BACKGROUND_SUPPORTED
+static int
+image_transform_default_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(colour_type)
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   return 1;
+}
+#endif
+
+#ifdef PNG_READ_EXPAND_SUPPORTED
+/* png_set_palette_to_rgb */
+static void
+image_transform_png_set_palette_to_rgb_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_palette_to_rgb(pp);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_palette_to_rgb_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->colour_type == PNG_COLOR_TYPE_PALETTE)
+      image_pixel_convert_PLTE(that);
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_palette_to_rgb_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   return colour_type == PNG_COLOR_TYPE_PALETTE;
+}
+
+IT(palette_to_rgb);
+#undef PT
+#define PT ITSTRUCT(palette_to_rgb)
+#endif /* PNG_READ_EXPAND_SUPPORTED */
+
+#ifdef PNG_READ_EXPAND_SUPPORTED
+/* png_set_tRNS_to_alpha */
+static void
+image_transform_png_set_tRNS_to_alpha_set(const image_transform *this,
+   transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_tRNS_to_alpha(pp);
+
+   /* If there was a tRNS chunk that would get expanded and add an alpha
+    * channel is_transparent must be updated:
+    */
+   if (that->this.has_tRNS)
+      that->this.is_transparent = 1;
+
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_tRNS_to_alpha_mod(const image_transform *this,
+   image_pixel *that, png_const_structp pp,
+   const transform_display *display)
+{
+#if PNG_LIBPNG_VER < 10700
+   /* LIBPNG BUG: this always forces palette images to RGB. */
+   if (that->colour_type == PNG_COLOR_TYPE_PALETTE)
+      image_pixel_convert_PLTE(that);
+#endif
+
+   /* This effectively does an 'expand' only if there is some transparency to
+    * convert to an alpha channel.
+    */
+   if (that->have_tRNS)
+#     if PNG_LIBPNG_VER >= 10700
+         if (that->colour_type != PNG_COLOR_TYPE_PALETTE &&
+             (that->colour_type & PNG_COLOR_MASK_ALPHA) == 0)
+#     endif
+      image_pixel_add_alpha(that, &display->this, 0/*!for background*/);
+
+#if PNG_LIBPNG_VER < 10700
+   /* LIBPNG BUG: otherwise libpng still expands to 8 bits! */
+   else
+   {
+      if (that->bit_depth < 8)
+         that->bit_depth =8;
+      if (that->sample_depth < 8)
+         that->sample_depth = 8;
+   }
+#endif
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_tRNS_to_alpha_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   /* We don't know yet whether there will be a tRNS chunk, but we know that
+    * this transformation should do nothing if there already is an alpha
+    * channel.  In addition, after the bug fix in 1.7.0, there is no longer
+    * any action on a palette image.
+    */
+   return
+#  if PNG_LIBPNG_VER >= 10700
+      colour_type != PNG_COLOR_TYPE_PALETTE &&
+#  endif
+   (colour_type & PNG_COLOR_MASK_ALPHA) == 0;
+}
+
+IT(tRNS_to_alpha);
+#undef PT
+#define PT ITSTRUCT(tRNS_to_alpha)
+#endif /* PNG_READ_EXPAND_SUPPORTED */
+
+#ifdef PNG_READ_GRAY_TO_RGB_SUPPORTED
+/* png_set_gray_to_rgb */
+static void
+image_transform_png_set_gray_to_rgb_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_gray_to_rgb(pp);
+   /* NOTE: this doesn't result in tRNS expansion. */
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_gray_to_rgb_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   /* NOTE: we can actually pend the tRNS processing at this point because we
+    * can correctly recognize the original pixel value even though we have
+    * mapped the one gray channel to the three RGB ones, but in fact libpng
+    * doesn't do this, so we don't either.
+    */
+   if ((that->colour_type & PNG_COLOR_MASK_COLOR) == 0 && that->have_tRNS)
+      image_pixel_add_alpha(that, &display->this, 0/*!for background*/);
+
+   /* Simply expand the bit depth and alter the colour type as required. */
+   if (that->colour_type == PNG_COLOR_TYPE_GRAY)
+   {
+      /* RGB images have a bit depth at least equal to '8' */
+      if (that->bit_depth < 8)
+         that->sample_depth = that->bit_depth = 8;
+
+      /* And just changing the colour type works here because the green and blue
+       * channels are being maintained in lock-step with the red/gray:
+       */
+      that->colour_type = PNG_COLOR_TYPE_RGB;
+   }
+
+   else if (that->colour_type == PNG_COLOR_TYPE_GRAY_ALPHA)
+      that->colour_type = PNG_COLOR_TYPE_RGB_ALPHA;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_gray_to_rgb_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   return (colour_type & PNG_COLOR_MASK_COLOR) == 0;
+}
+
+IT(gray_to_rgb);
+#undef PT
+#define PT ITSTRUCT(gray_to_rgb)
+#endif /* PNG_READ_GRAY_TO_RGB_SUPPORTED */
+
+#ifdef PNG_READ_EXPAND_SUPPORTED
+/* png_set_expand */
+static void
+image_transform_png_set_expand_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_expand(pp);
+
+   if (that->this.has_tRNS)
+      that->this.is_transparent = 1;
+
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_expand_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   /* The general expand case depends on what the colour type is: */
+   if (that->colour_type == PNG_COLOR_TYPE_PALETTE)
+      image_pixel_convert_PLTE(that);
+   else if (that->bit_depth < 8) /* grayscale */
+      that->sample_depth = that->bit_depth = 8;
+
+   if (that->have_tRNS)
+      image_pixel_add_alpha(that, &display->this, 0/*!for background*/);
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_expand_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   /* 'expand' should do nothing for RGBA or GA input - no tRNS and the bit
+    * depth is at least 8 already.
+    */
+   return (colour_type & PNG_COLOR_MASK_ALPHA) == 0;
+}
+
+IT(expand);
+#undef PT
+#define PT ITSTRUCT(expand)
+#endif /* PNG_READ_EXPAND_SUPPORTED */
+
+#ifdef PNG_READ_EXPAND_SUPPORTED
+/* png_set_expand_gray_1_2_4_to_8
+ * Pre 1.7.0 LIBPNG BUG: this just does an 'expand'
+ */
+static void
+image_transform_png_set_expand_gray_1_2_4_to_8_set(
+    const image_transform *this, transform_display *that, png_structp pp,
+    png_infop pi)
+{
+   png_set_expand_gray_1_2_4_to_8(pp);
+   /* NOTE: don't expect this to expand tRNS */
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_expand_gray_1_2_4_to_8_mod(
+    const image_transform *this, image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+#if PNG_LIBPNG_VER < 10700
+   image_transform_png_set_expand_mod(this, that, pp, display);
+#else
+   /* Only expand grayscale of bit depth less than 8: */
+   if (that->colour_type == PNG_COLOR_TYPE_GRAY &&
+       that->bit_depth < 8)
+      that->sample_depth = that->bit_depth = 8;
+
+   this->next->mod(this->next, that, pp, display);
+#endif /* 1.7 or later */
+}
+
+static int
+image_transform_png_set_expand_gray_1_2_4_to_8_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+#if PNG_LIBPNG_VER < 10700
+   return image_transform_png_set_expand_add(this, that, colour_type,
+      bit_depth);
+#else
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   /* This should do nothing unless the color type is gray and the bit depth is
+    * less than 8:
+    */
+   return colour_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8;
+#endif /* 1.7 or later */
+}
+
+IT(expand_gray_1_2_4_to_8);
+#undef PT
+#define PT ITSTRUCT(expand_gray_1_2_4_to_8)
+#endif /* PNG_READ_EXPAND_SUPPORTED */
+
+#ifdef PNG_READ_EXPAND_16_SUPPORTED
+/* png_set_expand_16 */
+static void
+image_transform_png_set_expand_16_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_expand_16(pp);
+
+   /* NOTE: prior to 1.7 libpng does SET_EXPAND as well, so tRNS is expanded. */
+#  if PNG_LIBPNG_VER < 10700
+      if (that->this.has_tRNS)
+         that->this.is_transparent = 1;
+#  endif
+
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_expand_16_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   /* Expect expand_16 to expand everything to 16 bits as a result of also
+    * causing 'expand' to happen.
+    */
+   if (that->colour_type == PNG_COLOR_TYPE_PALETTE)
+      image_pixel_convert_PLTE(that);
+
+   if (that->have_tRNS)
+      image_pixel_add_alpha(that, &display->this, 0/*!for background*/);
+
+   if (that->bit_depth < 16)
+      that->sample_depth = that->bit_depth = 16;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_expand_16_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(colour_type)
+
+   this->next = *that;
+   *that = this;
+
+   /* expand_16 does something unless the bit depth is already 16. */
+   return bit_depth < 16;
+}
+
+IT(expand_16);
+#undef PT
+#define PT ITSTRUCT(expand_16)
+#endif /* PNG_READ_EXPAND_16_SUPPORTED */
+
+#ifdef PNG_READ_SCALE_16_TO_8_SUPPORTED  /* API added in 1.5.4 */
+/* png_set_scale_16 */
+static void
+image_transform_png_set_scale_16_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_scale_16(pp);
+#  if PNG_LIBPNG_VER < 10700
+      /* libpng will limit the gamma table size: */
+      that->max_gamma_8 = PNG_MAX_GAMMA_8;
+#  endif
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_scale_16_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->bit_depth == 16)
+   {
+      that->sample_depth = that->bit_depth = 8;
+      if (that->red_sBIT > 8) that->red_sBIT = 8;
+      if (that->green_sBIT > 8) that->green_sBIT = 8;
+      if (that->blue_sBIT > 8) that->blue_sBIT = 8;
+      if (that->alpha_sBIT > 8) that->alpha_sBIT = 8;
+   }
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_scale_16_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(colour_type)
+
+   this->next = *that;
+   *that = this;
+
+   return bit_depth > 8;
+}
+
+IT(scale_16);
+#undef PT
+#define PT ITSTRUCT(scale_16)
+#endif /* PNG_READ_SCALE_16_TO_8_SUPPORTED (1.5.4 on) */
+
+#ifdef PNG_READ_16_TO_8_SUPPORTED /* the default before 1.5.4 */
+/* png_set_strip_16 */
+static void
+image_transform_png_set_strip_16_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_strip_16(pp);
+#  if PNG_LIBPNG_VER < 10700
+      /* libpng will limit the gamma table size: */
+      that->max_gamma_8 = PNG_MAX_GAMMA_8;
+#  endif
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_strip_16_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->bit_depth == 16)
+   {
+      that->sample_depth = that->bit_depth = 8;
+      if (that->red_sBIT > 8) that->red_sBIT = 8;
+      if (that->green_sBIT > 8) that->green_sBIT = 8;
+      if (that->blue_sBIT > 8) that->blue_sBIT = 8;
+      if (that->alpha_sBIT > 8) that->alpha_sBIT = 8;
+
+      /* Prior to 1.5.4 png_set_strip_16 would use an 'accurate' method if this
+       * configuration option is set.  From 1.5.4 the flag is never set and the
+       * 'scale' API (above) must be used.
+       */
+#     ifdef PNG_READ_ACCURATE_SCALE_SUPPORTED
+#        if PNG_LIBPNG_VER >= 10504
+#           error PNG_READ_ACCURATE_SCALE should not be set
+#        endif
+
+         /* The strip 16 algorithm drops the low 8 bits rather than calculating
+          * 1/257, so we need to adjust the permitted errors appropriately:
+          * Notice that this is only relevant prior to the addition of the
+          * png_set_scale_16 API in 1.5.4 (but 1.5.4+ always defines the above!)
+          */
+         {
+            const double d = (255-128.5)/65535;
+            that->rede += d;
+            that->greene += d;
+            that->bluee += d;
+            that->alphae += d;
+         }
+#     endif
+   }
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_strip_16_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(colour_type)
+
+   this->next = *that;
+   *that = this;
+
+   return bit_depth > 8;
+}
+
+IT(strip_16);
+#undef PT
+#define PT ITSTRUCT(strip_16)
+#endif /* PNG_READ_16_TO_8_SUPPORTED */
+
+#ifdef PNG_READ_STRIP_ALPHA_SUPPORTED
+/* png_set_strip_alpha */
+static void
+image_transform_png_set_strip_alpha_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_strip_alpha(pp);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_strip_alpha_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->colour_type == PNG_COLOR_TYPE_GRAY_ALPHA)
+      that->colour_type = PNG_COLOR_TYPE_GRAY;
+   else if (that->colour_type == PNG_COLOR_TYPE_RGB_ALPHA)
+      that->colour_type = PNG_COLOR_TYPE_RGB;
+
+   that->have_tRNS = 0;
+   that->alphaf = 1;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_strip_alpha_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   return (colour_type & PNG_COLOR_MASK_ALPHA) != 0;
+}
+
+IT(strip_alpha);
+#undef PT
+#define PT ITSTRUCT(strip_alpha)
+#endif /* PNG_READ_STRIP_ALPHA_SUPPORTED */
+
+#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
+/* png_set_rgb_to_gray(png_structp, int err_action, double red, double green)
+ * png_set_rgb_to_gray_fixed(png_structp, int err_action, png_fixed_point red,
+ *    png_fixed_point green)
+ * png_get_rgb_to_gray_status
+ *
+ * The 'default' test here uses values known to be used inside libpng prior to
+ * 1.7.0:
+ *
+ *   red:    6968
+ *   green: 23434
+ *   blue:   2366
+ *
+ * These values are being retained for compatibility, along with the somewhat
+ * broken truncation calculation in the fast-and-inaccurate code path.  Older
+ * versions of libpng will fail the accuracy tests below because they use the
+ * truncation algorithm everywhere.
+ */
+#define data ITDATA(rgb_to_gray)
+static struct
+{
+   double gamma;      /* File gamma to use in processing */
+
+   /* The following are the parameters for png_set_rgb_to_gray: */
+#  ifdef PNG_FLOATING_POINT_SUPPORTED
+      double red_to_set;
+      double green_to_set;
+#  else
+      png_fixed_point red_to_set;
+      png_fixed_point green_to_set;
+#  endif
+
+   /* The actual coefficients: */
+   double red_coefficient;
+   double green_coefficient;
+   double blue_coefficient;
+
+   /* Set if the coeefficients have been overridden. */
+   int coefficients_overridden;
+} data;
+
+#undef image_transform_ini
+#define image_transform_ini image_transform_png_set_rgb_to_gray_ini
+static void
+image_transform_png_set_rgb_to_gray_ini(const image_transform *this,
+    transform_display *that)
+{
+   png_modifier *pm = that->pm;
+   const color_encoding *e = pm->current_encoding;
+
+   UNUSED(this)
+
+   /* Since we check the encoding this flag must be set: */
+   pm->test_uses_encoding = 1;
+
+   /* If 'e' is not NULL chromaticity information is present and either a cHRM
+    * or an sRGB chunk will be inserted.
+    */
+   if (e != 0)
+   {
+      /* Coefficients come from the encoding, but may need to be normalized to a
+       * white point Y of 1.0
+       */
+      const double whiteY = e->red.Y + e->green.Y + e->blue.Y;
+
+      data.red_coefficient = e->red.Y;
+      data.green_coefficient = e->green.Y;
+      data.blue_coefficient = e->blue.Y;
+
+      if (whiteY != 1)
+      {
+         data.red_coefficient /= whiteY;
+         data.green_coefficient /= whiteY;
+         data.blue_coefficient /= whiteY;
+      }
+   }
+
+   else
+   {
+      /* The default (built in) coeffcients, as above: */
+#     if PNG_LIBPNG_VER < 10700
+         data.red_coefficient = 6968 / 32768.;
+         data.green_coefficient = 23434 / 32768.;
+         data.blue_coefficient = 2366 / 32768.;
+#     else
+         data.red_coefficient = .2126;
+         data.green_coefficient = .7152;
+         data.blue_coefficient = .0722;
+#     endif
+   }
+
+   data.gamma = pm->current_gamma;
+
+   /* If not set then the calculations assume linear encoding (implicitly): */
+   if (data.gamma == 0)
+      data.gamma = 1;
+
+   /* The arguments to png_set_rgb_to_gray can override the coefficients implied
+    * by the color space encoding.  If doing exhaustive checks do the override
+    * in each case, otherwise do it randomly.
+    */
+   if (pm->test_exhaustive)
+   {
+      /* First time in coefficients_overridden is 0, the following sets it to 1,
+       * so repeat if it is set.  If a test fails this may mean we subsequently
+       * skip a non-override test, ignore that.
+       */
+      data.coefficients_overridden = !data.coefficients_overridden;
+      pm->repeat = data.coefficients_overridden != 0;
+   }
+
+   else
+      data.coefficients_overridden = random_choice();
+
+   if (data.coefficients_overridden)
+   {
+      /* These values override the color encoding defaults, simply use random
+       * numbers.
+       */
+      png_uint_32 ru;
+      double total;
+
+      R32_1(ru);
+      data.green_coefficient = total = (ru & 0xffff) / 65535.;
+      ru >>= 16;
+      data.red_coefficient = (1 - total) * (ru & 0xffff) / 65535.;
+      total += data.red_coefficient;
+      data.blue_coefficient = 1 - total;
+
+#     ifdef PNG_FLOATING_POINT_SUPPORTED
+         data.red_to_set = data.red_coefficient;
+         data.green_to_set = data.green_coefficient;
+#     else
+         data.red_to_set = fix(data.red_coefficient);
+         data.green_to_set = fix(data.green_coefficient);
+#     endif
+
+      /* The following just changes the error messages: */
+      pm->encoding_ignored = 1;
+   }
+
+   else
+   {
+      data.red_to_set = -1;
+      data.green_to_set = -1;
+   }
+
+   /* Adjust the error limit in the png_modifier because of the larger errors
+    * produced in the digitization during the gamma handling.
+    */
+   if (data.gamma != 1) /* Use gamma tables */
+   {
+      if (that->this.bit_depth == 16 || pm->assume_16_bit_calculations)
+      {
+         /* The computations have the form:
+          *
+          *    r * rc + g * gc + b * bc
+          *
+          *  Each component of which is +/-1/65535 from the gamma_to_1 table
+          *  lookup, resulting in a base error of +/-6.  The gamma_from_1
+          *  conversion adds another +/-2 in the 16-bit case and
+          *  +/-(1<<(15-PNG_MAX_GAMMA_8)) in the 8-bit case.
+          */
+#        if PNG_LIBPNG_VER < 10700
+            if (that->this.bit_depth < 16)
+               that->max_gamma_8 = PNG_MAX_GAMMA_8;
+#        endif
+         that->pm->limit += pow(
+            (that->this.bit_depth == 16 || that->max_gamma_8 > 14 ?
+               8. :
+               6. + (1<<(15-that->max_gamma_8))
+            )/65535, data.gamma);
+      }
+
+      else
+      {
+         /* Rounding to 8 bits in the linear space causes massive errors which
+          * will trigger the error check in transform_range_check.  Fix that
+          * here by taking the gamma encoding into account.
+          *
+          * When DIGITIZE is set because a pre-1.7 version of libpng is being
+          * tested allow a bigger slack.
+          *
+          * NOTE: this number only affects the internal limit check in pngvalid,
+          * it has no effect on the limits applied to the libpng values.
+          */
+         that->pm->limit += pow(
+#        if DIGITIZE
+            2.0
+#        else
+            1.0
+#        endif
+            /255, data.gamma);
+      }
+   }
+
+   else
+   {
+      /* With no gamma correction a large error comes from the truncation of the
+       * calculation in the 8 bit case, allow for that here.
+       */
+      if (that->this.bit_depth != 16 && !pm->assume_16_bit_calculations)
+         that->pm->limit += 4E-3;
+   }
+}
+
+static void
+image_transform_png_set_rgb_to_gray_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   const int error_action = 1; /* no error, no defines in png.h */
+
+#  ifdef PNG_FLOATING_POINT_SUPPORTED
+      png_set_rgb_to_gray(pp, error_action, data.red_to_set, data.green_to_set);
+#  else
+      png_set_rgb_to_gray_fixed(pp, error_action, data.red_to_set,
+         data.green_to_set);
+#  endif
+
+#  ifdef PNG_READ_cHRM_SUPPORTED
+      if (that->pm->current_encoding != 0)
+      {
+         /* We have an encoding so a cHRM chunk may have been set; if so then
+          * check that the libpng APIs give the correct (X,Y,Z) values within
+          * some margin of error for the round trip through the chromaticity
+          * form.
+          */
+#        ifdef PNG_FLOATING_POINT_SUPPORTED
+#           define API_function png_get_cHRM_XYZ
+#           define API_form "FP"
+#           define API_type double
+#           define API_cvt(x) (x)
+#        else
+#           define API_function png_get_cHRM_XYZ_fixed
+#           define API_form "fixed"
+#           define API_type png_fixed_point
+#           define API_cvt(x) ((double)(x)/PNG_FP_1)
+#        endif
+
+         API_type rX, gX, bX;
+         API_type rY, gY, bY;
+         API_type rZ, gZ, bZ;
+
+         if ((API_function(pp, pi, &rX, &rY, &rZ, &gX, &gY, &gZ, &bX, &bY, &bZ)
+               & PNG_INFO_cHRM) != 0)
+         {
+            double maxe;
+            const char *el;
+            color_encoding e, o;
+
+            /* Expect libpng to return a normalized result, but the original
+             * color space encoding may not be normalized.
+             */
+            modifier_current_encoding(that->pm, &o);
+            normalize_color_encoding(&o);
+
+            /* Sanity check the pngvalid code - the coefficients should match
+             * the normalized Y values of the encoding unless they were
+             * overridden.
+             */
+            if (data.red_to_set == -1 && data.green_to_set == -1 &&
+               (fabs(o.red.Y - data.red_coefficient) > DBL_EPSILON ||
+               fabs(o.green.Y - data.green_coefficient) > DBL_EPSILON ||
+               fabs(o.blue.Y - data.blue_coefficient) > DBL_EPSILON))
+               png_error(pp, "internal pngvalid cHRM coefficient error");
+
+            /* Generate a colour space encoding. */
+            e.gamma = o.gamma; /* not used */
+            e.red.X = API_cvt(rX);
+            e.red.Y = API_cvt(rY);
+            e.red.Z = API_cvt(rZ);
+            e.green.X = API_cvt(gX);
+            e.green.Y = API_cvt(gY);
+            e.green.Z = API_cvt(gZ);
+            e.blue.X = API_cvt(bX);
+            e.blue.Y = API_cvt(bY);
+            e.blue.Z = API_cvt(bZ);
+
+            /* This should match the original one from the png_modifier, within
+             * the range permitted by the libpng fixed point representation.
+             */
+            maxe = 0;
+            el = "-"; /* Set to element name with error */
+
+#           define CHECK(col,x)\
+            {\
+               double err = fabs(o.col.x - e.col.x);\
+               if (err > maxe)\
+               {\
+                  maxe = err;\
+                  el = #col "(" #x ")";\
+               }\
+            }
+
+            CHECK(red,X)
+            CHECK(red,Y)
+            CHECK(red,Z)
+            CHECK(green,X)
+            CHECK(green,Y)
+            CHECK(green,Z)
+            CHECK(blue,X)
+            CHECK(blue,Y)
+            CHECK(blue,Z)
+
+            /* Here in both fixed and floating cases to check the values read
+             * from the cHRm chunk.  PNG uses fixed point in the cHRM chunk, so
+             * we can't expect better than +/-.5E-5 on the result, allow 1E-5.
+             */
+            if (maxe >= 1E-5)
+            {
+               size_t pos = 0;
+               char buffer[256];
+
+               pos = safecat(buffer, sizeof buffer, pos, API_form);
+               pos = safecat(buffer, sizeof buffer, pos, " cHRM ");
+               pos = safecat(buffer, sizeof buffer, pos, el);
+               pos = safecat(buffer, sizeof buffer, pos, " error: ");
+               pos = safecatd(buffer, sizeof buffer, pos, maxe, 7);
+               pos = safecat(buffer, sizeof buffer, pos, " ");
+               /* Print the color space without the gamma value: */
+               pos = safecat_color_encoding(buffer, sizeof buffer, pos, &o, 0);
+               pos = safecat(buffer, sizeof buffer, pos, " -> ");
+               pos = safecat_color_encoding(buffer, sizeof buffer, pos, &e, 0);
+
+               png_error(pp, buffer);
+            }
+         }
+      }
+#  endif /* READ_cHRM */
+
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_rgb_to_gray_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if ((that->colour_type & PNG_COLOR_MASK_COLOR) != 0)
+   {
+      double gray, err;
+
+#     if PNG_LIBPNG_VER < 10700
+         if (that->colour_type == PNG_COLOR_TYPE_PALETTE)
+            image_pixel_convert_PLTE(that);
+#     endif
+
+      /* Image now has RGB channels... */
+#  if DIGITIZE
+      {
+         png_modifier *pm = display->pm;
+         const unsigned int sample_depth = that->sample_depth;
+         const unsigned int calc_depth = (pm->assume_16_bit_calculations ? 16 :
+            sample_depth);
+         const unsigned int gamma_depth =
+            (sample_depth == 16 ?
+               display->max_gamma_8 :
+               (pm->assume_16_bit_calculations ?
+                  display->max_gamma_8 :
+                  sample_depth));
+         int isgray;
+         double r, g, b;
+         double rlo, rhi, glo, ghi, blo, bhi, graylo, grayhi;
+
+         /* Do this using interval arithmetic, otherwise it is too difficult to
+          * handle the errors correctly.
+          *
+          * To handle the gamma correction work out the upper and lower bounds
+          * of the digitized value.  Assume rounding here - normally the values
+          * will be identical after this operation if there is only one
+          * transform, feel free to delete the png_error checks on this below in
+          * the future (this is just me trying to ensure it works!)
+          *
+          * Interval arithmetic is exact, but to implement it it must be
+          * possible to control the floating point implementation rounding mode.
+          * This cannot be done in ANSI-C, so instead I reduce the 'lo' values
+          * by DBL_EPSILON and increase the 'hi' values by the same.
+          */
+#        define DD(v,d,r) (digitize(v*(1-DBL_EPSILON), d, r) * (1-DBL_EPSILON))
+#        define DU(v,d,r) (digitize(v*(1+DBL_EPSILON), d, r) * (1+DBL_EPSILON))
+
+         r = rlo = rhi = that->redf;
+         rlo -= that->rede;
+         rlo = DD(rlo, calc_depth, 1/*round*/);
+         rhi += that->rede;
+         rhi = DU(rhi, calc_depth, 1/*round*/);
+
+         g = glo = ghi = that->greenf;
+         glo -= that->greene;
+         glo = DD(glo, calc_depth, 1/*round*/);
+         ghi += that->greene;
+         ghi = DU(ghi, calc_depth, 1/*round*/);
+
+         b = blo = bhi = that->bluef;
+         blo -= that->bluee;
+         blo = DD(blo, calc_depth, 1/*round*/);
+         bhi += that->bluee;
+         bhi = DU(bhi, calc_depth, 1/*round*/);
+
+         isgray = r==g && g==b;
+
+         if (data.gamma != 1)
+         {
+            const double power = 1/data.gamma;
+            const double abse = .5/(sample_depth == 16 ? 65535 : 255);
+
+            /* If a gamma calculation is done it is done using lookup tables of
+             * precision gamma_depth, so the already digitized value above may
+             * need to be further digitized here.
+             */
+            if (gamma_depth != calc_depth)
+            {
+               rlo = DD(rlo, gamma_depth, 0/*truncate*/);
+               rhi = DU(rhi, gamma_depth, 0/*truncate*/);
+               glo = DD(glo, gamma_depth, 0/*truncate*/);
+               ghi = DU(ghi, gamma_depth, 0/*truncate*/);
+               blo = DD(blo, gamma_depth, 0/*truncate*/);
+               bhi = DU(bhi, gamma_depth, 0/*truncate*/);
+            }
+
+            /* 'abse' is the error in the gamma table calculation itself. */
+            r = pow(r, power);
+            rlo = DD(pow(rlo, power)-abse, calc_depth, 1);
+            rhi = DU(pow(rhi, power)+abse, calc_depth, 1);
+
+            g = pow(g, power);
+            glo = DD(pow(glo, power)-abse, calc_depth, 1);
+            ghi = DU(pow(ghi, power)+abse, calc_depth, 1);
+
+            b = pow(b, power);
+            blo = DD(pow(blo, power)-abse, calc_depth, 1);
+            bhi = DU(pow(bhi, power)+abse, calc_depth, 1);
+         }
+
+         /* Now calculate the actual gray values.  Although the error in the
+          * coefficients depends on whether they were specified on the command
+          * line (in which case truncation to 15 bits happened) or not (rounding
+          * was used) the maxium error in an individual coefficient is always
+          * 2/32768, because even in the rounding case the requirement that
+          * coefficients add up to 32768 can cause a larger rounding error.
+          *
+          * The only time when rounding doesn't occur in 1.5.5 and later is when
+          * the non-gamma code path is used for less than 16 bit data.
+          */
+         gray = r * data.red_coefficient + g * data.green_coefficient +
+            b * data.blue_coefficient;
+
+         {
+            const int do_round = data.gamma != 1 || calc_depth == 16;
+            const double ce = 2. / 32768;
+
+            graylo = DD(rlo * (data.red_coefficient-ce) +
+               glo * (data.green_coefficient-ce) +
+               blo * (data.blue_coefficient-ce), calc_depth, do_round);
+            if (graylo > gray) /* always accept the right answer */
+               graylo = gray;
+
+            grayhi = DU(rhi * (data.red_coefficient+ce) +
+               ghi * (data.green_coefficient+ce) +
+               bhi * (data.blue_coefficient+ce), calc_depth, do_round);
+            if (grayhi < gray)
+               grayhi = gray;
+         }
+
+         /* And invert the gamma. */
+         if (data.gamma != 1)
+         {
+            const double power = data.gamma;
+
+            /* And this happens yet again, shifting the values once more. */
+            if (gamma_depth != sample_depth)
+            {
+               rlo = DD(rlo, gamma_depth, 0/*truncate*/);
+               rhi = DU(rhi, gamma_depth, 0/*truncate*/);
+               glo = DD(glo, gamma_depth, 0/*truncate*/);
+               ghi = DU(ghi, gamma_depth, 0/*truncate*/);
+               blo = DD(blo, gamma_depth, 0/*truncate*/);
+               bhi = DU(bhi, gamma_depth, 0/*truncate*/);
+            }
+
+            gray = pow(gray, power);
+            graylo = DD(pow(graylo, power), sample_depth, 1);
+            grayhi = DU(pow(grayhi, power), sample_depth, 1);
+         }
+
+#        undef DD
+#        undef DU
+
+         /* Now the error can be calculated.
+          *
+          * If r==g==b because there is no overall gamma correction libpng
+          * currently preserves the original value.
+          */
+         if (isgray)
+            err = (that->rede + that->greene + that->bluee)/3;
+
+         else
+         {
+            err = fabs(grayhi-gray);
+
+            if (fabs(gray - graylo) > err)
+               err = fabs(graylo-gray);
+
+#if !RELEASE_BUILD
+            /* Check that this worked: */
+            if (err > pm->limit)
+            {
+               size_t pos = 0;
+               char buffer[128];
+
+               pos = safecat(buffer, sizeof buffer, pos, "rgb_to_gray error ");
+               pos = safecatd(buffer, sizeof buffer, pos, err, 6);
+               pos = safecat(buffer, sizeof buffer, pos, " exceeds limit ");
+               pos = safecatd(buffer, sizeof buffer, pos, pm->limit, 6);
+               png_warning(pp, buffer);
+               pm->limit = err;
+            }
+#endif /* !RELEASE_BUILD */
+         }
+      }
+#  else  /* !DIGITIZE */
+      {
+         double r = that->redf;
+         double re = that->rede;
+         double g = that->greenf;
+         double ge = that->greene;
+         double b = that->bluef;
+         double be = that->bluee;
+
+#        if PNG_LIBPNG_VER < 10700
+            /* The true gray case involves no math in earlier versions (not
+             * true, there was some if gamma correction was happening too.)
+             */
+            if (r == g && r == b)
+            {
+               gray = r;
+               err = re;
+               if (err < ge) err = ge;
+               if (err < be) err = be;
+            }
+
+            else
+#        endif /* before 1.7 */
+         if (data.gamma == 1)
+         {
+            /* There is no need to do the conversions to and from linear space,
+             * so the calculation should be a lot more accurate.  There is a
+             * built in error in the coefficients because they only have 15 bits
+             * and are adjusted to make sure they add up to 32768.  This
+             * involves a integer calculation with truncation of the form:
+             *
+             *     ((int)(coefficient * 100000) * 32768)/100000
+             *
+             * This is done to the red and green coefficients (the ones
+             * provided to the API) then blue is calculated from them so the
+             * result adds up to 32768.  In the worst case this can result in
+             * a -1 error in red and green and a +2 error in blue.  Consequently
+             * the worst case in the calculation below is 2/32768 error.
+             *
+             * TODO: consider fixing this in libpng by rounding the calculation
+             * limiting the error to 1/32768.
+             *
+             * Handling this by adding 2/32768 here avoids needing to increase
+             * the global error limits to take this into account.)
+             */
+            gray = r * data.red_coefficient + g * data.green_coefficient +
+               b * data.blue_coefficient;
+            err = re * data.red_coefficient + ge * data.green_coefficient +
+               be * data.blue_coefficient + 2./32768 + gray * 5 * DBL_EPSILON;
+         }
+
+         else
+         {
+            /* The calculation happens in linear space, and this produces much
+             * wider errors in the encoded space.  These are handled here by
+             * factoring the errors in to the calculation.  There are two table
+             * lookups in the calculation and each introduces a quantization
+             * error defined by the table size.
+             */
+            png_modifier *pm = display->pm;
+            double in_qe = (that->sample_depth > 8 ? .5/65535 : .5/255);
+            double out_qe = (that->sample_depth > 8 ? .5/65535 :
+               (pm->assume_16_bit_calculations ? .5/(1<<display->max_gamma_8) :
+               .5/255));
+            double rhi, ghi, bhi, grayhi;
+            double g1 = 1/data.gamma;
+
+            rhi = r + re + in_qe; if (rhi > 1) rhi = 1;
+            r -= re + in_qe; if (r < 0) r = 0;
+            ghi = g + ge + in_qe; if (ghi > 1) ghi = 1;
+            g -= ge + in_qe; if (g < 0) g = 0;
+            bhi = b + be + in_qe; if (bhi > 1) bhi = 1;
+            b -= be + in_qe; if (b < 0) b = 0;
+
+            r = pow(r, g1)*(1-DBL_EPSILON); rhi = pow(rhi, g1)*(1+DBL_EPSILON);
+            g = pow(g, g1)*(1-DBL_EPSILON); ghi = pow(ghi, g1)*(1+DBL_EPSILON);
+            b = pow(b, g1)*(1-DBL_EPSILON); bhi = pow(bhi, g1)*(1+DBL_EPSILON);
+
+            /* Work out the lower and upper bounds for the gray value in the
+             * encoded space, then work out an average and error.  Remove the
+             * previously added input quantization error at this point.
+             */
+            gray = r * data.red_coefficient + g * data.green_coefficient +
+               b * data.blue_coefficient - 2./32768 - out_qe;
+            if (gray <= 0)
+               gray = 0;
+            else
+            {
+               gray *= (1 - 6 * DBL_EPSILON);
+               gray = pow(gray, data.gamma) * (1-DBL_EPSILON);
+            }
+
+            grayhi = rhi * data.red_coefficient + ghi * data.green_coefficient +
+               bhi * data.blue_coefficient + 2./32768 + out_qe;
+            grayhi *= (1 + 6 * DBL_EPSILON);
+            if (grayhi >= 1)
+               grayhi = 1;
+            else
+               grayhi = pow(grayhi, data.gamma) * (1+DBL_EPSILON);
+
+            err = (grayhi - gray) / 2;
+            gray = (grayhi + gray) / 2;
+
+            if (err <= in_qe)
+               err = gray * DBL_EPSILON;
+
+            else
+               err -= in_qe;
+
+#if !RELEASE_BUILD
+            /* Validate that the error is within limits (this has caused
+             * problems before, it's much easier to detect them here.)
+             */
+            if (err > pm->limit)
+            {
+               size_t pos = 0;
+               char buffer[128];
+
+               pos = safecat(buffer, sizeof buffer, pos, "rgb_to_gray error ");
+               pos = safecatd(buffer, sizeof buffer, pos, err, 6);
+               pos = safecat(buffer, sizeof buffer, pos, " exceeds limit ");
+               pos = safecatd(buffer, sizeof buffer, pos, pm->limit, 6);
+               png_warning(pp, buffer);
+               pm->limit = err;
+            }
+#endif /* !RELEASE_BUILD */
+         }
+      }
+#  endif /* !DIGITIZE */
+
+      that->bluef = that->greenf = that->redf = gray;
+      that->bluee = that->greene = that->rede = err;
+
+      /* The sBIT is the minium of the three colour channel sBITs. */
+      if (that->red_sBIT > that->green_sBIT)
+         that->red_sBIT = that->green_sBIT;
+      if (that->red_sBIT > that->blue_sBIT)
+         that->red_sBIT = that->blue_sBIT;
+      that->blue_sBIT = that->green_sBIT = that->red_sBIT;
+
+      /* And remove the colour bit in the type: */
+      if (that->colour_type == PNG_COLOR_TYPE_RGB)
+         that->colour_type = PNG_COLOR_TYPE_GRAY;
+      else if (that->colour_type == PNG_COLOR_TYPE_RGB_ALPHA)
+         that->colour_type = PNG_COLOR_TYPE_GRAY_ALPHA;
+   }
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_rgb_to_gray_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   return (colour_type & PNG_COLOR_MASK_COLOR) != 0;
+}
+
+#undef data
+IT(rgb_to_gray);
+#undef PT
+#define PT ITSTRUCT(rgb_to_gray)
+#undef image_transform_ini
+#define image_transform_ini image_transform_default_ini
+#endif /* PNG_READ_RGB_TO_GRAY_SUPPORTED */
+
+#ifdef PNG_READ_BACKGROUND_SUPPORTED
+/* png_set_background(png_structp, png_const_color_16p background_color,
+ *    int background_gamma_code, int need_expand, double background_gamma)
+ * png_set_background_fixed(png_structp, png_const_color_16p background_color,
+ *    int background_gamma_code, int need_expand,
+ *    png_fixed_point background_gamma)
+ *
+ * This ignores the gamma (at present.)
+*/
+#define data ITDATA(background)
+static image_pixel data;
+
+static void
+image_transform_png_set_background_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_byte colour_type, bit_depth;
+   png_byte random_bytes[8]; /* 8 bytes - 64 bits - the biggest pixel */
+   int expand;
+   png_color_16 back;
+
+   /* We need a background colour, because we don't know exactly what transforms
+    * have been set we have to supply the colour in the original file format and
+    * so we need to know what that is!  The background colour is stored in the
+    * transform_display.
+    */
+   R8(random_bytes);
+
+   /* Read the random value, for colour type 3 the background colour is actually
+    * expressed as a 24bit rgb, not an index.
+    */
+   colour_type = that->this.colour_type;
+   if (colour_type == 3)
+   {
+      colour_type = PNG_COLOR_TYPE_RGB;
+      bit_depth = 8;
+      expand = 0; /* passing in an RGB not a pixel index */
+   }
+
+   else
+   {
+      if (that->this.has_tRNS)
+         that->this.is_transparent = 1;
+
+      bit_depth = that->this.bit_depth;
+      expand = 1;
+   }
+
+   image_pixel_init(&data, random_bytes, colour_type,
+      bit_depth, 0/*x*/, 0/*unused: palette*/, NULL/*format*/);
+
+   /* Extract the background colour from this image_pixel, but make sure the
+    * unused fields of 'back' are garbage.
+    */
+   R8(back);
+
+   if (colour_type & PNG_COLOR_MASK_COLOR)
+   {
+      back.red = (png_uint_16)data.red;
+      back.green = (png_uint_16)data.green;
+      back.blue = (png_uint_16)data.blue;
+   }
+
+   else
+      back.gray = (png_uint_16)data.red;
+
+#ifdef PNG_FLOATING_POINT_SUPPORTED
+   png_set_background(pp, &back, PNG_BACKGROUND_GAMMA_FILE, expand, 0);
+#else
+   png_set_background_fixed(pp, &back, PNG_BACKGROUND_GAMMA_FILE, expand, 0);
+#endif
+
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_background_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   /* Check for tRNS first: */
+   if (that->have_tRNS && that->colour_type != PNG_COLOR_TYPE_PALETTE)
+      image_pixel_add_alpha(that, &display->this, 1/*for background*/);
+
+   /* This is only necessary if the alpha value is less than 1. */
+   if (that->alphaf < 1)
+   {
+      /* Now we do the background calculation without any gamma correction. */
+      if (that->alphaf <= 0)
+      {
+         that->redf = data.redf;
+         that->greenf = data.greenf;
+         that->bluef = data.bluef;
+
+         that->rede = data.rede;
+         that->greene = data.greene;
+         that->bluee = data.bluee;
+
+         that->red_sBIT= data.red_sBIT;
+         that->green_sBIT= data.green_sBIT;
+         that->blue_sBIT= data.blue_sBIT;
+      }
+
+      else /* 0 < alpha < 1 */
+      {
+         double alf = 1 - that->alphaf;
+
+         that->redf = that->redf * that->alphaf + data.redf * alf;
+         that->rede = that->rede * that->alphaf + data.rede * alf +
+            DBL_EPSILON;
+         that->greenf = that->greenf * that->alphaf + data.greenf * alf;
+         that->greene = that->greene * that->alphaf + data.greene * alf +
+            DBL_EPSILON;
+         that->bluef = that->bluef * that->alphaf + data.bluef * alf;
+         that->bluee = that->bluee * that->alphaf + data.bluee * alf +
+            DBL_EPSILON;
+      }
+
+      /* Remove the alpha type and set the alpha (not in that order.) */
+      that->alphaf = 1;
+      that->alphae = 0;
+   }
+
+   if (that->colour_type == PNG_COLOR_TYPE_RGB_ALPHA)
+      that->colour_type = PNG_COLOR_TYPE_RGB;
+   else if (that->colour_type == PNG_COLOR_TYPE_GRAY_ALPHA)
+      that->colour_type = PNG_COLOR_TYPE_GRAY;
+   /* PNG_COLOR_TYPE_PALETTE is not changed */
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+#define image_transform_png_set_background_add image_transform_default_add
+
+#undef data
+IT(background);
+#undef PT
+#define PT ITSTRUCT(background)
+#endif /* PNG_READ_BACKGROUND_SUPPORTED */
+
+/* png_set_quantize(png_structp, png_colorp palette, int num_palette,
+ *    int maximum_colors, png_const_uint_16p histogram, int full_quantize)
+ *
+ * Very difficult to validate this!
+ */
+/*NOTE: TBD NYI */
+
+/* The data layout transforms are handled by swapping our own channel data,
+ * necessarily these need to happen at the end of the transform list because the
+ * semantic of the channels changes after these are executed.  Some of these,
+ * like set_shift and set_packing, can't be done at present because they change
+ * the layout of the data at the sub-sample level so sample() won't get the
+ * right answer.
+ */
+/* png_set_invert_alpha */
+#ifdef PNG_READ_INVERT_ALPHA_SUPPORTED
+/* Invert the alpha channel
+ *
+ *  png_set_invert_alpha(png_structrp png_ptr)
+ */
+static void
+image_transform_png_set_invert_alpha_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_invert_alpha(pp);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_invert_alpha_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->colour_type & 4)
+      that->alpha_inverted = 1;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_invert_alpha_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   /* Only has an effect on pixels with alpha: */
+   return (colour_type & 4) != 0;
+}
+
+IT(invert_alpha);
+#undef PT
+#define PT ITSTRUCT(invert_alpha)
+
+#endif /* PNG_READ_INVERT_ALPHA_SUPPORTED */
+
+/* png_set_bgr */
+#ifdef PNG_READ_BGR_SUPPORTED
+/* Swap R,G,B channels to order B,G,R.
+ *
+ *  png_set_bgr(png_structrp png_ptr)
+ *
+ * This only has an effect on RGB and RGBA pixels.
+ */
+static void
+image_transform_png_set_bgr_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_bgr(pp);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_bgr_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->colour_type == PNG_COLOR_TYPE_RGB ||
+       that->colour_type == PNG_COLOR_TYPE_RGBA)
+       that->swap_rgb = 1;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_bgr_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   return colour_type == PNG_COLOR_TYPE_RGB ||
+       colour_type == PNG_COLOR_TYPE_RGBA;
+}
+
+IT(bgr);
+#undef PT
+#define PT ITSTRUCT(bgr)
+
+#endif /* PNG_READ_BGR_SUPPORTED */
+
+/* png_set_swap_alpha */
+#ifdef PNG_READ_SWAP_ALPHA_SUPPORTED
+/* Put the alpha channel first.
+ *
+ *  png_set_swap_alpha(png_structrp png_ptr)
+ *
+ * This only has an effect on GA and RGBA pixels.
+ */
+static void
+image_transform_png_set_swap_alpha_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_swap_alpha(pp);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_swap_alpha_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->colour_type == PNG_COLOR_TYPE_GA ||
+       that->colour_type == PNG_COLOR_TYPE_RGBA)
+      that->alpha_first = 1;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_swap_alpha_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   return colour_type == PNG_COLOR_TYPE_GA ||
+       colour_type == PNG_COLOR_TYPE_RGBA;
+}
+
+IT(swap_alpha);
+#undef PT
+#define PT ITSTRUCT(swap_alpha)
+
+#endif /* PNG_READ_SWAP_ALPHA_SUPPORTED */
+
+/* png_set_swap */
+#ifdef PNG_READ_SWAP_SUPPORTED
+/* Byte swap 16-bit components.
+ *
+ *  png_set_swap(png_structrp png_ptr)
+ */
+static void
+image_transform_png_set_swap_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_swap(pp);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_swap_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->bit_depth == 16)
+      that->swap16 = 1;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_swap_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(colour_type)
+
+   this->next = *that;
+   *that = this;
+
+   return bit_depth == 16;
+}
+
+IT(swap);
+#undef PT
+#define PT ITSTRUCT(swap)
+
+#endif /* PNG_READ_SWAP_SUPPORTED */
+
+#ifdef PNG_READ_FILLER_SUPPORTED
+/* Add a filler byte to 8-bit Gray or 24-bit RGB images.
+ *
+ *  png_set_filler, (png_structp png_ptr, png_uint_32 filler, int flags));
+ *
+ * Flags:
+ *
+ *  PNG_FILLER_BEFORE
+ *  PNG_FILLER_AFTER
+ */
+#define data ITDATA(filler)
+static struct
+{
+   png_uint_32 filler;
+   int         flags;
+} data;
+
+static void
+image_transform_png_set_filler_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   /* Need a random choice for 'before' and 'after' as well as for the
+    * filler.  The 'filler' value has all 32 bits set, but only bit_depth
+    * will be used.  At this point we don't know bit_depth.
+    */
+   R32(data.filler);
+   data.flags = random_choice();
+
+   png_set_filler(pp, data.filler, data.flags);
+
+   /* The standard display handling stuff also needs to know that
+    * there is a filler, so set that here.
+    */
+   that->this.filler = 1;
+
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_filler_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->bit_depth >= 8 &&
+       (that->colour_type == PNG_COLOR_TYPE_RGB ||
+        that->colour_type == PNG_COLOR_TYPE_GRAY))
+   {
+      const unsigned int max = (1U << that->bit_depth)-1;
+      that->alpha = data.filler & max;
+      that->alphaf = ((double)that->alpha) / max;
+      that->alphae = 0;
+
+      /* The filler has been stored in the alpha channel, we must record
+       * that this has been done for the checking later on, the color
+       * type is faked to have an alpha channel, but libpng won't report
+       * this; the app has to know the extra channel is there and this
+       * was recording in standard_display::filler above.
+       */
+      that->colour_type |= 4; /* alpha added */
+      that->alpha_first = data.flags == PNG_FILLER_BEFORE;
+   }
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_filler_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   this->next = *that;
+   *that = this;
+
+   return bit_depth >= 8 && (colour_type == PNG_COLOR_TYPE_RGB ||
+           colour_type == PNG_COLOR_TYPE_GRAY);
+}
+
+#undef data
+IT(filler);
+#undef PT
+#define PT ITSTRUCT(filler)
+
+/* png_set_add_alpha, (png_structp png_ptr, png_uint_32 filler, int flags)); */
+/* Add an alpha byte to 8-bit Gray or 24-bit RGB images. */
+#define data ITDATA(add_alpha)
+static struct
+{
+   png_uint_32 filler;
+   int         flags;
+} data;
+
+static void
+image_transform_png_set_add_alpha_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   /* Need a random choice for 'before' and 'after' as well as for the
+    * filler.  The 'filler' value has all 32 bits set, but only bit_depth
+    * will be used.  At this point we don't know bit_depth.
+    */
+   R32(data.filler);
+   data.flags = random_choice();
+
+   png_set_add_alpha(pp, data.filler, data.flags);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_add_alpha_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->bit_depth >= 8 &&
+       (that->colour_type == PNG_COLOR_TYPE_RGB ||
+        that->colour_type == PNG_COLOR_TYPE_GRAY))
+   {
+      const unsigned int max = (1U << that->bit_depth)-1;
+      that->alpha = data.filler & max;
+      that->alphaf = ((double)that->alpha) / max;
+      that->alphae = 0;
+
+      that->colour_type |= 4; /* alpha added */
+      that->alpha_first = data.flags == PNG_FILLER_BEFORE;
+   }
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_add_alpha_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   this->next = *that;
+   *that = this;
+
+   return bit_depth >= 8 && (colour_type == PNG_COLOR_TYPE_RGB ||
+           colour_type == PNG_COLOR_TYPE_GRAY);
+}
+
+#undef data
+IT(add_alpha);
+#undef PT
+#define PT ITSTRUCT(add_alpha)
+
+#endif /* PNG_READ_FILLER_SUPPORTED */
+
+/* png_set_packing */
+#ifdef PNG_READ_PACK_SUPPORTED
+/* Use 1 byte per pixel in 1, 2, or 4-bit depth files.
+ *
+ *  png_set_packing(png_structrp png_ptr)
+ *
+ * This should only affect grayscale and palette images with less than 8 bits
+ * per pixel.
+ */
+static void
+image_transform_png_set_packing_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_packing(pp);
+   that->unpacked = 1;
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_packing_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   /* The general expand case depends on what the colour type is,
+    * low bit-depth pixel values are unpacked into bytes without
+    * scaling, so sample_depth is not changed.
+    */
+   if (that->bit_depth < 8) /* grayscale or palette */
+      that->bit_depth = 8;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_packing_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(colour_type)
+
+   this->next = *that;
+   *that = this;
+
+   /* Nothing should happen unless the bit depth is less than 8: */
+   return bit_depth < 8;
+}
+
+IT(packing);
+#undef PT
+#define PT ITSTRUCT(packing)
+
+#endif /* PNG_READ_PACK_SUPPORTED */
+
+/* png_set_packswap */
+#ifdef PNG_READ_PACKSWAP_SUPPORTED
+/* Swap pixels packed into bytes; reverses the order on screen so that
+ * the high order bits correspond to the rightmost pixels.
+ *
+ *  png_set_packswap(png_structrp png_ptr)
+ */
+static void
+image_transform_png_set_packswap_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_packswap(pp);
+   that->this.littleendian = 1;
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_packswap_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->bit_depth < 8)
+      that->littleendian = 1;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_packswap_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(colour_type)
+
+   this->next = *that;
+   *that = this;
+
+   return bit_depth < 8;
+}
+
+IT(packswap);
+#undef PT
+#define PT ITSTRUCT(packswap)
+
+#endif /* PNG_READ_PACKSWAP_SUPPORTED */
+
+
+/* png_set_invert_mono */
+#ifdef PNG_READ_INVERT_MONO_SUPPORTED
+/* Invert the gray channel
+ *
+ *  png_set_invert_mono(png_structrp png_ptr)
+ */
+static void
+image_transform_png_set_invert_mono_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_invert_mono(pp);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_invert_mono_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   if (that->colour_type & 4)
+      that->mono_inverted = 1;
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_invert_mono_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   /* Only has an effect on pixels with no colour: */
+   return (colour_type & 2) == 0;
+}
+
+IT(invert_mono);
+#undef PT
+#define PT ITSTRUCT(invert_mono)
+
+#endif /* PNG_READ_INVERT_MONO_SUPPORTED */
+
+#ifdef PNG_READ_SHIFT_SUPPORTED
+/* png_set_shift(png_structp, png_const_color_8p true_bits)
+ *
+ * The output pixels will be shifted by the given true_bits
+ * values.
+ */
+#define data ITDATA(shift)
+static png_color_8 data;
+
+static void
+image_transform_png_set_shift_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   /* Get a random set of shifts.  The shifts need to do something
+    * to test the transform, so they are limited to the bit depth
+    * of the input image.  Notice that in the following the 'gray'
+    * field is randomized independently.  This acts as a check that
+    * libpng does use the correct field.
+    */
+   const unsigned int depth = that->this.bit_depth;
+
+   data.red = (png_byte)/*SAFE*/(random_mod(depth)+1);
+   data.green = (png_byte)/*SAFE*/(random_mod(depth)+1);
+   data.blue = (png_byte)/*SAFE*/(random_mod(depth)+1);
+   data.gray = (png_byte)/*SAFE*/(random_mod(depth)+1);
+   data.alpha = (png_byte)/*SAFE*/(random_mod(depth)+1);
+
+   png_set_shift(pp, &data);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_shift_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   /* Copy the correct values into the sBIT fields, libpng does not do
+    * anything to palette data:
+    */
+   if (that->colour_type != PNG_COLOR_TYPE_PALETTE)
+   {
+       that->sig_bits = 1;
+
+       /* The sBIT fields are reset to the values previously sent to
+        * png_set_shift according to the colour type.
+        * does.
+        */
+       if (that->colour_type & 2) /* RGB channels */
+       {
+          that->red_sBIT = data.red;
+          that->green_sBIT = data.green;
+          that->blue_sBIT = data.blue;
+       }
+
+       else /* One grey channel */
+          that->red_sBIT = that->green_sBIT = that->blue_sBIT = data.gray;
+
+       that->alpha_sBIT = data.alpha;
+   }
+
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_shift_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   UNUSED(bit_depth)
+
+   this->next = *that;
+   *that = this;
+
+   return colour_type != PNG_COLOR_TYPE_PALETTE;
+}
+
+IT(shift);
+#undef PT
+#define PT ITSTRUCT(shift)
+
+#endif /* PNG_READ_SHIFT_SUPPORTED */
+
+#ifdef THIS_IS_THE_PROFORMA
+static void
+image_transform_png_set_@_set(const image_transform *this,
+    transform_display *that, png_structp pp, png_infop pi)
+{
+   png_set_@(pp);
+   this->next->set(this->next, that, pp, pi);
+}
+
+static void
+image_transform_png_set_@_mod(const image_transform *this,
+    image_pixel *that, png_const_structp pp,
+    const transform_display *display)
+{
+   this->next->mod(this->next, that, pp, display);
+}
+
+static int
+image_transform_png_set_@_add(image_transform *this,
+    const image_transform **that, png_byte colour_type, png_byte bit_depth)
+{
+   this->next = *that;
+   *that = this;
+
+   return 1;
+}
+
+IT(@);
+#endif
+
+
+/* This may just be 'end' if all the transforms are disabled! */
+static image_transform *const image_transform_first = &PT;
+
+static void
+transform_enable(const char *name)
+{
+   /* Everything starts out enabled, so if we see an 'enable' disabled
+    * everything else the first time round.
+    */
+   static int all_disabled = 0;
+   int found_it = 0;
+   image_transform *list = image_transform_first;
+
+   while (list != &image_transform_end)
+   {
+      if (strcmp(list->name, name) == 0)
+      {
+         list->enable = 1;
+         found_it = 1;
+      }
+      else if (!all_disabled)
+         list->enable = 0;
+
+      list = list->list;
+   }
+
+   all_disabled = 1;
+
+   if (!found_it)
+   {
+      fprintf(stderr, "pngvalid: --transform-enable=%s: unknown transform\n",
+         name);
+      exit(99);
+   }
+}
+
+static void
+transform_disable(const char *name)
+{
+   image_transform *list = image_transform_first;
+
+   while (list != &image_transform_end)
+   {
+      if (strcmp(list->name, name) == 0)
+      {
+         list->enable = 0;
+         return;
+      }
+
+      list = list->list;
+   }
+
+   fprintf(stderr, "pngvalid: --transform-disable=%s: unknown transform\n",
+      name);
+   exit(99);
+}
+
+static void
+image_transform_reset_count(void)
+{
+   image_transform *next = image_transform_first;
+   int count = 0;
+
+   while (next != &image_transform_end)
+   {
+      next->local_use = 0;
+      next->next = 0;
+      next = next->list;
+      ++count;
+   }
+
+   /* This can only happen if we every have more than 32 transforms (excluding
+    * the end) in the list.
+    */
+   if (count > 32) abort();
+}
+
+static int
+image_transform_test_counter(png_uint_32 counter, unsigned int max)
+{
+   /* Test the list to see if there is any point contining, given a current
+    * counter and a 'max' value.
+    */
+   image_transform *next = image_transform_first;
+
+   while (next != &image_transform_end)
+   {
+      /* For max 0 or 1 continue until the counter overflows: */
+      counter >>= 1;
+
+      /* Continue if any entry hasn't reacked the max. */
+      if (max > 1 && next->local_use < max)
+         return 1;
+      next = next->list;
+   }
+
+   return max <= 1 && counter == 0;
+}
+
+static png_uint_32
+image_transform_add(const image_transform **this, unsigned int max,
+   png_uint_32 counter, char *name, size_t sizeof_name, size_t *pos,
+   png_byte colour_type, png_byte bit_depth)
+{
+   for (;;) /* until we manage to add something */
+   {
+      png_uint_32 mask;
+      image_transform *list;
+
+      /* Find the next counter value, if the counter is zero this is the start
+       * of the list.  This routine always returns the current counter (not the
+       * next) so it returns 0 at the end and expects 0 at the beginning.
+       */
+      if (counter == 0) /* first time */
+      {
+         image_transform_reset_count();
+         if (max <= 1)
+            counter = 1;
+         else
+            counter = random_32();
+      }
+      else /* advance the counter */
+      {
+         switch (max)
+         {
+            case 0:  ++counter; break;
+            case 1:  counter <<= 1; break;
+            default: counter = random_32(); break;
+         }
+      }
+
+      /* Now add all these items, if possible */
+      *this = &image_transform_end;
+      list = image_transform_first;
+      mask = 1;
+
+      /* Go through the whole list adding anything that the counter selects: */
+      while (list != &image_transform_end)
+      {
+         if ((counter & mask) != 0 && list->enable &&
+             (max == 0 || list->local_use < max))
+         {
+            /* Candidate to add: */
+            if (list->add(list, this, colour_type, bit_depth) || max == 0)
+            {
+               /* Added, so add to the name too. */
+               *pos = safecat(name, sizeof_name, *pos, " +");
+               *pos = safecat(name, sizeof_name, *pos, list->name);
+            }
+
+            else
+            {
+               /* Not useful and max>0, so remove it from *this: */
+               *this = list->next;
+               list->next = 0;
+
+               /* And, since we know it isn't useful, stop it being added again
+                * in this run:
+                */
+               list->local_use = max;
+            }
+         }
+
+         mask <<= 1;
+         list = list->list;
+      }
+
+      /* Now if anything was added we have something to do. */
+      if (*this != &image_transform_end)
+         return counter;
+
+      /* Nothing added, but was there anything in there to add? */
+      if (!image_transform_test_counter(counter, max))
+         return 0;
+   }
+}
+
+static void
+perform_transform_test(png_modifier *pm)
+{
+   png_byte colour_type = 0;
+   png_byte bit_depth = 0;
+   unsigned int palette_number = 0;
+
+   while (next_format(&colour_type, &bit_depth, &palette_number, pm->test_lbg,
+            pm->test_tRNS))
+   {
+      png_uint_32 counter = 0;
+      size_t base_pos;
+      char name[64];
+
+      base_pos = safecat(name, sizeof name, 0, "transform:");
+
+      for (;;)
+      {
+         size_t pos = base_pos;
+         const image_transform *list = 0;
+
+         /* 'max' is currently hardwired to '1'; this should be settable on the
+          * command line.
+          */
+         counter = image_transform_add(&list, 1/*max*/, counter,
+            name, sizeof name, &pos, colour_type, bit_depth);
+
+         if (counter == 0)
+            break;
+
+         /* The command line can change this to checking interlaced images. */
+         do
+         {
+            pm->repeat = 0;
+            transform_test(pm, FILEID(colour_type, bit_depth, palette_number,
+               pm->interlace_type, 0, 0, 0), list, name);
+
+            if (fail(pm))
+               return;
+         }
+         while (pm->repeat);
+      }
+   }
+}
+#endif /* PNG_READ_TRANSFORMS_SUPPORTED */
+
+/********************************* GAMMA TESTS ********************************/
+#ifdef PNG_READ_GAMMA_SUPPORTED
+/* Reader callbacks and implementations, where they differ from the standard
+ * ones.
+ */
+typedef struct gamma_display
+{
+   standard_display this;
+
+   /* Parameters */
+   png_modifier*    pm;
+   double           file_gamma;
+   double           screen_gamma;
+   double           background_gamma;
+   png_byte         sbit;
+   int              threshold_test;
+   int              use_input_precision;
+   int              scale16;
+   int              expand16;
+   int              do_background;
+   png_color_16     background_color;
+
+   /* Local variables */
+   double       maxerrout;
+   double       maxerrpc;
+   double       maxerrabs;
+} gamma_display;
+
+#define ALPHA_MODE_OFFSET 4
+
+static void
+gamma_display_init(gamma_display *dp, png_modifier *pm, png_uint_32 id,
+    double file_gamma, double screen_gamma, png_byte sbit, int threshold_test,
+    int use_input_precision, int scale16, int expand16,
+    int do_background, const png_color_16 *pointer_to_the_background_color,
+    double background_gamma)
+{
+   /* Standard fields */
+   standard_display_init(&dp->this, &pm->this, id, do_read_interlace,
+      pm->use_update_info);
+
+   /* Parameter fields */
+   dp->pm = pm;
+   dp->file_gamma = file_gamma;
+   dp->screen_gamma = screen_gamma;
+   dp->background_gamma = background_gamma;
+   dp->sbit = sbit;
+   dp->threshold_test = threshold_test;
+   dp->use_input_precision = use_input_precision;
+   dp->scale16 = scale16;
+   dp->expand16 = expand16;
+   dp->do_background = do_background;
+   if (do_background && pointer_to_the_background_color != 0)
+      dp->background_color = *pointer_to_the_background_color;
+   else
+      memset(&dp->background_color, 0, sizeof dp->background_color);
+
+   /* Local variable fields */
+   dp->maxerrout = dp->maxerrpc = dp->maxerrabs = 0;
+}
+
+static void
+gamma_info_imp(gamma_display *dp, png_structp pp, png_infop pi)
+{
+   /* Reuse the standard stuff as appropriate. */
+   standard_info_part1(&dp->this, pp, pi);
+
+   /* If requested strip 16 to 8 bits - this is handled automagically below
+    * because the output bit depth is read from the library.  Note that there
+    * are interactions with sBIT but, internally, libpng makes sbit at most
+    * PNG_MAX_GAMMA_8 prior to 1.7 when doing the following.
+    */
+   if (dp->scale16)
+#     ifdef PNG_READ_SCALE_16_TO_8_SUPPORTED
+         png_set_scale_16(pp);
+#     else
+         /* The following works both in 1.5.4 and earlier versions: */
+#        ifdef PNG_READ_16_TO_8_SUPPORTED
+            png_set_strip_16(pp);
+#        else
+            png_error(pp, "scale16 (16 to 8 bit conversion) not supported");
+#        endif
+#     endif
+
+   if (dp->expand16)
+#     ifdef PNG_READ_EXPAND_16_SUPPORTED
+         png_set_expand_16(pp);
+#     else
+         png_error(pp, "expand16 (8 to 16 bit conversion) not supported");
+#     endif
+
+   if (dp->do_background >= ALPHA_MODE_OFFSET)
+   {
+#     ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+      {
+         /* This tests the alpha mode handling, if supported. */
+         int mode = dp->do_background - ALPHA_MODE_OFFSET;
+
+         /* The gamma value is the output gamma, and is in the standard,
+          * non-inverted, represenation.  It provides a default for the PNG file
+          * gamma, but since the file has a gAMA chunk this does not matter.
+          */
+         const double sg = dp->screen_gamma;
+#        ifndef PNG_FLOATING_POINT_SUPPORTED
+            const png_fixed_point g = fix(sg);
+#        endif
+
+#        ifdef PNG_FLOATING_POINT_SUPPORTED
+            png_set_alpha_mode(pp, mode, sg);
+#        else
+            png_set_alpha_mode_fixed(pp, mode, g);
+#        endif
+
+         /* However, for the standard Porter-Duff algorithm the output defaults
+          * to be linear, so if the test requires non-linear output it must be
+          * corrected here.
+          */
+         if (mode == PNG_ALPHA_STANDARD && sg != 1)
+         {
+#           ifdef PNG_FLOATING_POINT_SUPPORTED
+               png_set_gamma(pp, sg, dp->file_gamma);
+#           else
+               png_fixed_point f = fix(dp->file_gamma);
+               png_set_gamma_fixed(pp, g, f);
+#           endif
+         }
+      }
+#     else
+         png_error(pp, "alpha mode handling not supported");
+#     endif
+   }
+
+   else
+   {
+      /* Set up gamma processing. */
+#     ifdef PNG_FLOATING_POINT_SUPPORTED
+         png_set_gamma(pp, dp->screen_gamma, dp->file_gamma);
+#     else
+      {
+         png_fixed_point s = fix(dp->screen_gamma);
+         png_fixed_point f = fix(dp->file_gamma);
+         png_set_gamma_fixed(pp, s, f);
+      }
+#     endif
+
+      if (dp->do_background)
+      {
+#     ifdef PNG_READ_BACKGROUND_SUPPORTED
+         /* NOTE: this assumes the caller provided the correct background gamma!
+          */
+         const double bg = dp->background_gamma;
+#        ifndef PNG_FLOATING_POINT_SUPPORTED
+            const png_fixed_point g = fix(bg);
+#        endif
+
+#        ifdef PNG_FLOATING_POINT_SUPPORTED
+            png_set_background(pp, &dp->background_color, dp->do_background,
+               0/*need_expand*/, bg);
+#        else
+            png_set_background_fixed(pp, &dp->background_color,
+               dp->do_background, 0/*need_expand*/, g);
+#        endif
+#     else
+         png_error(pp, "png_set_background not supported");
+#     endif
+      }
+   }
+
+   {
+      int i = dp->this.use_update_info;
+      /* Always do one call, even if use_update_info is 0. */
+      do
+         png_read_update_info(pp, pi);
+      while (--i > 0);
+   }
+
+   /* Now we may get a different cbRow: */
+   standard_info_part2(&dp->this, pp, pi, 1 /*images*/);
+}
+
+static void PNGCBAPI
+gamma_info(png_structp pp, png_infop pi)
+{
+   gamma_info_imp(voidcast(gamma_display*, png_get_progressive_ptr(pp)), pp,
+      pi);
+}
+
+/* Validate a single component value - the routine gets the input and output
+ * sample values as unscaled PNG component values along with a cache of all the
+ * information required to validate the values.
+ */
+typedef struct validate_info
+{
+   png_const_structp  pp;
+   gamma_display *dp;
+   png_byte sbit;
+   int use_input_precision;
+   int do_background;
+   int scale16;
+   unsigned int sbit_max;
+   unsigned int isbit_shift;
+   unsigned int outmax;
+
+   double gamma_correction; /* Overall correction required. */
+   double file_inverse;     /* Inverse of file gamma. */
+   double screen_gamma;
+   double screen_inverse;   /* Inverse of screen gamma. */
+
+   double background_red;   /* Linear background value, red or gray. */
+   double background_green;
+   double background_blue;
+
+   double maxabs;
+   double maxpc;
+   double maxcalc;
+   double maxout;
+   double maxout_total;     /* Total including quantization error */
+   double outlog;
+   int    outquant;
+}
+validate_info;
+
+static void
+init_validate_info(validate_info *vi, gamma_display *dp, png_const_structp pp,
+    int in_depth, int out_depth)
+{
+   const unsigned int outmax = (1U<<out_depth)-1;
+
+   vi->pp = pp;
+   vi->dp = dp;
+
+   if (dp->sbit > 0 && dp->sbit < in_depth)
+   {
+      vi->sbit = dp->sbit;
+      vi->isbit_shift = in_depth - dp->sbit;
+   }
+
+   else
+   {
+      vi->sbit = (png_byte)in_depth;
+      vi->isbit_shift = 0;
+   }
+
+   vi->sbit_max = (1U << vi->sbit)-1;
+
+   /* This mimics the libpng threshold test, '0' is used to prevent gamma
+    * correction in the validation test.
+    */
+   vi->screen_gamma = dp->screen_gamma;
+   if (fabs(vi->screen_gamma-1) < PNG_GAMMA_THRESHOLD)
+      vi->screen_gamma = vi->screen_inverse = 0;
+   else
+      vi->screen_inverse = 1/vi->screen_gamma;
+
+   vi->use_input_precision = dp->use_input_precision;
+   vi->outmax = outmax;
+   vi->maxabs = abserr(dp->pm, in_depth, out_depth);
+   vi->maxpc = pcerr(dp->pm, in_depth, out_depth);
+   vi->maxcalc = calcerr(dp->pm, in_depth, out_depth);
+   vi->maxout = outerr(dp->pm, in_depth, out_depth);
+   vi->outquant = output_quantization_factor(dp->pm, in_depth, out_depth);
+   vi->maxout_total = vi->maxout + vi->outquant * .5;
+   vi->outlog = outlog(dp->pm, in_depth, out_depth);
+
+   if ((dp->this.colour_type & PNG_COLOR_MASK_ALPHA) != 0 ||
+      (dp->this.colour_type == 3 && dp->this.is_transparent) ||
+      ((dp->this.colour_type == 0 || dp->this.colour_type == 2) &&
+       dp->this.has_tRNS))
+   {
+      vi->do_background = dp->do_background;
+
+      if (vi->do_background != 0)
+      {
+         const double bg_inverse = 1/dp->background_gamma;
+         double r, g, b;
+
+         /* Caller must at least put the gray value into the red channel */
+         r = dp->background_color.red; r /= outmax;
+         g = dp->background_color.green; g /= outmax;
+         b = dp->background_color.blue; b /= outmax;
+
+#     if 0
+         /* libpng doesn't do this optimization, if we do pngvalid will fail.
+          */
+         if (fabs(bg_inverse-1) >= PNG_GAMMA_THRESHOLD)
+#     endif
+         {
+            r = pow(r, bg_inverse);
+            g = pow(g, bg_inverse);
+            b = pow(b, bg_inverse);
+         }
+
+         vi->background_red = r;
+         vi->background_green = g;
+         vi->background_blue = b;
+      }
+   }
+   else /* Do not expect any background processing */
+      vi->do_background = 0;
+
+   if (vi->do_background == 0)
+      vi->background_red = vi->background_green = vi->background_blue = 0;
+
+   vi->gamma_correction = 1/(dp->file_gamma*dp->screen_gamma);
+   if (fabs(vi->gamma_correction-1) < PNG_GAMMA_THRESHOLD)
+      vi->gamma_correction = 0;
+
+   vi->file_inverse = 1/dp->file_gamma;
+   if (fabs(vi->file_inverse-1) < PNG_GAMMA_THRESHOLD)
+      vi->file_inverse = 0;
+
+   vi->scale16 = dp->scale16;
+}
+
+/* This function handles composition of a single non-alpha component.  The
+ * argument is the input sample value, in the range 0..1, and the alpha value.
+ * The result is the composed, linear, input sample.  If alpha is less than zero
+ * this is the alpha component and the function should not be called!
+ */
+static double
+gamma_component_compose(int do_background, double input_sample, double alpha,
+   double background, int *compose)
+{
+   switch (do_background)
+   {
+#ifdef PNG_READ_BACKGROUND_SUPPORTED
+      case PNG_BACKGROUND_GAMMA_SCREEN:
+      case PNG_BACKGROUND_GAMMA_FILE:
+      case PNG_BACKGROUND_GAMMA_UNIQUE:
+         /* Standard PNG background processing. */
+         if (alpha < 1)
+         {
+            if (alpha > 0)
+            {
+               input_sample = input_sample * alpha + background * (1-alpha);
+               if (compose != NULL)
+                  *compose = 1;
+            }
+
+            else
+               input_sample = background;
+         }
+         break;
+#endif
+
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+      case ALPHA_MODE_OFFSET + PNG_ALPHA_STANDARD:
+      case ALPHA_MODE_OFFSET + PNG_ALPHA_BROKEN:
+         /* The components are premultiplied in either case and the output is
+          * gamma encoded (to get standard Porter-Duff we expect the output
+          * gamma to be set to 1.0!)
+          */
+      case ALPHA_MODE_OFFSET + PNG_ALPHA_OPTIMIZED:
+         /* The optimization is that the partial-alpha entries are linear
+          * while the opaque pixels are gamma encoded, but this only affects the
+          * output encoding.
+          */
+         if (alpha < 1)
+         {
+            if (alpha > 0)
+            {
+               input_sample *= alpha;
+               if (compose != NULL)
+                  *compose = 1;
+            }
+
+            else
+               input_sample = 0;
+         }
+         break;
+#endif
+
+      default:
+         /* Standard cases where no compositing is done (so the component
+          * value is already correct.)
+          */
+         UNUSED(alpha)
+         UNUSED(background)
+         UNUSED(compose)
+         break;
+   }
+
+   return input_sample;
+}
+
+/* This API returns the encoded *input* component, in the range 0..1 */
+static double
+gamma_component_validate(const char *name, const validate_info *vi,
+    const unsigned int id, const unsigned int od,
+    const double alpha /* <0 for the alpha channel itself */,
+    const double background /* component background value */)
+{
+   const unsigned int isbit = id >> vi->isbit_shift;
+   const unsigned int sbit_max = vi->sbit_max;
+   const unsigned int outmax = vi->outmax;
+   const int do_background = vi->do_background;
+
+   double i;
+
+   /* First check on the 'perfect' result obtained from the digitized input
+    * value, id, and compare this against the actual digitized result, 'od'.
+    * 'i' is the input result in the range 0..1:
+    */
+   i = isbit; i /= sbit_max;
+
+   /* Check for the fast route: if we don't do any background composition or if
+    * this is the alpha channel ('alpha' < 0) or if the pixel is opaque then
+    * just use the gamma_correction field to correct to the final output gamma.
+    */
+   if (alpha == 1 /* opaque pixel component */ || !do_background
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+      || do_background == ALPHA_MODE_OFFSET + PNG_ALPHA_PNG
+#endif
+      || (alpha < 0 /* alpha channel */
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+      && do_background != ALPHA_MODE_OFFSET + PNG_ALPHA_BROKEN
+#endif
+      ))
+   {
+      /* Then get the gamma corrected version of 'i' and compare to 'od', any
+       * error less than .5 is insignificant - just quantization of the output
+       * value to the nearest digital value (nevertheless the error is still
+       * recorded - it's interesting ;-)
+       */
+      double encoded_sample = i;
+      double encoded_error;
+
+      /* alpha less than 0 indicates the alpha channel, which is always linear
+       */
+      if (alpha >= 0 && vi->gamma_correction > 0)
+         encoded_sample = pow(encoded_sample, vi->gamma_correction);
+      encoded_sample *= outmax;
+
+      encoded_error = fabs(od-encoded_sample);
+
+      if (encoded_error > vi->dp->maxerrout)
+         vi->dp->maxerrout = encoded_error;
+
+      if (encoded_error < vi->maxout_total && encoded_error < vi->outlog)
+         return i;
+   }
+
+   /* The slow route - attempt to do linear calculations. */
+   /* There may be an error, or background processing is required, so calculate
+    * the actual sample values - unencoded light intensity values.  Note that in
+    * practice these are not completely unencoded because they include a
+    * 'viewing correction' to decrease or (normally) increase the perceptual
+    * contrast of the image.  There's nothing we can do about this - we don't
+    * know what it is - so assume the unencoded value is perceptually linear.
+    */
+   {
+      double input_sample = i; /* In range 0..1 */
+      double output, error, encoded_sample, encoded_error;
+      double es_lo, es_hi;
+      int compose = 0;           /* Set to one if composition done */
+      int output_is_encoded;     /* Set if encoded to screen gamma */
+      int log_max_error = 1;     /* Check maximum error values */
+      png_const_charp pass = 0;  /* Reason test passes (or 0 for fail) */
+
+      /* Convert to linear light (with the above caveat.)  The alpha channel is
+       * already linear.
+       */
+      if (alpha >= 0)
+      {
+         int tcompose;
+
+         if (vi->file_inverse > 0)
+            input_sample = pow(input_sample, vi->file_inverse);
+
+         /* Handle the compose processing: */
+         tcompose = 0;
+         input_sample = gamma_component_compose(do_background, input_sample,
+            alpha, background, &tcompose);
+
+         if (tcompose)
+            compose = 1;
+      }
+
+      /* And similarly for the output value, but we need to check the background
+       * handling to linearize it correctly.
+       */
+      output = od;
+      output /= outmax;
+
+      output_is_encoded = vi->screen_gamma > 0;
+
+      if (alpha < 0) /* The alpha channel */
+      {
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+         if (do_background != ALPHA_MODE_OFFSET + PNG_ALPHA_BROKEN)
+#endif
+         {
+            /* In all other cases the output alpha channel is linear already,
+             * don't log errors here, they are much larger in linear data.
+             */
+            output_is_encoded = 0;
+            log_max_error = 0;
+         }
+      }
+
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+      else /* A component */
+      {
+         if (do_background == ALPHA_MODE_OFFSET + PNG_ALPHA_OPTIMIZED &&
+            alpha < 1) /* the optimized case - linear output */
+         {
+            if (alpha > 0) log_max_error = 0;
+            output_is_encoded = 0;
+         }
+      }
+#endif
+
+      if (output_is_encoded)
+         output = pow(output, vi->screen_gamma);
+
+      /* Calculate (or recalculate) the encoded_sample value and repeat the
+       * check above (unnecessary if we took the fast route, but harmless.)
+       */
+      encoded_sample = input_sample;
+      if (output_is_encoded)
+         encoded_sample = pow(encoded_sample, vi->screen_inverse);
+      encoded_sample *= outmax;
+
+      encoded_error = fabs(od-encoded_sample);
+
+      /* Don't log errors in the alpha channel, or the 'optimized' case,
+       * neither are significant to the overall perception.
+       */
+      if (log_max_error && encoded_error > vi->dp->maxerrout)
+         vi->dp->maxerrout = encoded_error;
+
+      if (encoded_error < vi->maxout_total)
+      {
+         if (encoded_error < vi->outlog)
+            return i;
+
+         /* Test passed but error is bigger than the log limit, record why the
+          * test passed:
+          */
+         pass = "less than maxout:\n";
+      }
+
+      /* i: the original input value in the range 0..1
+       *
+       * pngvalid calculations:
+       *  input_sample: linear result; i linearized and composed, range 0..1
+       *  encoded_sample: encoded result; input_sample scaled to ouput bit depth
+       *
+       * libpng calculations:
+       *  output: linear result; od scaled to 0..1 and linearized
+       *  od: encoded result from libpng
+       */
+
+      /* Now we have the numbers for real errors, both absolute values as as a
+       * percentage of the correct value (output):
+       */
+      error = fabs(input_sample-output);
+
+      if (log_max_error && error > vi->dp->maxerrabs)
+         vi->dp->maxerrabs = error;
+
+      /* The following is an attempt to ignore the tendency of quantization to
+       * dominate the percentage errors for lower result values:
+       */
+      if (log_max_error && input_sample > .5)
+      {
+         double percentage_error = error/input_sample;
+         if (percentage_error > vi->dp->maxerrpc)
+            vi->dp->maxerrpc = percentage_error;
+      }
+
+      /* Now calculate the digitization limits for 'encoded_sample' using the
+       * 'max' values.  Note that maxout is in the encoded space but maxpc and
+       * maxabs are in linear light space.
+       *
+       * First find the maximum error in linear light space, range 0..1:
+       */
+      {
+         double tmp = input_sample * vi->maxpc;
+         if (tmp < vi->maxabs) tmp = vi->maxabs;
+         /* If 'compose' is true the composition was done in linear space using
+          * integer arithmetic.  This introduces an extra error of +/- 0.5 (at
+          * least) in the integer space used.  'maxcalc' records this, taking
+          * into account the possibility that even for 16 bit output 8 bit space
+          * may have been used.
+          */
+         if (compose && tmp < vi->maxcalc) tmp = vi->maxcalc;
+
+         /* The 'maxout' value refers to the encoded result, to compare with
+          * this encode input_sample adjusted by the maximum error (tmp) above.
+          */
+         es_lo = encoded_sample - vi->maxout;
+
+         if (es_lo > 0 && input_sample-tmp > 0)
+         {
+            double low_value = input_sample-tmp;
+            if (output_is_encoded)
+               low_value = pow(low_value, vi->screen_inverse);
+            low_value *= outmax;
+            if (low_value < es_lo) es_lo = low_value;
+
+            /* Quantize this appropriately: */
+            es_lo = ceil(es_lo / vi->outquant - .5) * vi->outquant;
+         }
+
+         else
+            es_lo = 0;
+
+         es_hi = encoded_sample + vi->maxout;
+
+         if (es_hi < outmax && input_sample+tmp < 1)
+         {
+            double high_value = input_sample+tmp;
+            if (output_is_encoded)
+               high_value = pow(high_value, vi->screen_inverse);
+            high_value *= outmax;
+            if (high_value > es_hi) es_hi = high_value;
+
+            es_hi = floor(es_hi / vi->outquant + .5) * vi->outquant;
+         }
+
+         else
+            es_hi = outmax;
+      }
+
+      /* The primary test is that the final encoded value returned by the
+       * library should be between the two limits (inclusive) that were
+       * calculated above.
+       */
+      if (od >= es_lo && od <= es_hi)
+      {
+         /* The value passes, but we may need to log the information anyway. */
+         if (encoded_error < vi->outlog)
+            return i;
+
+         if (pass == 0)
+            pass = "within digitization limits:\n";
+      }
+
+      {
+         /* There has been an error in processing, or we need to log this
+          * value.
+          */
+         double is_lo, is_hi;
+
+         /* pass is set at this point if either of the tests above would have
+          * passed.  Don't do these additional tests here - just log the
+          * original [es_lo..es_hi] values.
+          */
+         if (pass == 0 && vi->use_input_precision && vi->dp->sbit)
+         {
+            /* Ok, something is wrong - this actually happens in current libpng
+             * 16-to-8 processing.  Assume that the input value (id, adjusted
+             * for sbit) can be anywhere between value-.5 and value+.5 - quite a
+             * large range if sbit is low.
+             *
+             * NOTE: at present because the libpng gamma table stuff has been
+             * changed to use a rounding algorithm to correct errors in 8-bit
+             * calculations the precise sbit calculation (a shift) has been
+             * lost.  This can result in up to a +/-1 error in the presence of
+             * an sbit less than the bit depth.
+             */
+#           if PNG_LIBPNG_VER < 10700
+#              define SBIT_ERROR .5
+#           else
+#              define SBIT_ERROR 1.
+#           endif
+            double tmp = (isbit - SBIT_ERROR)/sbit_max;
+
+            if (tmp <= 0)
+               tmp = 0;
+
+            else if (alpha >= 0 && vi->file_inverse > 0 && tmp < 1)
+               tmp = pow(tmp, vi->file_inverse);
+
+            tmp = gamma_component_compose(do_background, tmp, alpha, background,
+               NULL);
+
+            if (output_is_encoded && tmp > 0 && tmp < 1)
+               tmp = pow(tmp, vi->screen_inverse);
+
+            is_lo = ceil(outmax * tmp - vi->maxout_total);
+
+            if (is_lo < 0)
+               is_lo = 0;
+
+            tmp = (isbit + SBIT_ERROR)/sbit_max;
+
+            if (tmp >= 1)
+               tmp = 1;
+
+            else if (alpha >= 0 && vi->file_inverse > 0 && tmp < 1)
+               tmp = pow(tmp, vi->file_inverse);
+
+            tmp = gamma_component_compose(do_background, tmp, alpha, background,
+               NULL);
+
+            if (output_is_encoded && tmp > 0 && tmp < 1)
+               tmp = pow(tmp, vi->screen_inverse);
+
+            is_hi = floor(outmax * tmp + vi->maxout_total);
+
+            if (is_hi > outmax)
+               is_hi = outmax;
+
+            if (!(od < is_lo || od > is_hi))
+            {
+               if (encoded_error < vi->outlog)
+                  return i;
+
+               pass = "within input precision limits:\n";
+            }
+
+            /* One last chance.  If this is an alpha channel and the 16to8
+             * option has been used and 'inaccurate' scaling is used then the
+             * bit reduction is obtained by simply using the top 8 bits of the
+             * value.
+             *
+             * This is only done for older libpng versions when the 'inaccurate'
+             * (chop) method of scaling was used.
+             */
+#           ifndef PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
+#              if PNG_LIBPNG_VER < 10504
+                  /* This may be required for other components in the future,
+                   * but at present the presence of gamma correction effectively
+                   * prevents the errors in the component scaling (I don't quite
+                   * understand why, but since it's better this way I care not
+                   * to ask, JB 20110419.)
+                   */
+                  if (pass == 0 && alpha < 0 && vi->scale16 && vi->sbit > 8 &&
+                     vi->sbit + vi->isbit_shift == 16)
+                  {
+                     tmp = ((id >> 8) - .5)/255;
+
+                     if (tmp > 0)
+                     {
+                        is_lo = ceil(outmax * tmp - vi->maxout_total);
+                        if (is_lo < 0) is_lo = 0;
+                     }
+
+                     else
+                        is_lo = 0;
+
+                     tmp = ((id >> 8) + .5)/255;
+
+                     if (tmp < 1)
+                     {
+                        is_hi = floor(outmax * tmp + vi->maxout_total);
+                        if (is_hi > outmax) is_hi = outmax;
+                     }
+
+                     else
+                        is_hi = outmax;
+
+                     if (!(od < is_lo || od > is_hi))
+                     {
+                        if (encoded_error < vi->outlog)
+                           return i;
+
+                        pass = "within 8 bit limits:\n";
+                     }
+                  }
+#              endif
+#           endif
+         }
+         else /* !use_input_precision */
+            is_lo = es_lo, is_hi = es_hi;
+
+         /* Attempt to output a meaningful error/warning message: the message
+          * output depends on the background/composite operation being performed
+          * because this changes what parameters were actually used above.
+          */
+         {
+            size_t pos = 0;
+            /* Need either 1/255 or 1/65535 precision here; 3 or 6 decimal
+             * places.  Just use outmax to work out which.
+             */
+            int precision = (outmax >= 1000 ? 6 : 3);
+            int use_input=1, use_background=0, do_compose=0;
+            char msg[256];
+
+            if (pass != 0)
+               pos = safecat(msg, sizeof msg, pos, "\n\t");
+
+            /* Set up the various flags, the output_is_encoded flag above
+             * is also used below.  do_compose is just a double check.
+             */
+            switch (do_background)
+            {
+#           ifdef PNG_READ_BACKGROUND_SUPPORTED
+               case PNG_BACKGROUND_GAMMA_SCREEN:
+               case PNG_BACKGROUND_GAMMA_FILE:
+               case PNG_BACKGROUND_GAMMA_UNIQUE:
+                  use_background = (alpha >= 0 && alpha < 1);
+                  /*FALL THROUGH*/
+#           endif
+#           ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+               case ALPHA_MODE_OFFSET + PNG_ALPHA_STANDARD:
+               case ALPHA_MODE_OFFSET + PNG_ALPHA_BROKEN:
+               case ALPHA_MODE_OFFSET + PNG_ALPHA_OPTIMIZED:
+#           endif /* ALPHA_MODE_SUPPORTED */
+               do_compose = (alpha > 0 && alpha < 1);
+               use_input = (alpha != 0);
+               break;
+
+            default:
+               break;
+            }
+
+            /* Check the 'compose' flag */
+            if (compose != do_compose)
+               png_error(vi->pp, "internal error (compose)");
+
+            /* 'name' is the component name */
+            pos = safecat(msg, sizeof msg, pos, name);
+            pos = safecat(msg, sizeof msg, pos, "(");
+            pos = safecatn(msg, sizeof msg, pos, id);
+            if (use_input || pass != 0/*logging*/)
+            {
+               if (isbit != id)
+               {
+                  /* sBIT has reduced the precision of the input: */
+                  pos = safecat(msg, sizeof msg, pos, ", sbit(");
+                  pos = safecatn(msg, sizeof msg, pos, vi->sbit);
+                  pos = safecat(msg, sizeof msg, pos, "): ");
+                  pos = safecatn(msg, sizeof msg, pos, isbit);
+               }
+               pos = safecat(msg, sizeof msg, pos, "/");
+               /* The output is either "id/max" or "id sbit(sbit): isbit/max" */
+               pos = safecatn(msg, sizeof msg, pos, vi->sbit_max);
+            }
+            pos = safecat(msg, sizeof msg, pos, ")");
+
+            /* A component may have been multiplied (in linear space) by the
+             * alpha value, 'compose' says whether this is relevant.
+             */
+            if (compose || pass != 0)
+            {
+               /* If any form of composition is being done report our
+                * calculated linear value here (the code above doesn't record
+                * the input value before composition is performed, so what
+                * gets reported is the value after composition.)
+                */
+               if (use_input || pass != 0)
+               {
+                  if (vi->file_inverse > 0)
+                  {
+                     pos = safecat(msg, sizeof msg, pos, "^");
+                     pos = safecatd(msg, sizeof msg, pos, vi->file_inverse, 2);
+                  }
+
+                  else
+                     pos = safecat(msg, sizeof msg, pos, "[linear]");
+
+                  pos = safecat(msg, sizeof msg, pos, "*(alpha)");
+                  pos = safecatd(msg, sizeof msg, pos, alpha, precision);
+               }
+
+               /* Now record the *linear* background value if it was used
+                * (this function is not passed the original, non-linear,
+                * value but it is contained in the test name.)
+                */
+               if (use_background)
+               {
+                  pos = safecat(msg, sizeof msg, pos, use_input ? "+" : " ");
+                  pos = safecat(msg, sizeof msg, pos, "(background)");
+                  pos = safecatd(msg, sizeof msg, pos, background, precision);
+                  pos = safecat(msg, sizeof msg, pos, "*");
+                  pos = safecatd(msg, sizeof msg, pos, 1-alpha, precision);
+               }
+            }
+
+            /* Report the calculated value (input_sample) and the linearized
+             * libpng value (output) unless this is just a component gamma
+             * correction.
+             */
+            if (compose || alpha < 0 || pass != 0)
+            {
+               pos = safecat(msg, sizeof msg, pos,
+                  pass != 0 ? " =\n\t" : " = ");
+               pos = safecatd(msg, sizeof msg, pos, input_sample, precision);
+               pos = safecat(msg, sizeof msg, pos, " (libpng: ");
+               pos = safecatd(msg, sizeof msg, pos, output, precision);
+               pos = safecat(msg, sizeof msg, pos, ")");
+
+               /* Finally report the output gamma encoding, if any. */
+               if (output_is_encoded)
+               {
+                  pos = safecat(msg, sizeof msg, pos, " ^");
+                  pos = safecatd(msg, sizeof msg, pos, vi->screen_inverse, 2);
+                  pos = safecat(msg, sizeof msg, pos, "(to screen) =");
+               }
+
+               else
+                  pos = safecat(msg, sizeof msg, pos, " [screen is linear] =");
+            }
+
+            if ((!compose && alpha >= 0) || pass != 0)
+            {
+               if (pass != 0) /* logging */
+                  pos = safecat(msg, sizeof msg, pos, "\n\t[overall:");
+
+               /* This is the non-composition case, the internal linear
+                * values are irrelevant (though the log below will reveal
+                * them.)  Output a much shorter warning/error message and report
+                * the overall gamma correction.
+                */
+               if (vi->gamma_correction > 0)
+               {
+                  pos = safecat(msg, sizeof msg, pos, " ^");
+                  pos = safecatd(msg, sizeof msg, pos, vi->gamma_correction, 2);
+                  pos = safecat(msg, sizeof msg, pos, "(gamma correction) =");
+               }
+
+               else
+                  pos = safecat(msg, sizeof msg, pos,
+                     " [no gamma correction] =");
+
+               if (pass != 0)
+                  pos = safecat(msg, sizeof msg, pos, "]");
+            }
+
+            /* This is our calculated encoded_sample which should (but does
+             * not) match od:
+             */
+            pos = safecat(msg, sizeof msg, pos, pass != 0 ? "\n\t" : " ");
+            pos = safecatd(msg, sizeof msg, pos, is_lo, 1);
+            pos = safecat(msg, sizeof msg, pos, " < ");
+            pos = safecatd(msg, sizeof msg, pos, encoded_sample, 1);
+            pos = safecat(msg, sizeof msg, pos, " (libpng: ");
+            pos = safecatn(msg, sizeof msg, pos, od);
+            pos = safecat(msg, sizeof msg, pos, ")");
+            pos = safecat(msg, sizeof msg, pos, "/");
+            pos = safecatn(msg, sizeof msg, pos, outmax);
+            pos = safecat(msg, sizeof msg, pos, " < ");
+            pos = safecatd(msg, sizeof msg, pos, is_hi, 1);
+
+            if (pass == 0) /* The error condition */
+            {
+#              ifdef PNG_WARNINGS_SUPPORTED
+                  png_warning(vi->pp, msg);
+#              else
+                  store_warning(vi->pp, msg);
+#              endif
+            }
+
+            else /* logging this value */
+               store_verbose(&vi->dp->pm->this, vi->pp, pass, msg);
+         }
+      }
+   }
+
+   return i;
+}
+
+static void
+gamma_image_validate(gamma_display *dp, png_const_structp pp,
+   png_infop pi)
+{
+   /* Get some constants derived from the input and output file formats: */
+   const png_store* const ps = dp->this.ps;
+   const png_byte in_ct = dp->this.colour_type;
+   const png_byte in_bd = dp->this.bit_depth;
+   const png_uint_32 w = dp->this.w;
+   const png_uint_32 h = dp->this.h;
+   const size_t cbRow = dp->this.cbRow;
+   const png_byte out_ct = png_get_color_type(pp, pi);
+   const png_byte out_bd = png_get_bit_depth(pp, pi);
+
+   /* There are three sources of error, firstly the quantization in the
+    * file encoding, determined by sbit and/or the file depth, secondly
+    * the output (screen) gamma and thirdly the output file encoding.
+    *
+    * Since this API receives the screen and file gamma in double
+    * precision it is possible to calculate an exact answer given an input
+    * pixel value.  Therefore we assume that the *input* value is exact -
+    * sample/maxsample - calculate the corresponding gamma corrected
+    * output to the limits of double precision arithmetic and compare with
+    * what libpng returns.
+    *
+    * Since the library must quantize the output to 8 or 16 bits there is
+    * a fundamental limit on the accuracy of the output of +/-.5 - this
+    * quantization limit is included in addition to the other limits
+    * specified by the paramaters to the API.  (Effectively, add .5
+    * everywhere.)
+    *
+    * The behavior of the 'sbit' paramter is defined by section 12.5
+    * (sample depth scaling) of the PNG spec.  That section forces the
+    * decoder to assume that the PNG values have been scaled if sBIT is
+    * present:
+    *
+    *     png-sample = floor( input-sample * (max-out/max-in) + .5);
+    *
+    * This means that only a subset of the possible PNG values should
+    * appear in the input. However, the spec allows the encoder to use a
+    * variety of approximations to the above and doesn't require any
+    * restriction of the values produced.
+    *
+    * Nevertheless the spec requires that the upper 'sBIT' bits of the
+    * value stored in a PNG file be the original sample bits.
+    * Consequently the code below simply scales the top sbit bits by
+    * (1<<sbit)-1 to obtain an original sample value.
+    *
+    * Because there is limited precision in the input it is arguable that
+    * an acceptable result is any valid result from input-.5 to input+.5.
+    * The basic tests below do not do this, however if 'use_input_precision'
+    * is set a subsequent test is performed above.
+    */
+   const unsigned int samples_per_pixel = (out_ct & 2U) ? 3U : 1U;
+   int processing;
+   png_uint_32 y;
+   const store_palette_entry *in_palette = dp->this.palette;
+   const int in_is_transparent = dp->this.is_transparent;
+   int process_tRNS;
+   int out_npalette = -1;
+   int out_is_transparent = 0; /* Just refers to the palette case */
+   store_palette out_palette;
+   validate_info vi;
+
+   /* Check for row overwrite errors */
+   store_image_check(dp->this.ps, pp, 0);
+
+   /* Supply the input and output sample depths here - 8 for an indexed image,
+    * otherwise the bit depth.
+    */
+   init_validate_info(&vi, dp, pp, in_ct==3?8:in_bd, out_ct==3?8:out_bd);
+
+   processing = (vi.gamma_correction > 0 && !dp->threshold_test)
+      || in_bd != out_bd || in_ct != out_ct || vi.do_background;
+   process_tRNS = dp->this.has_tRNS && vi.do_background;
+
+   /* TODO: FIX THIS: MAJOR BUG!  If the transformations all happen inside
+    * the palette there is no way of finding out, because libpng fails to
+    * update the palette on png_read_update_info.  Indeed, libpng doesn't
+    * even do the required work until much later, when it doesn't have any
+    * info pointer.  Oops.  For the moment 'processing' is turned off if
+    * out_ct is palette.
+    */
+   if (in_ct == 3 && out_ct == 3)
+      processing = 0;
+
+   if (processing && out_ct == 3)
+      out_is_transparent = read_palette(out_palette, &out_npalette, pp, pi);
+
+   for (y=0; y<h; ++y)
+   {
+      png_const_bytep pRow = store_image_row(ps, pp, 0, y);
+      png_byte std[STANDARD_ROWMAX];
+
+      transform_row(pp, std, in_ct, in_bd, y);
+
+      if (processing)
+      {
+         unsigned int x;
+
+         for (x=0; x<w; ++x)
+         {
+            double alpha = 1; /* serves as a flag value */
+
+            /* Record the palette index for index images. */
+            const unsigned int in_index =
+               in_ct == 3 ? sample(std, 3, in_bd, x, 0, 0, 0) : 256;
+            const unsigned int out_index =
+               out_ct == 3 ? sample(std, 3, out_bd, x, 0, 0, 0) : 256;
+
+            /* Handle input alpha - png_set_background will cause the output
+             * alpha to disappear so there is nothing to check.
+             */
+            if ((in_ct & PNG_COLOR_MASK_ALPHA) != 0 ||
+                (in_ct == 3 && in_is_transparent))
+            {
+               const unsigned int input_alpha = in_ct == 3 ?
+                  dp->this.palette[in_index].alpha :
+                  sample(std, in_ct, in_bd, x, samples_per_pixel, 0, 0);
+
+               unsigned int output_alpha = 65536 /* as a flag value */;
+
+               if (out_ct == 3)
+               {
+                  if (out_is_transparent)
+                     output_alpha = out_palette[out_index].alpha;
+               }
+
+               else if ((out_ct & PNG_COLOR_MASK_ALPHA) != 0)
+                  output_alpha = sample(pRow, out_ct, out_bd, x,
+                     samples_per_pixel, 0, 0);
+
+               if (output_alpha != 65536)
+                  alpha = gamma_component_validate("alpha", &vi, input_alpha,
+                     output_alpha, -1/*alpha*/, 0/*background*/);
+
+               else /* no alpha in output */
+               {
+                  /* This is a copy of the calculation of 'i' above in order to
+                   * have the alpha value to use in the background calculation.
+                   */
+                  alpha = input_alpha >> vi.isbit_shift;
+                  alpha /= vi.sbit_max;
+               }
+            }
+
+            else if (process_tRNS)
+            {
+               /* alpha needs to be set appropriately for this pixel, it is
+                * currently 1 and needs to be 0 for an input pixel which matches
+                * the values in tRNS.
+                */
+               switch (in_ct)
+               {
+                  case 0: /* gray */
+                     if (sample(std, in_ct, in_bd, x, 0, 0, 0) ==
+                           dp->this.transparent.red)
+                        alpha = 0;
+                     break;
+
+                  case 2: /* RGB */
+                     if (sample(std, in_ct, in_bd, x, 0, 0, 0) ==
+                           dp->this.transparent.red &&
+                         sample(std, in_ct, in_bd, x, 1, 0, 0) ==
+                           dp->this.transparent.green &&
+                         sample(std, in_ct, in_bd, x, 2, 0, 0) ==
+                           dp->this.transparent.blue)
+                        alpha = 0;
+                     break;
+
+                  default:
+                     break;
+               }
+            }
+
+            /* Handle grayscale or RGB components. */
+            if ((in_ct & PNG_COLOR_MASK_COLOR) == 0) /* grayscale */
+               (void)gamma_component_validate("gray", &vi,
+                  sample(std, in_ct, in_bd, x, 0, 0, 0),
+                  sample(pRow, out_ct, out_bd, x, 0, 0, 0),
+                  alpha/*component*/, vi.background_red);
+            else /* RGB or palette */
+            {
+               (void)gamma_component_validate("red", &vi,
+                  in_ct == 3 ? in_palette[in_index].red :
+                     sample(std, in_ct, in_bd, x, 0, 0, 0),
+                  out_ct == 3 ? out_palette[out_index].red :
+                     sample(pRow, out_ct, out_bd, x, 0, 0, 0),
+                  alpha/*component*/, vi.background_red);
+
+               (void)gamma_component_validate("green", &vi,
+                  in_ct == 3 ? in_palette[in_index].green :
+                     sample(std, in_ct, in_bd, x, 1, 0, 0),
+                  out_ct == 3 ? out_palette[out_index].green :
+                     sample(pRow, out_ct, out_bd, x, 1, 0, 0),
+                  alpha/*component*/, vi.background_green);
+
+               (void)gamma_component_validate("blue", &vi,
+                  in_ct == 3 ? in_palette[in_index].blue :
+                     sample(std, in_ct, in_bd, x, 2, 0, 0),
+                  out_ct == 3 ? out_palette[out_index].blue :
+                     sample(pRow, out_ct, out_bd, x, 2, 0, 0),
+                  alpha/*component*/, vi.background_blue);
+            }
+         }
+      }
+
+      else if (memcmp(std, pRow, cbRow) != 0)
+      {
+         char msg[64];
+
+         /* No transform is expected on the threshold tests. */
+         sprintf(msg, "gamma: below threshold row %lu changed",
+            (unsigned long)y);
+
+         png_error(pp, msg);
+      }
+   } /* row (y) loop */
+
+   dp->this.ps->validated = 1;
+}
+
+static void PNGCBAPI
+gamma_end(png_structp ppIn, png_infop pi)
+{
+   png_const_structp pp = ppIn;
+   gamma_display *dp = voidcast(gamma_display*, png_get_progressive_ptr(pp));
+
+   if (!dp->this.speed)
+      gamma_image_validate(dp, pp, pi);
+   else
+      dp->this.ps->validated = 1;
+}
+
+/* A single test run checking a gamma transformation.
+ *
+ * maxabs: maximum absolute error as a fraction
+ * maxout: maximum output error in the output units
+ * maxpc:  maximum percentage error (as a percentage)
+ */
+static void
+gamma_test(png_modifier *pmIn, const png_byte colour_typeIn,
+    const png_byte bit_depthIn, const int palette_numberIn,
+    const int interlace_typeIn,
+    const double file_gammaIn, const double screen_gammaIn,
+    const png_byte sbitIn, const int threshold_testIn,
+    const char *name,
+    const int use_input_precisionIn, const int scale16In,
+    const int expand16In, const int do_backgroundIn,
+    const png_color_16 *bkgd_colorIn, double bkgd_gammaIn)
+{
+   gamma_display d;
+   context(&pmIn->this, fault);
+
+   gamma_display_init(&d, pmIn, FILEID(colour_typeIn, bit_depthIn,
+      palette_numberIn, interlace_typeIn, 0, 0, 0),
+      file_gammaIn, screen_gammaIn, sbitIn,
+      threshold_testIn, use_input_precisionIn, scale16In,
+      expand16In, do_backgroundIn, bkgd_colorIn, bkgd_gammaIn);
+
+   Try
+   {
+      png_structp pp;
+      png_infop pi;
+      gama_modification gama_mod;
+      srgb_modification srgb_mod;
+      sbit_modification sbit_mod;
+
+      /* For the moment don't use the png_modifier support here. */
+      d.pm->encoding_counter = 0;
+      modifier_set_encoding(d.pm); /* Just resets everything */
+      d.pm->current_gamma = d.file_gamma;
+
+      /* Make an appropriate modifier to set the PNG file gamma to the
+       * given gamma value and the sBIT chunk to the given precision.
+       */
+      d.pm->modifications = NULL;
+      gama_modification_init(&gama_mod, d.pm, d.file_gamma);
+      srgb_modification_init(&srgb_mod, d.pm, 127 /*delete*/);
+      if (d.sbit > 0)
+         sbit_modification_init(&sbit_mod, d.pm, d.sbit);
+
+      modification_reset(d.pm->modifications);
+
+      /* Get a png_struct for reading the image. */
+      pp = set_modifier_for_read(d.pm, &pi, d.this.id, name);
+      standard_palette_init(&d.this);
+
+      /* Introduce the correct read function. */
+      if (d.pm->this.progressive)
+      {
+         /* Share the row function with the standard implementation. */
+         png_set_progressive_read_fn(pp, &d, gamma_info, progressive_row,
+            gamma_end);
+
+         /* Now feed data into the reader until we reach the end: */
+         modifier_progressive_read(d.pm, pp, pi);
+      }
+      else
+      {
+         /* modifier_read expects a png_modifier* */
+         png_set_read_fn(pp, d.pm, modifier_read);
+
+         /* Check the header values: */
+         png_read_info(pp, pi);
+
+         /* Process the 'info' requirements. Only one image is generated */
+         gamma_info_imp(&d, pp, pi);
+
+         sequential_row(&d.this, pp, pi, -1, 0);
+
+         if (!d.this.speed)
+            gamma_image_validate(&d, pp, pi);
+         else
+            d.this.ps->validated = 1;
+      }
+
+      modifier_reset(d.pm);
+
+      if (d.pm->log && !d.threshold_test && !d.this.speed)
+         fprintf(stderr, "%d bit %s %s: max error %f (%.2g, %2g%%)\n",
+            d.this.bit_depth, colour_types[d.this.colour_type], name,
+            d.maxerrout, d.maxerrabs, 100*d.maxerrpc);
+
+      /* Log the summary values too. */
+      if (d.this.colour_type == 0 || d.this.colour_type == 4)
+      {
+         switch (d.this.bit_depth)
+         {
+         case 1:
+            break;
+
+         case 2:
+            if (d.maxerrout > d.pm->error_gray_2)
+               d.pm->error_gray_2 = d.maxerrout;
+
+            break;
+
+         case 4:
+            if (d.maxerrout > d.pm->error_gray_4)
+               d.pm->error_gray_4 = d.maxerrout;
+
+            break;
+
+         case 8:
+            if (d.maxerrout > d.pm->error_gray_8)
+               d.pm->error_gray_8 = d.maxerrout;
+
+            break;
+
+         case 16:
+            if (d.maxerrout > d.pm->error_gray_16)
+               d.pm->error_gray_16 = d.maxerrout;
+
+            break;
+
+         default:
+            png_error(pp, "bad bit depth (internal: 1)");
+         }
+      }
+
+      else if (d.this.colour_type == 2 || d.this.colour_type == 6)
+      {
+         switch (d.this.bit_depth)
+         {
+         case 8:
+
+            if (d.maxerrout > d.pm->error_color_8)
+               d.pm->error_color_8 = d.maxerrout;
+
+            break;
+
+         case 16:
+
+            if (d.maxerrout > d.pm->error_color_16)
+               d.pm->error_color_16 = d.maxerrout;
+
+            break;
+
+         default:
+            png_error(pp, "bad bit depth (internal: 2)");
+         }
+      }
+
+      else if (d.this.colour_type == 3)
+      {
+         if (d.maxerrout > d.pm->error_indexed)
+            d.pm->error_indexed = d.maxerrout;
+      }
+   }
+
+   Catch(fault)
+      modifier_reset(voidcast(png_modifier*,(void*)fault));
+}
+
+static void gamma_threshold_test(png_modifier *pm, png_byte colour_type,
+    png_byte bit_depth, int interlace_type, double file_gamma,
+    double screen_gamma)
+{
+   size_t pos = 0;
+   char name[64];
+   pos = safecat(name, sizeof name, pos, "threshold ");
+   pos = safecatd(name, sizeof name, pos, file_gamma, 3);
+   pos = safecat(name, sizeof name, pos, "/");
+   pos = safecatd(name, sizeof name, pos, screen_gamma, 3);
+
+   (void)gamma_test(pm, colour_type, bit_depth, 0/*palette*/, interlace_type,
+      file_gamma, screen_gamma, 0/*sBIT*/, 1/*threshold test*/, name,
+      0 /*no input precision*/,
+      0 /*no scale16*/, 0 /*no expand16*/, 0 /*no background*/, 0 /*hence*/,
+      0 /*no background gamma*/);
+}
+
+static void
+perform_gamma_threshold_tests(png_modifier *pm)
+{
+   png_byte colour_type = 0;
+   png_byte bit_depth = 0;
+   unsigned int palette_number = 0;
+
+   /* Don't test more than one instance of each palette - it's pointless, in
+    * fact this test is somewhat excessive since libpng doesn't make this
+    * decision based on colour type or bit depth!
+    *
+    * CHANGED: now test two palettes and, as a side effect, images with and
+    * without tRNS.
+    */
+   while (next_format(&colour_type, &bit_depth, &palette_number,
+                      pm->test_lbg_gamma_threshold, pm->test_tRNS))
+      if (palette_number < 2)
+   {
+      double test_gamma = 1.0;
+      while (test_gamma >= .4)
+      {
+         /* There's little point testing the interlacing vs non-interlacing,
+          * but this can be set from the command line.
+          */
+         gamma_threshold_test(pm, colour_type, bit_depth, pm->interlace_type,
+            test_gamma, 1/test_gamma);
+         test_gamma *= .95;
+      }
+
+      /* And a special test for sRGB */
+      gamma_threshold_test(pm, colour_type, bit_depth, pm->interlace_type,
+          .45455, 2.2);
+
+      if (fail(pm))
+         return;
+   }
+}
+
+static void gamma_transform_test(png_modifier *pm,
+   const png_byte colour_type, const png_byte bit_depth,
+   const int palette_number,
+   const int interlace_type, const double file_gamma,
+   const double screen_gamma, const png_byte sbit,
+   const int use_input_precision, const int scale16)
+{
+   size_t pos = 0;
+   char name[64];
+
+   if (sbit != bit_depth && sbit != 0)
+   {
+      pos = safecat(name, sizeof name, pos, "sbit(");
+      pos = safecatn(name, sizeof name, pos, sbit);
+      pos = safecat(name, sizeof name, pos, ") ");
+   }
+
+   else
+      pos = safecat(name, sizeof name, pos, "gamma ");
+
+   if (scale16)
+      pos = safecat(name, sizeof name, pos, "16to8 ");
+
+   pos = safecatd(name, sizeof name, pos, file_gamma, 3);
+   pos = safecat(name, sizeof name, pos, "->");
+   pos = safecatd(name, sizeof name, pos, screen_gamma, 3);
+
+   gamma_test(pm, colour_type, bit_depth, palette_number, interlace_type,
+      file_gamma, screen_gamma, sbit, 0, name, use_input_precision,
+      scale16, pm->test_gamma_expand16, 0 , 0, 0);
+}
+
+static void perform_gamma_transform_tests(png_modifier *pm)
+{
+   png_byte colour_type = 0;
+   png_byte bit_depth = 0;
+   unsigned int palette_number = 0;
+
+   while (next_format(&colour_type, &bit_depth, &palette_number,
+                      pm->test_lbg_gamma_transform, pm->test_tRNS))
+   {
+      unsigned int i, j;
+
+      for (i=0; i<pm->ngamma_tests; ++i) for (j=0; j<pm->ngamma_tests; ++j)
+         if (i != j)
+         {
+            gamma_transform_test(pm, colour_type, bit_depth, palette_number,
+               pm->interlace_type, 1/pm->gammas[i], pm->gammas[j], 0/*sBIT*/,
+               pm->use_input_precision, 0 /*do not scale16*/);
+
+            if (fail(pm))
+               return;
+         }
+   }
+}
+
+static void perform_gamma_sbit_tests(png_modifier *pm)
+{
+   png_byte sbit;
+
+   /* The only interesting cases are colour and grayscale, alpha is ignored here
+    * for overall speed.  Only bit depths where sbit is less than the bit depth
+    * are tested.
+    */
+   for (sbit=pm->sbitlow; sbit<(1<<READ_BDHI); ++sbit)
+   {
+      png_byte colour_type = 0, bit_depth = 0;
+      unsigned int npalette = 0;
+
+      while (next_format(&colour_type, &bit_depth, &npalette,
+                         pm->test_lbg_gamma_sbit, pm->test_tRNS))
+         if ((colour_type & PNG_COLOR_MASK_ALPHA) == 0 &&
+            ((colour_type == 3 && sbit < 8) ||
+            (colour_type != 3 && sbit < bit_depth)))
+      {
+         unsigned int i;
+
+         for (i=0; i<pm->ngamma_tests; ++i)
+         {
+            unsigned int j;
+
+            for (j=0; j<pm->ngamma_tests; ++j) if (i != j)
+            {
+               gamma_transform_test(pm, colour_type, bit_depth, npalette,
+                  pm->interlace_type, 1/pm->gammas[i], pm->gammas[j],
+                  sbit, pm->use_input_precision_sbit, 0 /*scale16*/);
+
+               if (fail(pm))
+                  return;
+            }
+         }
+      }
+   }
+}
+
+/* Note that this requires a 16 bit source image but produces 8 bit output, so
+ * we only need the 16bit write support, but the 16 bit images are only
+ * generated if DO_16BIT is defined.
+ */
+#ifdef DO_16BIT
+static void perform_gamma_scale16_tests(png_modifier *pm)
+{
+#  ifndef PNG_MAX_GAMMA_8
+#     define PNG_MAX_GAMMA_8 11
+#  endif
+#  if defined PNG_MAX_GAMMA_8 || PNG_LIBPNG_VER < 10700
+#     define SBIT_16_TO_8 PNG_MAX_GAMMA_8
+#  else
+#     define SBIT_16_TO_8 16
+#  endif
+   /* Include the alpha cases here. Note that sbit matches the internal value
+    * used by the library - otherwise we will get spurious errors from the
+    * internal sbit style approximation.
+    *
+    * The threshold test is here because otherwise the 16 to 8 conversion will
+    * proceed *without* gamma correction, and the tests above will fail (but not
+    * by much) - this could be fixed, it only appears with the -g option.
+    */
+   unsigned int i, j;
+   for (i=0; i<pm->ngamma_tests; ++i)
+   {
+      for (j=0; j<pm->ngamma_tests; ++j)
+      {
+         if (i != j &&
+             fabs(pm->gammas[j]/pm->gammas[i]-1) >= PNG_GAMMA_THRESHOLD)
+         {
+            gamma_transform_test(pm, 0, 16, 0, pm->interlace_type,
+               1/pm->gammas[i], pm->gammas[j], SBIT_16_TO_8,
+               pm->use_input_precision_16to8, 1 /*scale16*/);
+
+            if (fail(pm))
+               return;
+
+            gamma_transform_test(pm, 2, 16, 0, pm->interlace_type,
+               1/pm->gammas[i], pm->gammas[j], SBIT_16_TO_8,
+               pm->use_input_precision_16to8, 1 /*scale16*/);
+
+            if (fail(pm))
+               return;
+
+            gamma_transform_test(pm, 4, 16, 0, pm->interlace_type,
+               1/pm->gammas[i], pm->gammas[j], SBIT_16_TO_8,
+               pm->use_input_precision_16to8, 1 /*scale16*/);
+
+            if (fail(pm))
+               return;
+
+            gamma_transform_test(pm, 6, 16, 0, pm->interlace_type,
+               1/pm->gammas[i], pm->gammas[j], SBIT_16_TO_8,
+               pm->use_input_precision_16to8, 1 /*scale16*/);
+
+            if (fail(pm))
+               return;
+         }
+      }
+   }
+}
+#endif /* 16 to 8 bit conversion */
+
+#if defined(PNG_READ_BACKGROUND_SUPPORTED) ||\
+   defined(PNG_READ_ALPHA_MODE_SUPPORTED)
+static void gamma_composition_test(png_modifier *pm,
+   const png_byte colour_type, const png_byte bit_depth,
+   const int palette_number,
+   const int interlace_type, const double file_gamma,
+   const double screen_gamma,
+   const int use_input_precision, const int do_background,
+   const int expand_16)
+{
+   size_t pos = 0;
+   png_const_charp base;
+   double bg;
+   char name[128];
+   png_color_16 background;
+
+   /* Make up a name and get an appropriate background gamma value. */
+   switch (do_background)
+   {
+      default:
+         base = "";
+         bg = 4; /* should not be used */
+         break;
+      case PNG_BACKGROUND_GAMMA_SCREEN:
+         base = " bckg(Screen):";
+         bg = 1/screen_gamma;
+         break;
+      case PNG_BACKGROUND_GAMMA_FILE:
+         base = " bckg(File):";
+         bg = file_gamma;
+         break;
+      case PNG_BACKGROUND_GAMMA_UNIQUE:
+         base = " bckg(Unique):";
+         /* This tests the handling of a unique value, the math is such that the
+          * value tends to be <1, but is neither screen nor file (even if they
+          * match!)
+          */
+         bg = (file_gamma + screen_gamma) / 3;
+         break;
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+      case ALPHA_MODE_OFFSET + PNG_ALPHA_PNG:
+         base = " alpha(PNG)";
+         bg = 4; /* should not be used */
+         break;
+      case ALPHA_MODE_OFFSET + PNG_ALPHA_STANDARD:
+         base = " alpha(Porter-Duff)";
+         bg = 4; /* should not be used */
+         break;
+      case ALPHA_MODE_OFFSET + PNG_ALPHA_OPTIMIZED:
+         base = " alpha(Optimized)";
+         bg = 4; /* should not be used */
+         break;
+      case ALPHA_MODE_OFFSET + PNG_ALPHA_BROKEN:
+         base = " alpha(Broken)";
+         bg = 4; /* should not be used */
+         break;
+#endif
+   }
+
+   /* Use random background values - the background is always presented in the
+    * output space (8 or 16 bit components).
+    */
+   if (expand_16 || bit_depth == 16)
+   {
+      png_uint_32 r = random_32();
+
+      background.red = (png_uint_16)r;
+      background.green = (png_uint_16)(r >> 16);
+      r = random_32();
+      background.blue = (png_uint_16)r;
+      background.gray = (png_uint_16)(r >> 16);
+
+      /* In earlier libpng versions, those where DIGITIZE is set, any background
+       * gamma correction in the expand16 case was done using 8-bit gamma
+       * correction tables, resulting in larger errors.  To cope with those
+       * cases use a 16-bit background value which will handle this gamma
+       * correction.
+       */
+#     if DIGITIZE
+         if (expand_16 && (do_background == PNG_BACKGROUND_GAMMA_UNIQUE ||
+                           do_background == PNG_BACKGROUND_GAMMA_FILE) &&
+            fabs(bg*screen_gamma-1) > PNG_GAMMA_THRESHOLD)
+         {
+            /* The background values will be looked up in an 8-bit table to do
+             * the gamma correction, so only select values which are an exact
+             * match for the 8-bit table entries:
+             */
+            background.red = (png_uint_16)((background.red >> 8) * 257);
+            background.green = (png_uint_16)((background.green >> 8) * 257);
+            background.blue = (png_uint_16)((background.blue >> 8) * 257);
+            background.gray = (png_uint_16)((background.gray >> 8) * 257);
+         }
+#     endif
+   }
+
+   else /* 8 bit colors */
+   {
+      png_uint_32 r = random_32();
+
+      background.red = (png_byte)r;
+      background.green = (png_byte)(r >> 8);
+      background.blue = (png_byte)(r >> 16);
+      background.gray = (png_byte)(r >> 24);
+   }
+
+   background.index = 193; /* rgb(193,193,193) to detect errors */
+
+   if (!(colour_type & PNG_COLOR_MASK_COLOR))
+   {
+      /* Because, currently, png_set_background is always called with
+       * 'need_expand' false in this case and because the gamma test itself
+       * doesn't cause an expand to 8-bit for lower bit depths the colour must
+       * be reduced to the correct range.
+       */
+      if (bit_depth < 8)
+         background.gray &= (png_uint_16)((1U << bit_depth)-1);
+
+      /* Grayscale input, we do not convert to RGB (TBD), so we must set the
+       * background to gray - else libpng seems to fail.
+       */
+      background.red = background.green = background.blue = background.gray;
+   }
+
+   pos = safecat(name, sizeof name, pos, "gamma ");
+   pos = safecatd(name, sizeof name, pos, file_gamma, 3);
+   pos = safecat(name, sizeof name, pos, "->");
+   pos = safecatd(name, sizeof name, pos, screen_gamma, 3);
+
+   pos = safecat(name, sizeof name, pos, base);
+   if (do_background < ALPHA_MODE_OFFSET)
+   {
+      /* Include the background color and gamma in the name: */
+      pos = safecat(name, sizeof name, pos, "(");
+      /* This assumes no expand gray->rgb - the current code won't handle that!
+       */
+      if (colour_type & PNG_COLOR_MASK_COLOR)
+      {
+         pos = safecatn(name, sizeof name, pos, background.red);
+         pos = safecat(name, sizeof name, pos, ",");
+         pos = safecatn(name, sizeof name, pos, background.green);
+         pos = safecat(name, sizeof name, pos, ",");
+         pos = safecatn(name, sizeof name, pos, background.blue);
+      }
+      else
+         pos = safecatn(name, sizeof name, pos, background.gray);
+      pos = safecat(name, sizeof name, pos, ")^");
+      pos = safecatd(name, sizeof name, pos, bg, 3);
+   }
+
+   gamma_test(pm, colour_type, bit_depth, palette_number, interlace_type,
+      file_gamma, screen_gamma, 0/*sBIT*/, 0, name, use_input_precision,
+      0/*strip 16*/, expand_16, do_background, &background, bg);
+}
+
+
+static void
+perform_gamma_composition_tests(png_modifier *pm, int do_background,
+   int expand_16)
+{
+   png_byte colour_type = 0;
+   png_byte bit_depth = 0;
+   unsigned int palette_number = 0;
+
+   /* Skip the non-alpha cases - there is no setting of a transparency colour at
+    * present.
+    *
+    * TODO: incorrect; the palette case sets tRNS and, now RGB and gray do,
+    * however the palette case fails miserably so is commented out below.
+    */
+   while (next_format(&colour_type, &bit_depth, &palette_number,
+                      pm->test_lbg_gamma_composition, pm->test_tRNS))
+      if ((colour_type & PNG_COLOR_MASK_ALPHA) != 0
+#if 0 /* TODO: FIXME */
+          /*TODO: FIXME: this should work */
+          || colour_type == 3
+#endif
+          || (colour_type != 3 && palette_number != 0))
+   {
+      unsigned int i, j;
+
+      /* Don't skip the i==j case here - it's relevant. */
+      for (i=0; i<pm->ngamma_tests; ++i) for (j=0; j<pm->ngamma_tests; ++j)
+      {
+         gamma_composition_test(pm, colour_type, bit_depth, palette_number,
+            pm->interlace_type, 1/pm->gammas[i], pm->gammas[j],
+            pm->use_input_precision, do_background, expand_16);
+
+         if (fail(pm))
+            return;
+      }
+   }
+}
+#endif /* READ_BACKGROUND || READ_ALPHA_MODE */
+
+static void
+init_gamma_errors(png_modifier *pm)
+{
+   /* Use -1 to catch tests that were not actually run */
+   pm->error_gray_2 = pm->error_gray_4 = pm->error_gray_8 = -1.;
+   pm->error_color_8 = -1.;
+   pm->error_indexed = -1.;
+   pm->error_gray_16 = pm->error_color_16 = -1.;
+}
+
+static void
+print_one(const char *leader, double err)
+{
+   if (err != -1.)
+      printf(" %s %.5f\n", leader, err);
+}
+
+static void
+summarize_gamma_errors(png_modifier *pm, png_const_charp who, int low_bit_depth,
+   int indexed)
+{
+   fflush(stderr);
+
+   if (who)
+      printf("\nGamma correction with %s:\n", who);
+
+   else
+      printf("\nBasic gamma correction:\n");
+
+   if (low_bit_depth)
+   {
+      print_one(" 2 bit gray: ", pm->error_gray_2);
+      print_one(" 4 bit gray: ", pm->error_gray_4);
+      print_one(" 8 bit gray: ", pm->error_gray_8);
+      print_one(" 8 bit color:", pm->error_color_8);
+      if (indexed)
+         print_one(" indexed:    ", pm->error_indexed);
+   }
+
+   print_one("16 bit gray: ", pm->error_gray_16);
+   print_one("16 bit color:", pm->error_color_16);
+
+   fflush(stdout);
+}
+
+static void
+perform_gamma_test(png_modifier *pm, int summary)
+{
+   /*TODO: remove this*/
+   /* Save certain values for the temporary overrides below. */
+   unsigned int calculations_use_input_precision =
+      pm->calculations_use_input_precision;
+#  ifdef PNG_READ_BACKGROUND_SUPPORTED
+      double maxout8 = pm->maxout8;
+#  endif
+
+   /* First some arbitrary no-transform tests: */
+   if (!pm->this.speed && pm->test_gamma_threshold)
+   {
+      perform_gamma_threshold_tests(pm);
+
+      if (fail(pm))
+         return;
+   }
+
+   /* Now some real transforms. */
+   if (pm->test_gamma_transform)
+   {
+      if (summary)
+      {
+         fflush(stderr);
+         printf("Gamma correction error summary\n\n");
+         printf("The printed value is the maximum error in the pixel values\n");
+         printf("calculated by the libpng gamma correction code.  The error\n");
+         printf("is calculated as the difference between the output pixel\n");
+         printf("value (always an integer) and the ideal value from the\n");
+         printf("libpng specification (typically not an integer).\n\n");
+
+         printf("Expect this value to be less than .5 for 8 bit formats,\n");
+         printf("less than 1 for formats with fewer than 8 bits and a small\n");
+         printf("number (typically less than 5) for the 16 bit formats.\n");
+         printf("For performance reasons the value for 16 bit formats\n");
+         printf("increases when the image file includes an sBIT chunk.\n");
+         fflush(stdout);
+      }
+
+      init_gamma_errors(pm);
+      /*TODO: remove this.  Necessary because the current libpng
+       * implementation works in 8 bits:
+       */
+      if (pm->test_gamma_expand16)
+         pm->calculations_use_input_precision = 1;
+      perform_gamma_transform_tests(pm);
+      if (!calculations_use_input_precision)
+         pm->calculations_use_input_precision = 0;
+
+      if (summary)
+         summarize_gamma_errors(pm, 0/*who*/, 1/*low bit depth*/, 1/*indexed*/);
+
+      if (fail(pm))
+         return;
+   }
+
+   /* The sbit tests produce much larger errors: */
+   if (pm->test_gamma_sbit)
+   {
+      init_gamma_errors(pm);
+      perform_gamma_sbit_tests(pm);
+
+      if (summary)
+         summarize_gamma_errors(pm, "sBIT", pm->sbitlow < 8U, 1/*indexed*/);
+
+      if (fail(pm))
+         return;
+   }
+
+#ifdef DO_16BIT /* Should be READ_16BIT_SUPPORTED */
+   if (pm->test_gamma_scale16)
+   {
+      /* The 16 to 8 bit strip operations: */
+      init_gamma_errors(pm);
+      perform_gamma_scale16_tests(pm);
+
+      if (summary)
+      {
+         fflush(stderr);
+         printf("\nGamma correction with 16 to 8 bit reduction:\n");
+         printf(" 16 bit gray:  %.5f\n", pm->error_gray_16);
+         printf(" 16 bit color: %.5f\n", pm->error_color_16);
+         fflush(stdout);
+      }
+
+      if (fail(pm))
+         return;
+   }
+#endif
+
+#ifdef PNG_READ_BACKGROUND_SUPPORTED
+   if (pm->test_gamma_background)
+   {
+      init_gamma_errors(pm);
+
+      /*TODO: remove this.  Necessary because the current libpng
+       * implementation works in 8 bits:
+       */
+      if (pm->test_gamma_expand16)
+      {
+         pm->calculations_use_input_precision = 1;
+         pm->maxout8 = .499; /* because the 16 bit background is smashed */
+      }
+      perform_gamma_composition_tests(pm, PNG_BACKGROUND_GAMMA_UNIQUE,
+         pm->test_gamma_expand16);
+      if (!calculations_use_input_precision)
+         pm->calculations_use_input_precision = 0;
+      pm->maxout8 = maxout8;
+
+      if (summary)
+         summarize_gamma_errors(pm, "background", 1, 0/*indexed*/);
+
+      if (fail(pm))
+         return;
+   }
+#endif
+
+#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
+   if (pm->test_gamma_alpha_mode)
+   {
+      int do_background;
+
+      init_gamma_errors(pm);
+
+      /*TODO: remove this.  Necessary because the current libpng
+       * implementation works in 8 bits:
+       */
+      if (pm->test_gamma_expand16)
+         pm->calculations_use_input_precision = 1;
+      for (do_background = ALPHA_MODE_OFFSET + PNG_ALPHA_STANDARD;
+         do_background <= ALPHA_MODE_OFFSET + PNG_ALPHA_BROKEN && !fail(pm);
+         ++do_background)
+         perform_gamma_composition_tests(pm, do_background,
+            pm->test_gamma_expand16);
+      if (!calculations_use_input_precision)
+         pm->calculations_use_input_precision = 0;
+
+      if (summary)
+         summarize_gamma_errors(pm, "alpha mode", 1, 0/*indexed*/);
+
+      if (fail(pm))
+         return;
+   }
+#endif
+}
+#endif /* PNG_READ_GAMMA_SUPPORTED */
+#endif /* PNG_READ_SUPPORTED */
+
+/* INTERLACE MACRO VALIDATION */
+/* This is copied verbatim from the specification, it is simply the pass
+ * number in which each pixel in each 8x8 tile appears.  The array must
+ * be indexed adam7[y][x] and notice that the pass numbers are based at
+ * 1, not 0 - the base libpng uses.
+ */
+static const
+png_byte adam7[8][8] =
+{
+   { 1,6,4,6,2,6,4,6 },
+   { 7,7,7,7,7,7,7,7 },
+   { 5,6,5,6,5,6,5,6 },
+   { 7,7,7,7,7,7,7,7 },
+   { 3,6,4,6,3,6,4,6 },
+   { 7,7,7,7,7,7,7,7 },
+   { 5,6,5,6,5,6,5,6 },
+   { 7,7,7,7,7,7,7,7 }
+};
+
+/* This routine validates all the interlace support macros in png.h for
+ * a variety of valid PNG widths and heights.  It uses a number of similarly
+ * named internal routines that feed off the above array.
+ */
+static png_uint_32
+png_pass_start_row(int pass)
+{
+   int x, y;
+   ++pass;
+   for (y=0; y<8; ++y) for (x=0; x<8; ++x) if (adam7[y][x] == pass)
+      return y;
+   return 0xf;
+}
+
+static png_uint_32
+png_pass_start_col(int pass)
+{
+   int x, y;
+   ++pass;
+   for (x=0; x<8; ++x) for (y=0; y<8; ++y) if (adam7[y][x] == pass)
+      return x;
+   return 0xf;
+}
+
+static int
+png_pass_row_shift(int pass)
+{
+   int x, y, base=(-1), inc=8;
+   ++pass;
+   for (y=0; y<8; ++y) for (x=0; x<8; ++x) if (adam7[y][x] == pass)
+   {
+      if (base == (-1))
+         base = y;
+      else if (base == y)
+         {}
+      else if (inc == y-base)
+         base=y;
+      else if (inc == 8)
+         inc = y-base, base=y;
+      else if (inc != y-base)
+         return 0xff; /* error - more than one 'inc' value! */
+   }
+
+   if (base == (-1)) return 0xfe; /* error - no row in pass! */
+
+   /* The shift is always 1, 2 or 3 - no pass has all the rows! */
+   switch (inc)
+   {
+case 2: return 1;
+case 4: return 2;
+case 8: return 3;
+default: break;
+   }
+
+   /* error - unrecognized 'inc' */
+   return (inc << 8) + 0xfd;
+}
+
+static int
+png_pass_col_shift(int pass)
+{
+   int x, y, base=(-1), inc=8;
+   ++pass;
+   for (x=0; x<8; ++x) for (y=0; y<8; ++y) if (adam7[y][x] == pass)
+   {
+      if (base == (-1))
+         base = x;
+      else if (base == x)
+         {}
+      else if (inc == x-base)
+         base=x;
+      else if (inc == 8)
+         inc = x-base, base=x;
+      else if (inc != x-base)
+         return 0xff; /* error - more than one 'inc' value! */
+   }
+
+   if (base == (-1)) return 0xfe; /* error - no row in pass! */
+
+   /* The shift is always 1, 2 or 3 - no pass has all the rows! */
+   switch (inc)
+   {
+case 1: return 0; /* pass 7 has all the columns */
+case 2: return 1;
+case 4: return 2;
+case 8: return 3;
+default: break;
+   }
+
+   /* error - unrecognized 'inc' */
+   return (inc << 8) + 0xfd;
+}
+
+static png_uint_32
+png_row_from_pass_row(png_uint_32 yIn, int pass)
+{
+   /* By examination of the array: */
+   switch (pass)
+   {
+case 0: return yIn * 8;
+case 1: return yIn * 8;
+case 2: return yIn * 8 + 4;
+case 3: return yIn * 4;
+case 4: return yIn * 4 + 2;
+case 5: return yIn * 2;
+case 6: return yIn * 2 + 1;
+default: break;
+   }
+
+   return 0xff; /* bad pass number */
+}
+
+static png_uint_32
+png_col_from_pass_col(png_uint_32 xIn, int pass)
+{
+   /* By examination of the array: */
+   switch (pass)
+   {
+case 0: return xIn * 8;
+case 1: return xIn * 8 + 4;
+case 2: return xIn * 4;
+case 3: return xIn * 4 + 2;
+case 4: return xIn * 2;
+case 5: return xIn * 2 + 1;
+case 6: return xIn;
+default: break;
+   }
+
+   return 0xff; /* bad pass number */
+}
+
+static int
+png_row_in_interlace_pass(png_uint_32 y, int pass)
+{
+   /* Is row 'y' in pass 'pass'? */
+   int x;
+   y &= 7;
+   ++pass;
+   for (x=0; x<8; ++x) if (adam7[y][x] == pass)
+      return 1;
+
+   return 0;
+}
+
+static int
+png_col_in_interlace_pass(png_uint_32 x, int pass)
+{
+   /* Is column 'x' in pass 'pass'? */
+   int y;
+   x &= 7;
+   ++pass;
+   for (y=0; y<8; ++y) if (adam7[y][x] == pass)
+      return 1;
+
+   return 0;
+}
+
+static png_uint_32
+png_pass_rows(png_uint_32 height, int pass)
+{
+   png_uint_32 tiles = height>>3;
+   png_uint_32 rows = 0;
+   unsigned int x, y;
+
+   height &= 7;
+   ++pass;
+   for (y=0; y<8; ++y) for (x=0; x<8; ++x) if (adam7[y][x] == pass)
+   {
+      rows += tiles;
+      if (y < height) ++rows;
+      break; /* i.e. break the 'x', column, loop. */
+   }
+
+   return rows;
+}
+
+static png_uint_32
+png_pass_cols(png_uint_32 width, int pass)
+{
+   png_uint_32 tiles = width>>3;
+   png_uint_32 cols = 0;
+   unsigned int x, y;
+
+   width &= 7;
+   ++pass;
+   for (x=0; x<8; ++x) for (y=0; y<8; ++y) if (adam7[y][x] == pass)
+   {
+      cols += tiles;
+      if (x < width) ++cols;
+      break; /* i.e. break the 'y', row, loop. */
+   }
+
+   return cols;
+}
+
+static void
+perform_interlace_macro_validation(void)
+{
+   /* The macros to validate, first those that depend only on pass:
+    *
+    * PNG_PASS_START_ROW(pass)
+    * PNG_PASS_START_COL(pass)
+    * PNG_PASS_ROW_SHIFT(pass)
+    * PNG_PASS_COL_SHIFT(pass)
+    */
+   int pass;
+
+   for (pass=0; pass<7; ++pass)
+   {
+      png_uint_32 m, f, v;
+
+      m = PNG_PASS_START_ROW(pass);
+      f = png_pass_start_row(pass);
+      if (m != f)
+      {
+         fprintf(stderr, "PNG_PASS_START_ROW(%d) = %u != %x\n", pass, m, f);
+         exit(99);
+      }
+
+      m = PNG_PASS_START_COL(pass);
+      f = png_pass_start_col(pass);
+      if (m != f)
+      {
+         fprintf(stderr, "PNG_PASS_START_COL(%d) = %u != %x\n", pass, m, f);
+         exit(99);
+      }
+
+      m = PNG_PASS_ROW_SHIFT(pass);
+      f = png_pass_row_shift(pass);
+      if (m != f)
+      {
+         fprintf(stderr, "PNG_PASS_ROW_SHIFT(%d) = %u != %x\n", pass, m, f);
+         exit(99);
+      }
+
+      m = PNG_PASS_COL_SHIFT(pass);
+      f = png_pass_col_shift(pass);
+      if (m != f)
+      {
+         fprintf(stderr, "PNG_PASS_COL_SHIFT(%d) = %u != %x\n", pass, m, f);
+         exit(99);
+      }
+
+      /* Macros that depend on the image or sub-image height too:
+       *
+       * PNG_PASS_ROWS(height, pass)
+       * PNG_PASS_COLS(width, pass)
+       * PNG_ROW_FROM_PASS_ROW(yIn, pass)
+       * PNG_COL_FROM_PASS_COL(xIn, pass)
+       * PNG_ROW_IN_INTERLACE_PASS(y, pass)
+       * PNG_COL_IN_INTERLACE_PASS(x, pass)
+       */
+      for (v=0;;)
+      {
+         /* First the base 0 stuff: */
+         m = PNG_ROW_FROM_PASS_ROW(v, pass);
+         f = png_row_from_pass_row(v, pass);
+         if (m != f)
+         {
+            fprintf(stderr, "PNG_ROW_FROM_PASS_ROW(%u, %d) = %u != %x\n",
+               v, pass, m, f);
+            exit(99);
+         }
+
+         m = PNG_COL_FROM_PASS_COL(v, pass);
+         f = png_col_from_pass_col(v, pass);
+         if (m != f)
+         {
+            fprintf(stderr, "PNG_COL_FROM_PASS_COL(%u, %d) = %u != %x\n",
+               v, pass, m, f);
+            exit(99);
+         }
+
+         m = PNG_ROW_IN_INTERLACE_PASS(v, pass);
+         f = png_row_in_interlace_pass(v, pass);
+         if (m != f)
+         {
+            fprintf(stderr, "PNG_ROW_IN_INTERLACE_PASS(%u, %d) = %u != %x\n",
+               v, pass, m, f);
+            exit(99);
+         }
+
+         m = PNG_COL_IN_INTERLACE_PASS(v, pass);
+         f = png_col_in_interlace_pass(v, pass);
+         if (m != f)
+         {
+            fprintf(stderr, "PNG_COL_IN_INTERLACE_PASS(%u, %d) = %u != %x\n",
+               v, pass, m, f);
+            exit(99);
+         }
+
+         /* Then the base 1 stuff: */
+         ++v;
+         m = PNG_PASS_ROWS(v, pass);
+         f = png_pass_rows(v, pass);
+         if (m != f)
+         {
+            fprintf(stderr, "PNG_PASS_ROWS(%u, %d) = %u != %x\n",
+               v, pass, m, f);
+            exit(99);
+         }
+
+         m = PNG_PASS_COLS(v, pass);
+         f = png_pass_cols(v, pass);
+         if (m != f)
+         {
+            fprintf(stderr, "PNG_PASS_COLS(%u, %d) = %u != %x\n",
+               v, pass, m, f);
+            exit(99);
+         }
+
+         /* Move to the next v - the stepping algorithm starts skipping
+          * values above 1024.
+          */
+         if (v > 1024)
+         {
+            if (v == PNG_UINT_31_MAX)
+               break;
+
+            v = (v << 1) ^ v;
+            if (v >= PNG_UINT_31_MAX)
+               v = PNG_UINT_31_MAX-1;
+         }
+      }
+   }
+}
+
+/* Test color encodings. These values are back-calculated from the published
+ * chromaticities.  The values are accurate to about 14 decimal places; 15 are
+ * given.  These values are much more accurate than the ones given in the spec,
+ * which typically don't exceed 4 decimal places.  This allows testing of the
+ * libpng code to its theoretical accuracy of 4 decimal places.  (If pngvalid
+ * used the published errors the 'slack' permitted would have to be +/-.5E-4 or
+ * more.)
+ *
+ * The png_modifier code assumes that encodings[0] is sRGB and treats it
+ * specially: do not change the first entry in this list!
+ */
+static const color_encoding test_encodings[] =
+{
+/* sRGB: must be first in this list! */
+/*gamma:*/ { 1/2.2,
+/*red:  */ { 0.412390799265959, 0.212639005871510, 0.019330818715592 },
+/*green:*/ { 0.357584339383878, 0.715168678767756, 0.119194779794626 },
+/*blue: */ { 0.180480788401834, 0.072192315360734, 0.950532152249660} },
+/* Kodak ProPhoto (wide gamut) */
+/*gamma:*/ { 1/1.6 /*approximate: uses 1.8 power law compared to sRGB 2.4*/,
+/*red:  */ { 0.797760489672303, 0.288071128229293, 0.000000000000000 },
+/*green:*/ { 0.135185837175740, 0.711843217810102, 0.000000000000000 },
+/*blue: */ { 0.031349349581525, 0.000085653960605, 0.825104602510460} },
+/* Adobe RGB (1998) */
+/*gamma:*/ { 1/(2+51./256),
+/*red:  */ { 0.576669042910131, 0.297344975250536, 0.027031361386412 },
+/*green:*/ { 0.185558237906546, 0.627363566255466, 0.070688852535827 },
+/*blue: */ { 0.188228646234995, 0.075291458493998, 0.991337536837639} },
+/* Adobe Wide Gamut RGB */
+/*gamma:*/ { 1/(2+51./256),
+/*red:  */ { 0.716500716779386, 0.258728243040113, 0.000000000000000 },
+/*green:*/ { 0.101020574397477, 0.724682314948566, 0.051211818965388 },
+/*blue: */ { 0.146774385252705, 0.016589442011321, 0.773892783545073} },
+/* Fake encoding which selects just the green channel */
+/*gamma:*/ { 1.45/2.2, /* the 'Mac' gamma */
+/*red:  */ { 0.716500716779386, 0.000000000000000, 0.000000000000000 },
+/*green:*/ { 0.101020574397477, 1.000000000000000, 0.051211818965388 },
+/*blue: */ { 0.146774385252705, 0.000000000000000, 0.773892783545073} },
+};
+
+/* signal handler
+ *
+ * This attempts to trap signals and escape without crashing.  It needs a
+ * context pointer so that it can throw an exception (call longjmp) to recover
+ * from the condition; this is handled by making the png_modifier used by 'main'
+ * into a global variable.
+ */
+static png_modifier pm;
+
+static void signal_handler(int signum)
+{
+
+   size_t pos = 0;
+   char msg[64];
+
+   pos = safecat(msg, sizeof msg, pos, "caught signal: ");
+
+   switch (signum)
+   {
+      case SIGABRT:
+         pos = safecat(msg, sizeof msg, pos, "abort");
+         break;
+
+      case SIGFPE:
+         pos = safecat(msg, sizeof msg, pos, "floating point exception");
+         break;
+
+      case SIGILL:
+         pos = safecat(msg, sizeof msg, pos, "illegal instruction");
+         break;
+
+      case SIGINT:
+         pos = safecat(msg, sizeof msg, pos, "interrupt");
+         break;
+
+      case SIGSEGV:
+         pos = safecat(msg, sizeof msg, pos, "invalid memory access");
+         break;
+
+      case SIGTERM:
+         pos = safecat(msg, sizeof msg, pos, "termination request");
+         break;
+
+      default:
+         pos = safecat(msg, sizeof msg, pos, "unknown ");
+         pos = safecatn(msg, sizeof msg, pos, signum);
+         break;
+   }
+
+   store_log(&pm.this, NULL/*png_structp*/, msg, 1/*error*/);
+
+   /* And finally throw an exception so we can keep going, unless this is
+    * SIGTERM in which case stop now.
+    */
+   if (signum != SIGTERM)
+   {
+      struct exception_context *the_exception_context =
+         &pm.this.exception_context;
+
+      Throw &pm.this;
+   }
+
+   else
+      exit(1);
+}
+
+/* main program */
+int main(int argc, char **argv)
+{
+   int summary = 1;  /* Print the error summary at the end */
+   int memstats = 0; /* Print memory statistics at the end */
+
+   /* Create the given output file on success: */
+   const char *touch = NULL;
+
+   /* This is an array of standard gamma values (believe it or not I've seen
+    * every one of these mentioned somewhere.)
+    *
+    * In the following list the most useful values are first!
+    */
+   static double
+      gammas[]={2.2, 1.0, 2.2/1.45, 1.8, 1.5, 2.4, 2.5, 2.62, 2.9};
+
+   /* This records the command and arguments: */
+   size_t cp = 0;
+   char command[1024];
+
+   anon_context(&pm.this);
+
+   gnu_volatile(summary)
+   gnu_volatile(memstats)
+   gnu_volatile(touch)
+
+   /* Add appropriate signal handlers, just the ANSI specified ones: */
+   signal(SIGABRT, signal_handler);
+   signal(SIGFPE, signal_handler);
+   signal(SIGILL, signal_handler);
+   signal(SIGINT, signal_handler);
+   signal(SIGSEGV, signal_handler);
+   signal(SIGTERM, signal_handler);
+
+#ifdef HAVE_FEENABLEEXCEPT
+   /* Only required to enable FP exceptions on platforms where they start off
+    * disabled; this is not necessary but if it is not done pngvalid will likely
+    * end up ignoring FP conditions that other platforms fault.
+    */
+   feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);
+#endif
+
+   modifier_init(&pm);
+
+   /* Preallocate the image buffer, because we know how big it needs to be,
+    * note that, for testing purposes, it is deliberately mis-aligned by tag
+    * bytes either side.  All rows have an additional five bytes of padding for
+    * overwrite checking.
+    */
+   store_ensure_image(&pm.this, NULL, 2, TRANSFORM_ROWMAX, TRANSFORM_HEIGHTMAX);
+
+   /* Don't give argv[0], it's normally some horrible libtool string: */
+   cp = safecat(command, sizeof command, cp, "pngvalid");
+
+   /* Default to error on warning: */
+   pm.this.treat_warnings_as_errors = 1;
+
+   /* Default assume_16_bit_calculations appropriately; this tells the checking
+    * code that 16-bit arithmetic is used for 8-bit samples when it would make a
+    * difference.
+    */
+   pm.assume_16_bit_calculations = PNG_LIBPNG_VER >= 10700;
+
+   /* Currently 16 bit expansion happens at the end of the pipeline, so the
+    * calculations are done in the input bit depth not the output.
+    *
+    * TODO: fix this
+    */
+   pm.calculations_use_input_precision = 1U;
+
+   /* Store the test gammas */
+   pm.gammas = gammas;
+   pm.ngammas = ARRAY_SIZE(gammas);
+   pm.ngamma_tests = 0; /* default to off */
+
+   /* Low bit depth gray images don't do well in the gamma tests, until
+    * this is fixed turn them off for some gamma cases:
+    */
+#  ifdef PNG_WRITE_tRNS_SUPPORTED
+      pm.test_tRNS = 1;
+#  endif
+   pm.test_lbg = PNG_LIBPNG_VER >= 10600;
+   pm.test_lbg_gamma_threshold = 1;
+   pm.test_lbg_gamma_transform = PNG_LIBPNG_VER >= 10600;
+   pm.test_lbg_gamma_sbit = 1;
+   pm.test_lbg_gamma_composition = PNG_LIBPNG_VER >= 10700;
+
+   /* And the test encodings */
+   pm.encodings = test_encodings;
+   pm.nencodings = ARRAY_SIZE(test_encodings);
+
+#  if PNG_LIBPNG_VER < 10700
+      pm.sbitlow = 8U; /* because libpng doesn't do sBIT below 8! */
+#  else
+      pm.sbitlow = 1U;
+#  endif
+
+   /* The following allows results to pass if they correspond to anything in the
+    * transformed range [input-.5,input+.5]; this is is required because of the
+    * way libpng treates the 16_TO_8 flag when building the gamma tables in
+    * releases up to 1.6.0.
+    *
+    * TODO: review this
+    */
+   pm.use_input_precision_16to8 = 1U;
+   pm.use_input_precision_sbit = 1U; /* because libpng now rounds sBIT */
+
+   /* Some default values (set the behavior for 'make check' here).
+    * These values simply control the maximum error permitted in the gamma
+    * transformations.  The practial limits for human perception are described
+    * below (the setting for maxpc16), however for 8 bit encodings it isn't
+    * possible to meet the accepted capabilities of human vision - i.e. 8 bit
+    * images can never be good enough, regardless of encoding.
+    */
+   pm.maxout8 = .1;     /* Arithmetic error in *encoded* value */
+   pm.maxabs8 = .00005; /* 1/20000 */
+   pm.maxcalc8 = 1./255;  /* +/-1 in 8 bits for compose errors */
+   pm.maxpc8 = .499;    /* I.e., .499% fractional error */
+   pm.maxout16 = .499;  /* Error in *encoded* value */
+   pm.maxabs16 = .00005;/* 1/20000 */
+   pm.maxcalc16 =1./65535;/* +/-1 in 16 bits for compose errors */
+#  if PNG_LIBPNG_VER < 10700
+      pm.maxcalcG = 1./((1<<PNG_MAX_GAMMA_8)-1);
+#  else
+      pm.maxcalcG = 1./((1<<16)-1);
+#  endif
+
+   /* NOTE: this is a reasonable perceptual limit. We assume that humans can
+    * perceive light level differences of 1% over a 100:1 range, so we need to
+    * maintain 1 in 10000 accuracy (in linear light space), which is what the
+    * following guarantees.  It also allows significantly higher errors at
+    * higher 16 bit values, which is important for performance.  The actual
+    * maximum 16 bit error is about +/-1.9 in the fixed point implementation but
+    * this is only allowed for values >38149 by the following:
+    */
+   pm.maxpc16 = .005;   /* I.e., 1/200% - 1/20000 */
+
+   /* Now parse the command line options. */
+   while (--argc >= 1)
+   {
+      int catmore = 0; /* Set if the argument has an argument. */
+
+      /* Record each argument for posterity: */
+      cp = safecat(command, sizeof command, cp, " ");
+      cp = safecat(command, sizeof command, cp, *++argv);
+
+      if (strcmp(*argv, "-v") == 0)
+         pm.this.verbose = 1;
+
+      else if (strcmp(*argv, "-l") == 0)
+         pm.log = 1;
+
+      else if (strcmp(*argv, "-q") == 0)
+         summary = pm.this.verbose = pm.log = 0;
+
+      else if (strcmp(*argv, "-w") == 0 ||
+               strcmp(*argv, "--strict") == 0)
+         pm.this.treat_warnings_as_errors = 1; /* NOTE: this is the default! */
+
+      else if (strcmp(*argv, "--nostrict") == 0)
+         pm.this.treat_warnings_as_errors = 0;
+
+      else if (strcmp(*argv, "--speed") == 0)
+         pm.this.speed = 1, pm.ngamma_tests = pm.ngammas, pm.test_standard = 0,
+            summary = 0;
+
+      else if (strcmp(*argv, "--memory") == 0)
+         memstats = 1;
+
+      else if (strcmp(*argv, "--size") == 0)
+         pm.test_size = 1;
+
+      else if (strcmp(*argv, "--nosize") == 0)
+         pm.test_size = 0;
+
+      else if (strcmp(*argv, "--standard") == 0)
+         pm.test_standard = 1;
+
+      else if (strcmp(*argv, "--nostandard") == 0)
+         pm.test_standard = 0;
+
+      else if (strcmp(*argv, "--transform") == 0)
+         pm.test_transform = 1;
+
+      else if (strcmp(*argv, "--notransform") == 0)
+         pm.test_transform = 0;
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+      else if (strncmp(*argv, "--transform-disable=",
+         sizeof "--transform-disable") == 0)
+         {
+         pm.test_transform = 1;
+         transform_disable(*argv + sizeof "--transform-disable");
+         }
+
+      else if (strncmp(*argv, "--transform-enable=",
+         sizeof "--transform-enable") == 0)
+         {
+         pm.test_transform = 1;
+         transform_enable(*argv + sizeof "--transform-enable");
+         }
+#endif /* PNG_READ_TRANSFORMS_SUPPORTED */
+
+      else if (strcmp(*argv, "--gamma") == 0)
+         {
+         /* Just do two gamma tests here (2.2 and linear) for speed: */
+         pm.ngamma_tests = 2U;
+         pm.test_gamma_threshold = 1;
+         pm.test_gamma_transform = 1;
+         pm.test_gamma_sbit = 1;
+         pm.test_gamma_scale16 = 1;
+         pm.test_gamma_background = 1; /* composition */
+         pm.test_gamma_alpha_mode = 1;
+         }
+
+      else if (strcmp(*argv, "--nogamma") == 0)
+         pm.ngamma_tests = 0;
+
+      else if (strcmp(*argv, "--gamma-threshold") == 0)
+         pm.ngamma_tests = 2U, pm.test_gamma_threshold = 1;
+
+      else if (strcmp(*argv, "--nogamma-threshold") == 0)
+         pm.test_gamma_threshold = 0;
+
+      else if (strcmp(*argv, "--gamma-transform") == 0)
+         pm.ngamma_tests = 2U, pm.test_gamma_transform = 1;
+
+      else if (strcmp(*argv, "--nogamma-transform") == 0)
+         pm.test_gamma_transform = 0;
+
+      else if (strcmp(*argv, "--gamma-sbit") == 0)
+         pm.ngamma_tests = 2U, pm.test_gamma_sbit = 1;
+
+      else if (strcmp(*argv, "--nogamma-sbit") == 0)
+         pm.test_gamma_sbit = 0;
+
+      else if (strcmp(*argv, "--gamma-16-to-8") == 0)
+         pm.ngamma_tests = 2U, pm.test_gamma_scale16 = 1;
+
+      else if (strcmp(*argv, "--nogamma-16-to-8") == 0)
+         pm.test_gamma_scale16 = 0;
+
+      else if (strcmp(*argv, "--gamma-background") == 0)
+         pm.ngamma_tests = 2U, pm.test_gamma_background = 1;
+
+      else if (strcmp(*argv, "--nogamma-background") == 0)
+         pm.test_gamma_background = 0;
+
+      else if (strcmp(*argv, "--gamma-alpha-mode") == 0)
+         pm.ngamma_tests = 2U, pm.test_gamma_alpha_mode = 1;
+
+      else if (strcmp(*argv, "--nogamma-alpha-mode") == 0)
+         pm.test_gamma_alpha_mode = 0;
+
+      else if (strcmp(*argv, "--expand16") == 0)
+         pm.test_gamma_expand16 = 1;
+
+      else if (strcmp(*argv, "--noexpand16") == 0)
+         pm.test_gamma_expand16 = 0;
+
+      else if (strcmp(*argv, "--low-depth-gray") == 0)
+         pm.test_lbg = pm.test_lbg_gamma_threshold =
+            pm.test_lbg_gamma_transform = pm.test_lbg_gamma_sbit =
+            pm.test_lbg_gamma_composition = 1;
+
+      else if (strcmp(*argv, "--nolow-depth-gray") == 0)
+         pm.test_lbg = pm.test_lbg_gamma_threshold =
+            pm.test_lbg_gamma_transform = pm.test_lbg_gamma_sbit =
+            pm.test_lbg_gamma_composition = 0;
+
+#     ifdef PNG_WRITE_tRNS_SUPPORTED
+         else if (strcmp(*argv, "--tRNS") == 0)
+            pm.test_tRNS = 1;
+#     endif
+
+      else if (strcmp(*argv, "--notRNS") == 0)
+         pm.test_tRNS = 0;
+
+      else if (strcmp(*argv, "--more-gammas") == 0)
+         pm.ngamma_tests = 3U;
+
+      else if (strcmp(*argv, "--all-gammas") == 0)
+         pm.ngamma_tests = pm.ngammas;
+
+      else if (strcmp(*argv, "--progressive-read") == 0)
+         pm.this.progressive = 1;
+
+      else if (strcmp(*argv, "--use-update-info") == 0)
+         ++pm.use_update_info; /* Can call multiple times */
+
+      else if (strcmp(*argv, "--interlace") == 0)
+      {
+#        if CAN_WRITE_INTERLACE
+            pm.interlace_type = PNG_INTERLACE_ADAM7;
+#        else /* !CAN_WRITE_INTERLACE */
+            fprintf(stderr, "pngvalid: no write interlace support\n");
+            return SKIP;
+#        endif /* !CAN_WRITE_INTERLACE */
+      }
+
+      else if (strcmp(*argv, "--use-input-precision") == 0)
+         pm.use_input_precision = 1U;
+
+      else if (strcmp(*argv, "--use-calculation-precision") == 0)
+         pm.use_input_precision = 0;
+
+      else if (strcmp(*argv, "--calculations-use-input-precision") == 0)
+         pm.calculations_use_input_precision = 1U;
+
+      else if (strcmp(*argv, "--assume-16-bit-calculations") == 0)
+         pm.assume_16_bit_calculations = 1U;
+
+      else if (strcmp(*argv, "--calculations-follow-bit-depth") == 0)
+         pm.calculations_use_input_precision =
+            pm.assume_16_bit_calculations = 0;
+
+      else if (strcmp(*argv, "--exhaustive") == 0)
+         pm.test_exhaustive = 1;
+
+      else if (argc > 1 && strcmp(*argv, "--sbitlow") == 0)
+         --argc, pm.sbitlow = (png_byte)atoi(*++argv), catmore = 1;
+
+      else if (argc > 1 && strcmp(*argv, "--touch") == 0)
+         --argc, touch = *++argv, catmore = 1;
+
+      else if (argc > 1 && strncmp(*argv, "--max", 5) == 0)
+      {
+         --argc;
+
+         if (strcmp(5+*argv, "abs8") == 0)
+            pm.maxabs8 = atof(*++argv);
+
+         else if (strcmp(5+*argv, "abs16") == 0)
+            pm.maxabs16 = atof(*++argv);
+
+         else if (strcmp(5+*argv, "calc8") == 0)
+            pm.maxcalc8 = atof(*++argv);
+
+         else if (strcmp(5+*argv, "calc16") == 0)
+            pm.maxcalc16 = atof(*++argv);
+
+         else if (strcmp(5+*argv, "out8") == 0)
+            pm.maxout8 = atof(*++argv);
+
+         else if (strcmp(5+*argv, "out16") == 0)
+            pm.maxout16 = atof(*++argv);
+
+         else if (strcmp(5+*argv, "pc8") == 0)
+            pm.maxpc8 = atof(*++argv);
+
+         else if (strcmp(5+*argv, "pc16") == 0)
+            pm.maxpc16 = atof(*++argv);
+
+         else
+         {
+            fprintf(stderr, "pngvalid: %s: unknown 'max' option\n", *argv);
+            exit(99);
+         }
+
+         catmore = 1;
+      }
+
+      else if (strcmp(*argv, "--log8") == 0)
+         --argc, pm.log8 = atof(*++argv), catmore = 1;
+
+      else if (strcmp(*argv, "--log16") == 0)
+         --argc, pm.log16 = atof(*++argv), catmore = 1;
+
+#ifdef PNG_SET_OPTION_SUPPORTED
+      else if (strncmp(*argv, "--option=", 9) == 0)
+      {
+         /* Syntax of the argument is <option>:{on|off} */
+         const char *arg = 9+*argv;
+         unsigned char option=0, setting=0;
+
+#ifdef PNG_ARM_NEON
+         if (strncmp(arg, "arm-neon:", 9) == 0)
+            option = PNG_ARM_NEON, arg += 9;
+
+         else
+#endif
+#ifdef PNG_EXTENSIONS
+         if (strncmp(arg, "extensions:", 11) == 0)
+            option = PNG_EXTENSIONS, arg += 11;
+
+         else
+#endif
+#ifdef PNG_MAXIMUM_INFLATE_WINDOW
+         if (strncmp(arg, "max-inflate-window:", 19) == 0)
+            option = PNG_MAXIMUM_INFLATE_WINDOW, arg += 19;
+
+         else
+#endif
+         {
+            fprintf(stderr, "pngvalid: %s: %s: unknown option\n", *argv, arg);
+            exit(99);
+         }
+
+         if (strcmp(arg, "off") == 0)
+            setting = PNG_OPTION_OFF;
+
+         else if (strcmp(arg, "on") == 0)
+            setting = PNG_OPTION_ON;
+
+         else
+         {
+            fprintf(stderr,
+               "pngvalid: %s: %s: unknown setting (use 'on' or 'off')\n",
+               *argv, arg);
+            exit(99);
+         }
+
+         pm.this.options[pm.this.noptions].option = option;
+         pm.this.options[pm.this.noptions++].setting = setting;
+      }
+#endif /* PNG_SET_OPTION_SUPPORTED */
+
+      else
+      {
+         fprintf(stderr, "pngvalid: %s: unknown argument\n", *argv);
+         exit(99);
+      }
+
+      if (catmore) /* consumed an extra *argv */
+      {
+         cp = safecat(command, sizeof command, cp, " ");
+         cp = safecat(command, sizeof command, cp, *argv);
+      }
+   }
+
+   /* If pngvalid is run with no arguments default to a reasonable set of the
+    * tests.
+    */
+   if (pm.test_standard == 0 && pm.test_size == 0 && pm.test_transform == 0 &&
+      pm.ngamma_tests == 0)
+   {
+      /* Make this do all the tests done in the test shell scripts with the same
+       * parameters, where possible.  The limitation is that all the progressive
+       * read and interlace stuff has to be done in separate runs, so only the
+       * basic 'standard' and 'size' tests are done.
+       */
+      pm.test_standard = 1;
+      pm.test_size = 1;
+      pm.test_transform = 1;
+      pm.ngamma_tests = 2U;
+   }
+
+   if (pm.ngamma_tests > 0 &&
+      pm.test_gamma_threshold == 0 && pm.test_gamma_transform == 0 &&
+      pm.test_gamma_sbit == 0 && pm.test_gamma_scale16 == 0 &&
+      pm.test_gamma_background == 0 && pm.test_gamma_alpha_mode == 0)
+   {
+      pm.test_gamma_threshold = 1;
+      pm.test_gamma_transform = 1;
+      pm.test_gamma_sbit = 1;
+      pm.test_gamma_scale16 = 1;
+      pm.test_gamma_background = 1;
+      pm.test_gamma_alpha_mode = 1;
+   }
+
+   else if (pm.ngamma_tests == 0)
+   {
+      /* Nothing to test so turn everything off: */
+      pm.test_gamma_threshold = 0;
+      pm.test_gamma_transform = 0;
+      pm.test_gamma_sbit = 0;
+      pm.test_gamma_scale16 = 0;
+      pm.test_gamma_background = 0;
+      pm.test_gamma_alpha_mode = 0;
+   }
+
+   Try
+   {
+      /* Make useful base images */
+      make_transform_images(&pm);
+
+      /* Perform the standard and gamma tests. */
+      if (pm.test_standard)
+      {
+         perform_interlace_macro_validation();
+         perform_formatting_test(&pm.this);
+#        ifdef PNG_READ_SUPPORTED
+            perform_standard_test(&pm);
+#        endif
+         perform_error_test(&pm);
+      }
+
+      /* Various oddly sized images: */
+      if (pm.test_size)
+      {
+         make_size_images(&pm.this);
+#        ifdef PNG_READ_SUPPORTED
+            perform_size_test(&pm);
+#        endif
+      }
+
+#ifdef PNG_READ_TRANSFORMS_SUPPORTED
+      /* Combinatorial transforms: */
+      if (pm.test_transform)
+         perform_transform_test(&pm);
+#endif /* PNG_READ_TRANSFORMS_SUPPORTED */
+
+#ifdef PNG_READ_GAMMA_SUPPORTED
+      if (pm.ngamma_tests > 0)
+         perform_gamma_test(&pm, summary);
+#endif
+   }
+
+   Catch_anonymous
+   {
+      fprintf(stderr, "pngvalid: test aborted (probably failed in cleanup)\n");
+      if (!pm.this.verbose)
+      {
+         if (pm.this.error[0] != 0)
+            fprintf(stderr, "pngvalid: first error: %s\n", pm.this.error);
+
+         fprintf(stderr, "pngvalid: run with -v to see what happened\n");
+      }
+      exit(1);
+   }
+
+   if (summary)
+   {
+      printf("%s: %s (%s point arithmetic)\n",
+         (pm.this.nerrors || (pm.this.treat_warnings_as_errors &&
+            pm.this.nwarnings)) ? "FAIL" : "PASS",
+         command,
+#if defined(PNG_FLOATING_ARITHMETIC_SUPPORTED) || PNG_LIBPNG_VER < 10500
+         "floating"
+#else
+         "fixed"
+#endif
+         );
+   }
+
+   if (memstats)
+   {
+      printf("Allocated memory statistics (in bytes):\n"
+         "\tread  %lu maximum single, %lu peak, %lu total\n"
+         "\twrite %lu maximum single, %lu peak, %lu total\n",
+         (unsigned long)pm.this.read_memory_pool.max_max,
+         (unsigned long)pm.this.read_memory_pool.max_limit,
+         (unsigned long)pm.this.read_memory_pool.max_total,
+         (unsigned long)pm.this.write_memory_pool.max_max,
+         (unsigned long)pm.this.write_memory_pool.max_limit,
+         (unsigned long)pm.this.write_memory_pool.max_total);
+   }
+
+   /* Do this here to provoke memory corruption errors in memory not directly
+    * allocated by libpng - not a complete test, but better than nothing.
+    */
+   store_delete(&pm.this);
+
+   /* Error exit if there are any errors, and maybe if there are any
+    * warnings.
+    */
+   if (pm.this.nerrors || (pm.this.treat_warnings_as_errors &&
+       pm.this.nwarnings))
+   {
+      if (!pm.this.verbose)
+         fprintf(stderr, "pngvalid: %s\n", pm.this.error);
+
+      fprintf(stderr, "pngvalid: %d errors, %d warnings\n", pm.this.nerrors,
+          pm.this.nwarnings);
+
+      exit(1);
+   }
+
+   /* Success case. */
+   if (touch != NULL)
+   {
+      FILE *fsuccess = fopen(touch, "wt");
+
+      if (fsuccess != NULL)
+      {
+         int error = 0;
+         fprintf(fsuccess, "PNG validation succeeded\n");
+         fflush(fsuccess);
+         error = ferror(fsuccess);
+
+         if (fclose(fsuccess) || error)
+         {
+            fprintf(stderr, "%s: write failed\n", touch);
+            exit(1);
+         }
+      }
+
+      else
+      {
+         fprintf(stderr, "%s: open failed\n", touch);
+         exit(1);
+      }
+   }
+
+   /* This is required because some very minimal configurations do not use it:
+    */
+   UNUSED(fail)
+   return 0;
+}
+#else /* write or low level APIs not supported */
+int main(void)
+{
+   fprintf(stderr,
+      "pngvalid: no low level write support in libpng, all tests skipped\n");
+   /* So the test is skipped: */
+   return SKIP;
+}
+#endif