Check libpng directly into third_party/

third_party/libpng/contrib/libtests/pngstest.c

+/*-
+ * pngstest.c
+ *
+ * Copyright (c) 2013-2016 John Cunningham Bowler
+ *
+ * Last changed in libpng 1.6.22 [(PENDING RELEASE)]
+ *
+ * This code is released under the libpng license.
+ * For conditions of distribution and use, see the disclaimer
+ * and license in png.h
+ *
+ * Test for the PNG 'simplified' APIs.
+ */
+#define _ISOC90_SOURCE 1
+#define MALLOC_CHECK_ 2/*glibc facility: turn on debugging*/
+
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+#include <errno.h>
+#include <ctype.h>
+#include <math.h>
+
+#if defined(HAVE_CONFIG_H) && !defined(PNG_NO_CONFIG_H)
+#  include <config.h>
+#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
+
+/* 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_SIMPLIFIED_READ_SUPPORTED /* Else nothing can be done */
+#include "../tools/sRGB.h"
+
+/* KNOWN ISSUES
+ *
+ * These defines switch on alternate algorithms for format conversions to match
+ * the current libpng implementation; they are set to allow pngstest to pass
+ * even though libpng is producing answers that are not as correct as they
+ * should be.
+ */
+#define ALLOW_UNUSED_GPC 0
+   /* If true include unused static GPC functions and declare an external array
+    * of them to hide the fact that they are unused.  This is for development
+    * use while testing the correct function to use to take into account libpng
+    * misbehavior, such as using a simple power law to correct sRGB to linear.
+    */
+
+/* The following is to support direct compilation of this file as C++ */
+#ifdef __cplusplus
+#  define voidcast(type, value) static_cast<type>(value)
+#  define aligncastconst(type, value) \
+      static_cast<type>(static_cast<const void*>(value))
+#else
+#  define voidcast(type, value) (value)
+#  define aligncastconst(type, value) ((const void*)(value))
+#endif /* __cplusplus */
+
+/* During parallel runs of pngstest each temporary file needs a unique name,
+ * this is used to permit uniqueness using a command line argument which can be
+ * up to 22 characters long.
+ */
+static char tmpf[23] = "TMP";
+
+/* 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".
+ */
+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 png_uint_32 color_seed[2];
+
+static void
+reseed(void)
+{
+   color_seed[0] = 0x12345678U;
+   color_seed[1] = 0x9abcdefU;
+}
+
+static void
+random_color(png_colorp color)
+{
+   make_random_bytes(color_seed, color, sizeof *color);
+}
+
+/* Math support - neither Cygwin nor Visual Studio have C99 support and we need
+ * a predictable rounding function, so make one here:
+ */
+static double
+closestinteger(double x)
+{
+   return floor(x + .5);
+}
+
+/* Cast support: remove GCC whines. */
+static png_byte
+u8d(double d)
+{
+   d = closestinteger(d);
+   return (png_byte)d;
+}
+
+static png_uint_16
+u16d(double d)
+{
+   d = closestinteger(d);
+   return (png_uint_16)d;
+}
+
+/* sRGB support: use exact calculations rounded to the nearest int, see the
+ * fesetround() call in main().  sRGB_to_d optimizes the 8 to 16-bit conversion.
+ */
+static double sRGB_to_d[256];
+static double g22_to_d[256];
+
+static void
+init_sRGB_to_d(void)
+{
+   int i;
+
+   sRGB_to_d[0] = 0;
+   for (i=1; i<255; ++i)
+      sRGB_to_d[i] = linear_from_sRGB(i/255.);
+   sRGB_to_d[255] = 1;
+
+   g22_to_d[0] = 0;
+   for (i=1; i<255; ++i)
+      g22_to_d[i] = pow(i/255., 1/.45455);
+   g22_to_d[255] = 1;
+}
+
+static png_byte
+sRGB(double linear /*range 0.0 .. 1.0*/)
+{
+   return u8d(255 * sRGB_from_linear(linear));
+}
+
+static png_byte
+isRGB(int fixed_linear)
+{
+   return sRGB(fixed_linear / 65535.);
+}
+
+#if 0 /* not used */
+static png_byte
+unpremultiply(int component, int alpha)
+{
+   if (alpha <= component)
+      return 255; /* Arbitrary, but consistent with the libpng code */
+
+   else if (alpha >= 65535)
+      return isRGB(component);
+
+   else
+      return sRGB((double)component / alpha);
+}
+#endif
+
+static png_uint_16
+ilinear(int fixed_srgb)
+{
+   return u16d(65535 * sRGB_to_d[fixed_srgb]);
+}
+
+static png_uint_16
+ilineara(int fixed_srgb, int alpha)
+{
+   return u16d((257 * alpha) * sRGB_to_d[fixed_srgb]);
+}
+
+static png_uint_16
+ilinear_g22(int fixed_srgb)
+{
+   return u16d(65535 * g22_to_d[fixed_srgb]);
+}
+
+#if ALLOW_UNUSED_GPC
+static png_uint_16
+ilineara_g22(int fixed_srgb, int alpha)
+{
+   return u16d((257 * alpha) * g22_to_d[fixed_srgb]);
+}
+#endif
+
+static double
+YfromRGBint(int ir, int ig, int ib)
+{
+   double r = ir;
+   double g = ig;
+   double b = ib;
+   return YfromRGB(r, g, b);
+}
+
+#if 0 /* unused */
+/* The error that results from using a 2.2 power law in place of the correct
+ * sRGB transform, given an 8-bit value which might be either sRGB or power-law.
+ */
+static int
+power_law_error8(int value)
+{
+   if (value > 0 && value < 255)
+   {
+      double vd = value / 255.;
+      double e = fabs(
+         pow(sRGB_to_d[value], 1/2.2) - sRGB_from_linear(pow(vd, 2.2)));
+
+      /* Always allow an extra 1 here for rounding errors */
+      e = 1+floor(255 * e);
+      return (int)e;
+   }
+
+   return 0;
+}
+
+static int error_in_sRGB_roundtrip = 56; /* by experiment */
+static int
+power_law_error16(int value)
+{
+   if (value > 0 && value < 65535)
+   {
+      /* Round trip the value through an 8-bit representation but using
+       * non-matching to/from conversions.
+       */
+      double vd = value / 65535.;
+      double e = fabs(
+         pow(sRGB_from_linear(vd), 2.2) - linear_from_sRGB(pow(vd, 1/2.2)));
+
+      /* Always allow an extra 1 here for rounding errors */
+      e = error_in_sRGB_roundtrip+floor(65535 * e);
+      return (int)e;
+   }
+
+   return 0;
+}
+
+static int
+compare_8bit(int v1, int v2, int error_limit, int multiple_algorithms)
+{
+   int e = abs(v1-v2);
+   int ev1, ev2;
+
+   if (e <= error_limit)
+      return 1;
+
+   if (!multiple_algorithms)
+      return 0;
+
+   ev1 = power_law_error8(v1);
+   if (e <= ev1)
+      return 1;
+
+   ev2 = power_law_error8(v2);
+   if (e <= ev2)
+      return 1;
+
+   return 0;
+}
+
+static int
+compare_16bit(int v1, int v2, int error_limit, int multiple_algorithms)
+{
+   int e = abs(v1-v2);
+   int ev1, ev2;
+
+   if (e <= error_limit)
+      return 1;
+
+   /* "multiple_algorithms" in this case means that a color-map has been
+    * involved somewhere, so we can deduce that the values were forced to 8-bit
+    * (like the via_linear case for 8-bit.)
+    */
+   if (!multiple_algorithms)
+      return 0;
+
+   ev1 = power_law_error16(v1);
+   if (e <= ev1)
+      return 1;
+
+   ev2 = power_law_error16(v2);
+   if (e <= ev2)
+      return 1;
+
+   return 0;
+}
+#endif /* unused */
+
+#define USE_FILE 1       /* else memory */
+#define USE_STDIO 2      /* else use file name */
+#define STRICT 4         /* fail on warnings too */
+#define VERBOSE 8
+#define KEEP_TMPFILES 16 /* else delete temporary files */
+#define KEEP_GOING 32
+#define ACCUMULATE 64
+#define FAST_WRITE 128
+#define sRGB_16BIT 256
+#define NO_RESEED  512   /* do not reseed on each new file */
+#define GBG_ERROR 1024   /* do not ignore the gamma+background_rgb_to_gray
+                          * libpng warning. */
+
+static void
+print_opts(png_uint_32 opts)
+{
+   if (opts & USE_FILE)
+      printf(" --file");
+   if (opts & USE_STDIO)
+      printf(" --stdio");
+   if (!(opts & STRICT))
+      printf(" --nostrict");
+   if (opts & VERBOSE)
+      printf(" --verbose");
+   if (opts & KEEP_TMPFILES)
+      printf(" --preserve");
+   if (opts & KEEP_GOING)
+      printf(" --keep-going");
+   if (opts & ACCUMULATE)
+      printf(" --accumulate");
+   if (!(opts & FAST_WRITE)) /* --fast is currently the default */
+      printf(" --slow");
+   if (opts & sRGB_16BIT)
+      printf(" --sRGB-16bit");
+   if (opts & NO_RESEED)
+      printf(" --noreseed");
+#if PNG_LIBPNG_VER < 10700 /* else on by default */
+   if (opts & GBG_ERROR)
+      printf(" --fault-gbg-warning");
+#endif
+}
+
+#define FORMAT_NO_CHANGE 0x80000000 /* additional flag */
+
+/* A name table for all the formats - defines the format of the '+' arguments to
+ * pngstest.
+ */
+#define FORMAT_COUNT 64
+#define FORMAT_MASK 0x3f
+static PNG_CONST char * PNG_CONST format_names[FORMAT_COUNT] =
+{
+   "sRGB-gray",
+   "sRGB-gray+alpha",
+   "sRGB-rgb",
+   "sRGB-rgb+alpha",
+   "linear-gray",
+   "linear-gray+alpha",
+   "linear-rgb",
+   "linear-rgb+alpha",
+
+   "color-mapped-sRGB-gray",
+   "color-mapped-sRGB-gray+alpha",
+   "color-mapped-sRGB-rgb",
+   "color-mapped-sRGB-rgb+alpha",
+   "color-mapped-linear-gray",
+   "color-mapped-linear-gray+alpha",
+   "color-mapped-linear-rgb",
+   "color-mapped-linear-rgb+alpha",
+
+   "sRGB-gray",
+   "sRGB-gray+alpha",
+   "sRGB-bgr",
+   "sRGB-bgr+alpha",
+   "linear-gray",
+   "linear-gray+alpha",
+   "linear-bgr",
+   "linear-bgr+alpha",
+
+   "color-mapped-sRGB-gray",
+   "color-mapped-sRGB-gray+alpha",
+   "color-mapped-sRGB-bgr",
+   "color-mapped-sRGB-bgr+alpha",
+   "color-mapped-linear-gray",
+   "color-mapped-linear-gray+alpha",
+   "color-mapped-linear-bgr",
+   "color-mapped-linear-bgr+alpha",
+
+   "sRGB-gray",
+   "alpha+sRGB-gray",
+   "sRGB-rgb",
+   "alpha+sRGB-rgb",
+   "linear-gray",
+   "alpha+linear-gray",
+   "linear-rgb",
+   "alpha+linear-rgb",
+
+   "color-mapped-sRGB-gray",
+   "color-mapped-alpha+sRGB-gray",
+   "color-mapped-sRGB-rgb",
+   "color-mapped-alpha+sRGB-rgb",
+   "color-mapped-linear-gray",
+   "color-mapped-alpha+linear-gray",
+   "color-mapped-linear-rgb",
+   "color-mapped-alpha+linear-rgb",
+
+   "sRGB-gray",
+   "alpha+sRGB-gray",
+   "sRGB-bgr",
+   "alpha+sRGB-bgr",
+   "linear-gray",
+   "alpha+linear-gray",
+   "linear-bgr",
+   "alpha+linear-bgr",
+
+   "color-mapped-sRGB-gray",
+   "color-mapped-alpha+sRGB-gray",
+   "color-mapped-sRGB-bgr",
+   "color-mapped-alpha+sRGB-bgr",
+   "color-mapped-linear-gray",
+   "color-mapped-alpha+linear-gray",
+   "color-mapped-linear-bgr",
+   "color-mapped-alpha+linear-bgr",
+};
+
+/* Decode an argument to a format number. */
+static png_uint_32
+formatof(const char *arg)
+{
+   char *ep;
+   unsigned long format = strtoul(arg, &ep, 0);
+
+   if (ep > arg && *ep == 0 && format < FORMAT_COUNT)
+      return (png_uint_32)format;
+
+   else for (format=0; format < FORMAT_COUNT; ++format)
+   {
+      if (strcmp(format_names[format], arg) == 0)
+         return (png_uint_32)format;
+   }
+
+   fprintf(stderr, "pngstest: format name '%s' invalid\n", arg);
+   return FORMAT_COUNT;
+}
+
+/* Bitset/test functions for formats */
+#define FORMAT_SET_COUNT (FORMAT_COUNT / 32)
+typedef struct
+{
+   png_uint_32 bits[FORMAT_SET_COUNT];
+}
+format_list;
+
+static void format_init(format_list *pf)
+{
+   int i;
+   for (i=0; i<FORMAT_SET_COUNT; ++i)
+      pf->bits[i] = 0; /* All off */
+}
+
+#if 0 /* currently unused */
+static void format_clear(format_list *pf)
+{
+   int i;
+   for (i=0; i<FORMAT_SET_COUNT; ++i)
+      pf->bits[i] = 0;
+}
+#endif
+
+static int format_is_initial(format_list *pf)
+{
+   int i;
+   for (i=0; i<FORMAT_SET_COUNT; ++i)
+      if (pf->bits[i] != 0)
+         return 0;
+
+   return 1;
+}
+
+static int format_set(format_list *pf, png_uint_32 format)
+{
+   if (format < FORMAT_COUNT)
+      return pf->bits[format >> 5] |= ((png_uint_32)1) << (format & 31);
+
+   return 0;
+}
+
+#if 0 /* currently unused */
+static int format_unset(format_list *pf, png_uint_32 format)
+{
+   if (format < FORMAT_COUNT)
+      return pf->bits[format >> 5] &= ~((png_uint_32)1) << (format & 31);
+
+   return 0;
+}
+#endif
+
+static int format_isset(format_list *pf, png_uint_32 format)
+{
+   return format < FORMAT_COUNT &&
+      (pf->bits[format >> 5] & (((png_uint_32)1) << (format & 31))) != 0;
+}
+
+static void format_default(format_list *pf, int redundant)
+{
+   if (redundant)
+   {
+      int i;
+
+      /* set everything, including flags that are pointless */
+      for (i=0; i<FORMAT_SET_COUNT; ++i)
+         pf->bits[i] = ~(png_uint_32)0;
+   }
+
+   else
+   {
+      png_uint_32 f;
+
+      for (f=0; f<FORMAT_COUNT; ++f)
+      {
+         /* Eliminate redundant and unsupported settings. */
+#        ifdef PNG_FORMAT_BGR_SUPPORTED
+            /* BGR is meaningless if no color: */
+            if ((f & PNG_FORMAT_FLAG_COLOR) == 0 &&
+               (f & PNG_FORMAT_FLAG_BGR) != 0)
+#        else
+            if ((f & 0x10U/*HACK: fixed value*/) != 0)
+#        endif
+            continue;
+
+         /* AFIRST is meaningless if no alpha: */
+#        ifdef PNG_FORMAT_AFIRST_SUPPORTED
+            if ((f & PNG_FORMAT_FLAG_ALPHA) == 0 &&
+               (f & PNG_FORMAT_FLAG_AFIRST) != 0)
+#        else
+            if ((f & 0x20U/*HACK: fixed value*/) != 0)
+#        endif
+            continue;
+
+         format_set(pf, f);
+      }
+   }
+}
+
+/* THE Image STRUCTURE */
+/* The super-class of a png_image, contains the decoded image plus the input
+ * data necessary to re-read the file with a different format.
+ */
+typedef struct
+{
+   png_image   image;
+   png_uint_32 opts;
+   const char *file_name;
+   int         stride_extra;
+   FILE       *input_file;
+   png_voidp   input_memory;
+   png_size_t  input_memory_size;
+   png_bytep   buffer;
+   ptrdiff_t   stride;
+   png_size_t  bufsize;
+   png_size_t  allocsize;
+   char        tmpfile_name[32];
+   png_uint_16 colormap[256*4];
+}
+Image;
+
+/* Initializer: also sets the permitted error limit for 16-bit operations. */
+static void
+newimage(Image *image)
+{
+   memset(image, 0, sizeof *image);
+}
+
+/* Reset the image to be read again - only needs to rewind the FILE* at present.
+ */
+static void
+resetimage(Image *image)
+{
+   if (image->input_file != NULL)
+      rewind(image->input_file);
+}
+
+/* Free the image buffer; the buffer is re-used on a re-read, this is just for
+ * cleanup.
+ */
+static void
+freebuffer(Image *image)
+{
+   if (image->buffer) free(image->buffer);
+   image->buffer = NULL;
+   image->bufsize = 0;
+   image->allocsize = 0;
+}
+
+/* Delete function; cleans out all the allocated data and the temporary file in
+ * the image.
+ */
+static void
+freeimage(Image *image)
+{
+   freebuffer(image);
+   png_image_free(&image->image);
+
+   if (image->input_file != NULL)
+   {
+      fclose(image->input_file);
+      image->input_file = NULL;
+   }
+
+   if (image->input_memory != NULL)
+   {
+      free(image->input_memory);
+      image->input_memory = NULL;
+      image->input_memory_size = 0;
+   }
+
+   if (image->tmpfile_name[0] != 0 && (image->opts & KEEP_TMPFILES) == 0)
+   {
+      (void)remove(image->tmpfile_name);
+      image->tmpfile_name[0] = 0;
+   }
+}
+
+/* This is actually a re-initializer; allows an image structure to be re-used by
+ * freeing everything that relates to an old image.
+ */
+static void initimage(Image *image, png_uint_32 opts, const char *file_name,
+   int stride_extra)
+{
+   freeimage(image);
+   memset(&image->image, 0, sizeof image->image);
+   image->opts = opts;
+   image->file_name = file_name;
+   image->stride_extra = stride_extra;
+}
+
+/* Make sure the image buffer is big enough; allows re-use of the buffer if the
+ * image is re-read.
+ */
+#define BUFFER_INIT8 73
+static void
+allocbuffer(Image *image)
+{
+   png_size_t size = PNG_IMAGE_BUFFER_SIZE(image->image, image->stride);
+
+   if (size+32 > image->bufsize)
+   {
+      freebuffer(image);
+      image->buffer = voidcast(png_bytep, malloc(size+32));
+      if (image->buffer == NULL)
+      {
+         fflush(stdout);
+         fprintf(stderr,
+            "simpletest: out of memory allocating %lu(+32) byte buffer\n",
+            (unsigned long)size);
+         exit(1);
+      }
+      image->bufsize = size+32;
+   }
+
+   memset(image->buffer, 95, image->bufsize);
+   memset(image->buffer+16, BUFFER_INIT8, size);
+   image->allocsize = size;
+}
+
+/* Make sure 16 bytes match the given byte. */
+static int
+check16(png_const_bytep bp, int b)
+{
+   int i = 16;
+
+   do
+      if (*bp != b) return 1;
+   while (--i);
+
+   return 0;
+}
+
+/* Check for overwrite in the image buffer. */
+static void
+checkbuffer(Image *image, const char *arg)
+{
+   if (check16(image->buffer, 95))
+   {
+      fflush(stdout);
+      fprintf(stderr, "%s: overwrite at start of image buffer\n", arg);
+      exit(1);
+   }
+
+   if (check16(image->buffer+16+image->allocsize, 95))
+   {
+      fflush(stdout);
+      fprintf(stderr, "%s: overwrite at end of image buffer\n", arg);
+      exit(1);
+   }
+}
+
+/* ERROR HANDLING */
+/* Log a terminal error, also frees the libpng part of the image if necessary.
+ */
+static int
+logerror(Image *image, const char *a1, const char *a2, const char *a3)
+{
+   fflush(stdout);
+   if (image->image.warning_or_error)
+      fprintf(stderr, "%s%s%s: %s\n", a1, a2, a3, image->image.message);
+
+   else
+      fprintf(stderr, "%s%s%s\n", a1, a2, a3);
+
+   if (image->image.opaque != NULL)
+   {
+      fprintf(stderr, "%s: image opaque pointer non-NULL on error\n",
+         image->file_name);
+      png_image_free(&image->image);
+   }
+
+   return 0;
+}
+
+/* Log an error and close a file (just a utility to do both things in one
+ * function call.)
+ */
+static int
+logclose(Image *image, FILE *f, const char *name, const char *operation)
+{
+   int e = errno;
+
+   fclose(f);
+   return logerror(image, name, operation, strerror(e));
+}
+
+/* Make sure the png_image has been freed - validates that libpng is doing what
+ * the spec says and freeing the image.
+ */
+static int
+checkopaque(Image *image)
+{
+   if (image->image.opaque != NULL)
+   {
+      png_image_free(&image->image);
+      return logerror(image, image->file_name, ": opaque not NULL", "");
+   }
+
+   /* Separate out the gamma+background_rgb_to_gray warning because it may
+    * produce opaque component errors:
+    */
+   else if (image->image.warning_or_error != 0 &&
+            (strcmp(image->image.message,
+               "libpng does not support gamma+background+rgb_to_gray") == 0 ?
+                  (image->opts & GBG_ERROR) != 0 : (image->opts & STRICT) != 0))
+      return logerror(image, image->file_name, (image->opts & GBG_ERROR) != 0 ?
+                      " --fault-gbg-warning" : " --strict", "");
+
+   else
+      return 1;
+}
+
+/* IMAGE COMPARISON/CHECKING */
+/* Compare the pixels of two images, which should be the same but aren't.  The
+ * images must have been checked for a size match.
+ */
+typedef struct
+{
+   /* The components, for grayscale images the gray value is in 'g' and if alpha
+    * is not present 'a' is set to 255 or 65535 according to format.
+    */
+   int         r, g, b, a;
+} Pixel;
+
+typedef struct
+{
+   /* The background as the original sRGB 8-bit value converted to the final
+    * integer format and as a double precision linear value in the range 0..1
+    * for with partially transparent pixels.
+    */
+   int ir, ig, ib;
+   double dr, dg, db; /* linear r,g,b scaled to 0..1 */
+} Background;
+
+/* Basic image formats; control the data but not the layout thereof. */
+#define BASE_FORMATS\
+   (PNG_FORMAT_FLAG_ALPHA|PNG_FORMAT_FLAG_COLOR|PNG_FORMAT_FLAG_LINEAR)
+
+/* Read a Pixel from a buffer.  The code below stores the correct routine for
+ * the format in a function pointer, these are the routines:
+ */
+static void
+gp_g8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = p->g = p->b = pp[0];
+   p->a = 255;
+}
+
+static void
+gp_ga8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = p->g = p->b = pp[0];
+   p->a = pp[1];
+}
+
+#ifdef PNG_FORMAT_AFIRST_SUPPORTED
+static void
+gp_ag8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = p->g = p->b = pp[1];
+   p->a = pp[0];
+}
+#endif
+
+static void
+gp_rgb8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = pp[0];
+   p->g = pp[1];
+   p->b = pp[2];
+   p->a = 255;
+}
+
+#ifdef PNG_FORMAT_BGR_SUPPORTED
+static void
+gp_bgr8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = pp[2];
+   p->g = pp[1];
+   p->b = pp[0];
+   p->a = 255;
+}
+#endif
+
+static void
+gp_rgba8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = pp[0];
+   p->g = pp[1];
+   p->b = pp[2];
+   p->a = pp[3];
+}
+
+#ifdef PNG_FORMAT_BGR_SUPPORTED
+static void
+gp_bgra8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = pp[2];
+   p->g = pp[1];
+   p->b = pp[0];
+   p->a = pp[3];
+}
+#endif
+
+#ifdef PNG_FORMAT_AFIRST_SUPPORTED
+static void
+gp_argb8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = pp[1];
+   p->g = pp[2];
+   p->b = pp[3];
+   p->a = pp[0];
+}
+#endif
+
+#if defined(PNG_FORMAT_AFIRST_SUPPORTED) && defined(PNG_FORMAT_BGR_SUPPORTED)
+static void
+gp_abgr8(Pixel *p, png_const_voidp pb)
+{
+   png_const_bytep pp = voidcast(png_const_bytep, pb);
+
+   p->r = pp[3];
+   p->g = pp[2];
+   p->b = pp[1];
+   p->a = pp[0];
+}
+#endif
+
+static void
+gp_g16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = p->g = p->b = pp[0];
+   p->a = 65535;
+}
+
+static void
+gp_ga16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = p->g = p->b = pp[0];
+   p->a = pp[1];
+}
+
+#ifdef PNG_FORMAT_AFIRST_SUPPORTED
+static void
+gp_ag16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = p->g = p->b = pp[1];
+   p->a = pp[0];
+}
+#endif
+
+static void
+gp_rgb16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = pp[0];
+   p->g = pp[1];
+   p->b = pp[2];
+   p->a = 65535;
+}
+
+#ifdef PNG_FORMAT_BGR_SUPPORTED
+static void
+gp_bgr16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = pp[2];
+   p->g = pp[1];
+   p->b = pp[0];
+   p->a = 65535;
+}
+#endif
+
+static void
+gp_rgba16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = pp[0];
+   p->g = pp[1];
+   p->b = pp[2];
+   p->a = pp[3];
+}
+
+#ifdef PNG_FORMAT_BGR_SUPPORTED
+static void
+gp_bgra16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = pp[2];
+   p->g = pp[1];
+   p->b = pp[0];
+   p->a = pp[3];
+}
+#endif
+
+#ifdef PNG_FORMAT_AFIRST_SUPPORTED
+static void
+gp_argb16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = pp[1];
+   p->g = pp[2];
+   p->b = pp[3];
+   p->a = pp[0];
+}
+#endif
+
+#if defined(PNG_FORMAT_AFIRST_SUPPORTED) && defined(PNG_FORMAT_BGR_SUPPORTED)
+static void
+gp_abgr16(Pixel *p, png_const_voidp pb)
+{
+   png_const_uint_16p pp = voidcast(png_const_uint_16p, pb);
+
+   p->r = pp[3];
+   p->g = pp[2];
+   p->b = pp[1];
+   p->a = pp[0];
+}
+#endif
+
+/* Given a format, return the correct one of the above functions. */
+static void (*
+get_pixel(png_uint_32 format))(Pixel *p, png_const_voidp pb)
+{
+   /* The color-map flag is irrelevant here - the caller of the function
+    * returned must either pass the buffer or, for a color-mapped image, the
+    * correct entry in the color-map.
+    */
+   if (format & PNG_FORMAT_FLAG_LINEAR)
+   {
+      if (format & PNG_FORMAT_FLAG_COLOR)
+      {
+#        ifdef PNG_FORMAT_BGR_SUPPORTED
+            if (format & PNG_FORMAT_FLAG_BGR)
+            {
+               if (format & PNG_FORMAT_FLAG_ALPHA)
+               {
+#                 ifdef PNG_FORMAT_AFIRST_SUPPORTED
+                     if (format & PNG_FORMAT_FLAG_AFIRST)
+                        return gp_abgr16;
+
+                     else
+#                 endif
+                     return gp_bgra16;
+               }
+
+               else
+                  return gp_bgr16;
+            }
+
+            else
+#        endif
+         {
+            if (format & PNG_FORMAT_FLAG_ALPHA)
+            {
+#              ifdef PNG_FORMAT_AFIRST_SUPPORTED
+                  if (format & PNG_FORMAT_FLAG_AFIRST)
+                     return gp_argb16;
+
+                  else
+#              endif
+                  return gp_rgba16;
+            }
+
+            else
+               return gp_rgb16;
+         }
+      }
+
+      else
+      {
+         if (format & PNG_FORMAT_FLAG_ALPHA)
+         {
+#           ifdef PNG_FORMAT_AFIRST_SUPPORTED
+               if (format & PNG_FORMAT_FLAG_AFIRST)
+                  return gp_ag16;
+
+               else
+#           endif
+               return gp_ga16;
+         }
+
+         else
+            return gp_g16;
+      }
+   }
+
+   else
+   {
+      if (format & PNG_FORMAT_FLAG_COLOR)
+      {
+#        ifdef PNG_FORMAT_BGR_SUPPORTED
+            if (format & PNG_FORMAT_FLAG_BGR)
+            {
+               if (format & PNG_FORMAT_FLAG_ALPHA)
+               {
+#                 ifdef PNG_FORMAT_AFIRST_SUPPORTED
+                     if (format & PNG_FORMAT_FLAG_AFIRST)
+                        return gp_abgr8;
+
+                     else
+#                 endif
+                     return gp_bgra8;
+               }
+
+               else
+                  return gp_bgr8;
+            }
+
+            else
+#        endif
+         {
+            if (format & PNG_FORMAT_FLAG_ALPHA)
+            {
+#              ifdef PNG_FORMAT_AFIRST_SUPPORTED
+                  if (format & PNG_FORMAT_FLAG_AFIRST)
+                     return gp_argb8;
+
+                  else
+#              endif
+                  return gp_rgba8;
+            }
+
+            else
+               return gp_rgb8;
+         }
+      }
+
+      else
+      {
+         if (format & PNG_FORMAT_FLAG_ALPHA)
+         {
+#           ifdef PNG_FORMAT_AFIRST_SUPPORTED
+               if (format & PNG_FORMAT_FLAG_AFIRST)
+                  return gp_ag8;
+
+               else
+#           endif
+               return gp_ga8;
+         }
+
+         else
+            return gp_g8;
+      }
+   }
+}
+
+/* Convertion between pixel formats.  The code above effectively eliminates the
+ * component ordering changes leaving three basic changes:
+ *
+ * 1) Remove an alpha channel by pre-multiplication or compositing on a
+ *    background color.  (Adding an alpha channel is a no-op.)
+ *
+ * 2) Remove color by mapping to grayscale.  (Grayscale to color is a no-op.)
+ *
+ * 3) Convert between 8-bit and 16-bit components.  (Both directtions are
+ *    relevant.)
+ *
+ * This gives the following base format conversion matrix:
+ *
+ *   OUT:    ----- 8-bit -----    ----- 16-bit -----
+ *   IN     G    GA   RGB  RGBA  G    GA   RGB  RGBA
+ *  8 G     .    .    .    .     lin  lin  lin  lin
+ *  8 GA    bckg .    bckc .     pre' pre  pre' pre
+ *  8 RGB   g8   g8   .    .     glin glin lin  lin
+ *  8 RGBA  g8b  g8   bckc .     gpr' gpre pre' pre
+ * 16 G     sRGB sRGB sRGB sRGB  .    .    .    .
+ * 16 GA    b16g unpg b16c unpc  A    .    A    .
+ * 16 RGB   sG   sG   sRGB sRGB  g16  g16  .    .
+ * 16 RGBA  gb16 sGp  cb16 sCp   g16  g16' A    .
+ *
+ *  8-bit to 8-bit:
+ * bckg: composite on gray background
+ * bckc: composite on color background
+ * g8:   convert sRGB components to sRGB grayscale
+ * g8b:  convert sRGB components to grayscale and composite on gray background
+ *
+ *  8-bit to 16-bit:
+ * lin:  make sRGB components linear, alpha := 65535
+ * pre:  make sRGB components linear and premultiply by alpha  (scale alpha)
+ * pre': as 'pre' but alpha := 65535
+ * glin: make sRGB components linear, convert to grayscale, alpha := 65535
+ * gpre: make sRGB components grayscale and linear and premultiply by alpha
+ * gpr': as 'gpre' but alpha := 65535
+ *
+ *  16-bit to 8-bit:
+ * sRGB: convert linear components to sRGB, alpha := 255
+ * unpg: unpremultiply gray component and convert to sRGB (scale alpha)
+ * unpc: unpremultiply color components and convert to sRGB (scale alpha)
+ * b16g: composite linear onto gray background and convert the result to sRGB
+ * b16c: composite linear onto color background and convert the result to sRGB
+ * sG:   convert linear RGB to sRGB grayscale
+ * sGp:  unpremultiply RGB then convert to sRGB grayscale
+ * sCp:  unpremultiply RGB then convert to sRGB
+ * gb16: composite linear onto background and convert to sRGB grayscale
+ *       (order doesn't matter, the composite and grayscale operations permute)
+ * cb16: composite linear onto background and convert to sRGB
+ *
+ *  16-bit to 16-bit:
+ * A:    set alpha to 65535
+ * g16:  convert linear RGB to linear grayscale (alpha := 65535)
+ * g16': as 'g16' but alpha is unchanged
+ */
+/* Simple copy: */
+static void
+gpc_noop(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+   out->r = in->r;
+   out->g = in->g;
+   out->b = in->b;
+   out->a = in->a;
+}
+
+#if ALLOW_UNUSED_GPC
+static void
+gpc_nop8(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+   if (in->a == 0)
+      out->r = out->g = out->b = 255;
+
+   else
+   {
+      out->r = in->r;
+      out->g = in->g;
+      out->b = in->b;
+   }
+
+   out->a = in->a;
+}
+#endif
+
+#if ALLOW_UNUSED_GPC
+static void
+gpc_nop6(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+   if (in->a == 0)
+      out->r = out->g = out->b = 65535;
+
+   else
+   {
+      out->r = in->r;
+      out->g = in->g;
+      out->b = in->b;
+   }
+
+   out->a = in->a;
+}
+#endif
+
+/* 8-bit to 8-bit conversions */
+/* bckg: composite on gray background */
+static void
+gpc_bckg(Pixel *out, const Pixel *in, const Background *back)
+{
+   if (in->a <= 0)
+      out->r = out->g = out->b = back->ig;
+
+   else if (in->a >= 255)
+      out->r = out->g = out->b = in->g;
+
+   else
+   {
+      double a = in->a / 255.;
+
+      out->r = out->g = out->b = sRGB(sRGB_to_d[in->g] * a + back->dg * (1-a));
+   }
+
+   out->a = 255;
+}
+
+/* bckc: composite on color background */
+static void
+gpc_bckc(Pixel *out, const Pixel *in, const Background *back)
+{
+   if (in->a <= 0)
+   {
+      out->r = back->ir;
+      out->g = back->ig;
+      out->b = back->ib;
+   }
+
+   else if (in->a >= 255)
+   {
+      out->r = in->r;
+      out->g = in->g;
+      out->b = in->b;
+   }
+
+   else
+   {
+      double a = in->a / 255.;
+
+      out->r = sRGB(sRGB_to_d[in->r] * a + back->dr * (1-a));
+      out->g = sRGB(sRGB_to_d[in->g] * a + back->dg * (1-a));
+      out->b = sRGB(sRGB_to_d[in->b] * a + back->db * (1-a));
+   }
+
+   out->a = 255;
+}
+
+/* g8: convert sRGB components to sRGB grayscale */
+static void
+gpc_g8(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->r == in->g && in->g == in->b)
+      out->r = out->g = out->b = in->g;
+
+   else
+      out->r = out->g = out->b =
+         sRGB(YfromRGB(sRGB_to_d[in->r], sRGB_to_d[in->g], sRGB_to_d[in->b]));
+
+   out->a = in->a;
+}
+
+/* g8b: convert sRGB components to grayscale and composite on gray background */
+static void
+gpc_g8b(Pixel *out, const Pixel *in, const Background *back)
+{
+   if (in->a <= 0)
+      out->r = out->g = out->b = back->ig;
+
+   else if (in->a >= 255)
+   {
+      if (in->r == in->g && in->g == in->b)
+         out->r = out->g = out->b = in->g;
+
+      else
+         out->r = out->g = out->b = sRGB(YfromRGB(
+            sRGB_to_d[in->r], sRGB_to_d[in->g], sRGB_to_d[in->b]));
+   }
+
+   else
+   {
+      double a = in->a/255.;
+
+      out->r = out->g = out->b = sRGB(a * YfromRGB(sRGB_to_d[in->r],
+         sRGB_to_d[in->g], sRGB_to_d[in->b]) + back->dg * (1-a));
+   }
+
+   out->a = 255;
+}
+
+/* 8-bit to 16-bit conversions */
+/* lin: make sRGB components linear, alpha := 65535 */
+static void
+gpc_lin(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   out->r = ilinear(in->r);
+
+   if (in->g == in->r)
+   {
+      out->g = out->r;
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else
+         out->b = ilinear(in->b);
+   }
+
+   else
+   {
+      out->g = ilinear(in->g);
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else if (in->b == in->g)
+         out->b = out->g;
+
+      else
+         out->b = ilinear(in->b);
+   }
+
+   out->a = 65535;
+}
+
+/* pre: make sRGB components linear and premultiply by alpha (scale alpha) */
+static void
+gpc_pre(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   out->r = ilineara(in->r, in->a);
+
+   if (in->g == in->r)
+   {
+      out->g = out->r;
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else
+         out->b = ilineara(in->b, in->a);
+   }
+
+   else
+   {
+      out->g = ilineara(in->g, in->a);
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else if (in->b == in->g)
+         out->b = out->g;
+
+      else
+         out->b = ilineara(in->b, in->a);
+   }
+
+   out->a = in->a * 257;
+}
+
+/* pre': as 'pre' but alpha := 65535 */
+static void
+gpc_preq(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   out->r = ilineara(in->r, in->a);
+
+   if (in->g == in->r)
+   {
+      out->g = out->r;
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else
+         out->b = ilineara(in->b, in->a);
+   }
+
+   else
+   {
+      out->g = ilineara(in->g, in->a);
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else if (in->b == in->g)
+         out->b = out->g;
+
+      else
+         out->b = ilineara(in->b, in->a);
+   }
+
+   out->a = 65535;
+}
+
+/* glin: make sRGB components linear, convert to grayscale, alpha := 65535 */
+static void
+gpc_glin(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->r == in->g && in->g == in->b)
+      out->r = out->g = out->b = ilinear(in->g);
+
+   else
+      out->r = out->g = out->b = u16d(65535 *
+         YfromRGB(sRGB_to_d[in->r], sRGB_to_d[in->g], sRGB_to_d[in->b]));
+
+   out->a = 65535;
+}
+
+/* gpre: make sRGB components grayscale and linear and premultiply by alpha */
+static void
+gpc_gpre(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->r == in->g && in->g == in->b)
+      out->r = out->g = out->b = ilineara(in->g, in->a);
+
+   else
+      out->r = out->g = out->b = u16d(in->a * 257 *
+         YfromRGB(sRGB_to_d[in->r], sRGB_to_d[in->g], sRGB_to_d[in->b]));
+
+   out->a = 257 * in->a;
+}
+
+/* gpr': as 'gpre' but alpha := 65535 */
+static void
+gpc_gprq(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->r == in->g && in->g == in->b)
+      out->r = out->g = out->b = ilineara(in->g, in->a);
+
+   else
+      out->r = out->g = out->b = u16d(in->a * 257 *
+         YfromRGB(sRGB_to_d[in->r], sRGB_to_d[in->g], sRGB_to_d[in->b]));
+
+   out->a = 65535;
+}
+
+/* 8-bit to 16-bit conversions for gAMA 45455 encoded values */
+/* Lin: make gAMA 45455 components linear, alpha := 65535 */
+static void
+gpc_Lin(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   out->r = ilinear_g22(in->r);
+
+   if (in->g == in->r)
+   {
+      out->g = out->r;
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else
+         out->b = ilinear_g22(in->b);
+   }
+
+   else
+   {
+      out->g = ilinear_g22(in->g);
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else if (in->b == in->g)
+         out->b = out->g;
+
+      else
+         out->b = ilinear_g22(in->b);
+   }
+
+   out->a = 65535;
+}
+
+#if ALLOW_UNUSED_GPC
+/* Pre: make gAMA 45455 components linear and premultiply by alpha (scale alpha)
+ */
+static void
+gpc_Pre(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   out->r = ilineara_g22(in->r, in->a);
+
+   if (in->g == in->r)
+   {
+      out->g = out->r;
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else
+         out->b = ilineara_g22(in->b, in->a);
+   }
+
+   else
+   {
+      out->g = ilineara_g22(in->g, in->a);
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else if (in->b == in->g)
+         out->b = out->g;
+
+      else
+         out->b = ilineara_g22(in->b, in->a);
+   }
+
+   out->a = in->a * 257;
+}
+#endif
+
+#if ALLOW_UNUSED_GPC
+/* Pre': as 'Pre' but alpha := 65535 */
+static void
+gpc_Preq(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   out->r = ilineara_g22(in->r, in->a);
+
+   if (in->g == in->r)
+   {
+      out->g = out->r;
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else
+         out->b = ilineara_g22(in->b, in->a);
+   }
+
+   else
+   {
+      out->g = ilineara_g22(in->g, in->a);
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else if (in->b == in->g)
+         out->b = out->g;
+
+      else
+         out->b = ilineara_g22(in->b, in->a);
+   }
+
+   out->a = 65535;
+}
+#endif
+
+#if ALLOW_UNUSED_GPC
+/* Glin: make gAMA 45455 components linear, convert to grayscale, alpha := 65535
+ */
+static void
+gpc_Glin(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->r == in->g && in->g == in->b)
+      out->r = out->g = out->b = ilinear_g22(in->g);
+
+   else
+      out->r = out->g = out->b = u16d(65535 *
+         YfromRGB(g22_to_d[in->r], g22_to_d[in->g], g22_to_d[in->b]));
+
+   out->a = 65535;
+}
+#endif
+
+#if ALLOW_UNUSED_GPC
+/* Gpre: make gAMA 45455 components grayscale and linear and premultiply by
+ * alpha.
+ */
+static void
+gpc_Gpre(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->r == in->g && in->g == in->b)
+      out->r = out->g = out->b = ilineara_g22(in->g, in->a);
+
+   else
+      out->r = out->g = out->b = u16d(in->a * 257 *
+         YfromRGB(g22_to_d[in->r], g22_to_d[in->g], g22_to_d[in->b]));
+
+   out->a = 257 * in->a;
+}
+#endif
+
+#if ALLOW_UNUSED_GPC
+/* Gpr': as 'Gpre' but alpha := 65535 */
+static void
+gpc_Gprq(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->r == in->g && in->g == in->b)
+      out->r = out->g = out->b = ilineara_g22(in->g, in->a);
+
+   else
+      out->r = out->g = out->b = u16d(in->a * 257 *
+         YfromRGB(g22_to_d[in->r], g22_to_d[in->g], g22_to_d[in->b]));
+
+   out->a = 65535;
+}
+#endif
+
+/* 16-bit to 8-bit conversions */
+/* sRGB: convert linear components to sRGB, alpha := 255 */
+static void
+gpc_sRGB(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   out->r = isRGB(in->r);
+
+   if (in->g == in->r)
+   {
+      out->g = out->r;
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else
+         out->b = isRGB(in->b);
+   }
+
+   else
+   {
+      out->g = isRGB(in->g);
+
+      if (in->b == in->r)
+         out->b = out->r;
+
+      else if (in->b == in->g)
+         out->b = out->g;
+
+      else
+         out->b = isRGB(in->b);
+   }
+
+   out->a = 255;
+}
+
+/* unpg: unpremultiply gray component and convert to sRGB (scale alpha) */
+static void
+gpc_unpg(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->a <= 128)
+   {
+      out->r = out->g = out->b = 255;
+      out->a = 0;
+   }
+
+   else
+   {
+      out->r = out->g = out->b = sRGB((double)in->g / in->a);
+      out->a = u8d(in->a / 257.);
+   }
+}
+
+/* unpc: unpremultiply color components and convert to sRGB (scale alpha) */
+static void
+gpc_unpc(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->a <= 128)
+   {
+      out->r = out->g = out->b = 255;
+      out->a = 0;
+   }
+
+   else
+   {
+      out->r = sRGB((double)in->r / in->a);
+      out->g = sRGB((double)in->g / in->a);
+      out->b = sRGB((double)in->b / in->a);
+      out->a = u8d(in->a / 257.);
+   }
+}
+
+/* b16g: composite linear onto gray background and convert the result to sRGB */
+static void
+gpc_b16g(Pixel *out, const Pixel *in, const Background *back)
+{
+   if (in->a <= 0)
+      out->r = out->g = out->b = back->ig;
+
+   else
+   {
+      double a = in->a/65535.;
+      double a1 = 1-a;
+
+      a /= 65535;
+      out->r = out->g = out->b = sRGB(in->g * a + back->dg * a1);
+   }
+
+   out->a = 255;
+}
+
+/* b16c: composite linear onto color background and convert the result to sRGB*/
+static void
+gpc_b16c(Pixel *out, const Pixel *in, const Background *back)
+{
+   if (in->a <= 0)
+   {
+      out->r = back->ir;
+      out->g = back->ig;
+      out->b = back->ib;
+   }
+
+   else
+   {
+      double a = in->a/65535.;
+      double a1 = 1-a;
+
+      a /= 65535;
+      out->r = sRGB(in->r * a + back->dr * a1);
+      out->g = sRGB(in->g * a + back->dg * a1);
+      out->b = sRGB(in->b * a + back->db * a1);
+   }
+
+   out->a = 255;
+}
+
+/* sG: convert linear RGB to sRGB grayscale */
+static void
+gpc_sG(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   out->r = out->g = out->b = sRGB(YfromRGBint(in->r, in->g, in->b)/65535);
+   out->a = 255;
+}
+
+/* sGp: unpremultiply RGB then convert to sRGB grayscale */
+static void
+gpc_sGp(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->a <= 128)
+   {
+      out->r = out->g = out->b = 255;
+      out->a = 0;
+   }
+
+   else
+   {
+      out->r = out->g = out->b = sRGB(YfromRGBint(in->r, in->g, in->b)/in->a);
+      out->a = u8d(in->a / 257.);
+   }
+}
+
+/* sCp: unpremultiply RGB then convert to sRGB */
+static void
+gpc_sCp(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+
+   if (in->a <= 128)
+   {
+      out->r = out->g = out->b = 255;
+      out->a = 0;
+   }
+
+   else
+   {
+      out->r = sRGB((double)in->r / in->a);
+      out->g = sRGB((double)in->g / in->a);
+      out->b = sRGB((double)in->b / in->a);
+      out->a = u8d(in->a / 257.);
+   }
+}
+
+/* gb16: composite linear onto background and convert to sRGB grayscale */
+/*  (order doesn't matter, the composite and grayscale operations permute) */
+static void
+gpc_gb16(Pixel *out, const Pixel *in, const Background *back)
+{
+   if (in->a <= 0)
+      out->r = out->g = out->b = back->ig;
+
+   else if (in->a >= 65535)
+      out->r = out->g = out->b = isRGB(in->g);
+
+   else
+   {
+      double a = in->a / 65535.;
+      double a1 = 1-a;
+
+      a /= 65535;
+      out->r = out->g = out->b = sRGB(in->g * a + back->dg * a1);
+   }
+
+   out->a = 255;
+}
+
+/* cb16: composite linear onto background and convert to sRGB */
+static void
+gpc_cb16(Pixel *out, const Pixel *in, const Background *back)
+{
+   if (in->a <= 0)
+   {
+      out->r = back->ir;
+      out->g = back->ig;
+      out->b = back->ib;
+   }
+
+   else if (in->a >= 65535)
+   {
+      out->r = isRGB(in->r);
+      out->g = isRGB(in->g);
+      out->b = isRGB(in->b);
+   }
+
+   else
+   {
+      double a = in->a / 65535.;
+      double a1 = 1-a;
+
+      a /= 65535;
+      out->r = sRGB(in->r * a + back->dr * a1);
+      out->g = sRGB(in->g * a + back->dg * a1);
+      out->b = sRGB(in->b * a + back->db * a1);
+   }
+
+   out->a = 255;
+}
+
+/* 16-bit to 16-bit conversions */
+/* A:    set alpha to 65535 */
+static void
+gpc_A(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+   out->r = in->r;
+   out->g = in->g;
+   out->b = in->b;
+   out->a = 65535;
+}
+
+/* g16:  convert linear RGB to linear grayscale (alpha := 65535) */
+static void
+gpc_g16(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+   out->r = out->g = out->b = u16d(YfromRGBint(in->r, in->g, in->b));
+   out->a = 65535;
+}
+
+/* g16': as 'g16' but alpha is unchanged */
+static void
+gpc_g16q(Pixel *out, const Pixel *in, const Background *back)
+{
+   (void)back;
+   out->r = out->g = out->b = u16d(YfromRGBint(in->r, in->g, in->b));
+   out->a = in->a;
+}
+
+#if ALLOW_UNUSED_GPC
+/* Unused functions (to hide them from GCC unused function warnings) */
+void (* const gpc_unused[])
+   (Pixel *out, const Pixel *in, const Background *back) =
+{
+   gpc_Pre, gpc_Preq, gpc_Glin, gpc_Gpre, gpc_Gprq, gpc_nop8, gpc_nop6
+};
+#endif
+
+/*   OUT:    ----- 8-bit -----    ----- 16-bit -----
+ *   IN     G    GA   RGB  RGBA  G    GA   RGB  RGBA
+ *  8 G     .    .    .    .     lin  lin  lin  lin
+ *  8 GA    bckg .    bckc .     pre' pre  pre' pre
+ *  8 RGB   g8   g8   .    .     glin glin lin  lin
+ *  8 RGBA  g8b  g8   bckc .     gpr' gpre pre' pre
+ * 16 G     sRGB sRGB sRGB sRGB  .    .    .    .
+ * 16 GA    b16g unpg b16c unpc  A    .    A    .
+ * 16 RGB   sG   sG   sRGB sRGB  g16  g16  .    .
+ * 16 RGBA  gb16 sGp  cb16 sCp   g16  g16' A    .
+ *
+ * The matrix is held in an array indexed thus:
+ *
+ *   gpc_fn[out_format & BASE_FORMATS][in_format & BASE_FORMATS];
+ */
+/* This will produce a compile time error if the FORMAT_FLAG values don't
+ * match the above matrix!
+ */
+#if PNG_FORMAT_FLAG_ALPHA == 1 && PNG_FORMAT_FLAG_COLOR == 2 &&\
+   PNG_FORMAT_FLAG_LINEAR == 4
+static void (* const gpc_fn[8/*in*/][8/*out*/])
+   (Pixel *out, const Pixel *in, const Background *back) =
+{
+/*out: G-8     GA-8     RGB-8    RGBA-8    G-16     GA-16   RGB-16  RGBA-16 */
+   {gpc_noop,gpc_noop,gpc_noop,gpc_noop, gpc_Lin, gpc_Lin, gpc_Lin, gpc_Lin },
+   {gpc_bckg,gpc_noop,gpc_bckc,gpc_noop, gpc_preq,gpc_pre, gpc_preq,gpc_pre },
+   {gpc_g8,  gpc_g8,  gpc_noop,gpc_noop, gpc_glin,gpc_glin,gpc_lin, gpc_lin },
+   {gpc_g8b, gpc_g8,  gpc_bckc,gpc_noop, gpc_gprq,gpc_gpre,gpc_preq,gpc_pre },
+   {gpc_sRGB,gpc_sRGB,gpc_sRGB,gpc_sRGB, gpc_noop,gpc_noop,gpc_noop,gpc_noop},
+   {gpc_b16g,gpc_unpg,gpc_b16c,gpc_unpc, gpc_A,   gpc_noop,gpc_A,   gpc_noop},
+   {gpc_sG,  gpc_sG,  gpc_sRGB,gpc_sRGB, gpc_g16, gpc_g16, gpc_noop,gpc_noop},
+   {gpc_gb16,gpc_sGp, gpc_cb16,gpc_sCp,  gpc_g16, gpc_g16q,gpc_A,   gpc_noop}
+};
+
+/* The array is repeated for the cases where both the input and output are color
+ * mapped because then different algorithms are used.
+ */
+static void (* const gpc_fn_colormapped[8/*in*/][8/*out*/])
+   (Pixel *out, const Pixel *in, const Background *back) =
+{
+/*out: G-8     GA-8     RGB-8    RGBA-8    G-16     GA-16   RGB-16  RGBA-16 */
+   {gpc_noop,gpc_noop,gpc_noop,gpc_noop, gpc_lin, gpc_lin, gpc_lin, gpc_lin },
+   {gpc_bckg,gpc_noop,gpc_bckc,gpc_noop, gpc_preq,gpc_pre, gpc_preq,gpc_pre },
+   {gpc_g8,  gpc_g8,  gpc_noop,gpc_noop, gpc_glin,gpc_glin,gpc_lin, gpc_lin },
+   {gpc_g8b, gpc_g8,  gpc_bckc,gpc_noop, gpc_gprq,gpc_gpre,gpc_preq,gpc_pre },
+   {gpc_sRGB,gpc_sRGB,gpc_sRGB,gpc_sRGB, gpc_noop,gpc_noop,gpc_noop,gpc_noop},
+   {gpc_b16g,gpc_unpg,gpc_b16c,gpc_unpc, gpc_A,   gpc_noop,gpc_A,   gpc_noop},
+   {gpc_sG,  gpc_sG,  gpc_sRGB,gpc_sRGB, gpc_g16, gpc_g16, gpc_noop,gpc_noop},
+   {gpc_gb16,gpc_sGp, gpc_cb16,gpc_sCp,  gpc_g16, gpc_g16q,gpc_A,   gpc_noop}
+};
+
+/* The error arrays record the error in the same matrix; 64 entries, however
+ * the different algorithms used in libpng for colormap and direct conversions
+ * mean that four separate matrices are used (for each combination of
+ * colormapped and direct.)
+ *
+ * In some cases the conversion between sRGB formats goes via a linear
+ * intermediate; an sRGB to linear conversion (as above) is followed by a simple
+ * linear to sRGB step with no other conversions.  This is done by a separate
+ * error array from an arbitrary 'in' format to one of the four basic outputs
+ * (since final output is always sRGB not colormapped).
+ *
+ * These arrays may be modified if the --accumulate flag is set during the run;
+ * then instead of logging errors they are simply added in.
+ *
+ * The three entries are currently for transparent, partially transparent and
+ * opaque input pixel values.  Notice that alpha should be exact in each case.
+ *
+ * Errors in alpha should only occur when converting from a direct format
+ * to a colormapped format, when alpha is effectively smashed (so large
+ * errors can occur.)  There should be no error in the '0' and 'opaque'
+ * values.  The fourth entry in the array is used for the alpha error (and it
+ * should always be zero for the 'via linear' case since this is never color
+ * mapped.)
+ *
+ * Mapping to a colormap smashes the colors, it is necessary to have separate
+ * values for these cases because they are much larger; it is very much
+ * impossible to obtain a reasonable result, these are held in
+ * gpc_error_to_colormap.
+ */
+#if PNG_FORMAT_FLAG_COLORMAP == 8 /* extra check also required */
+#  include "pngstest-errors.h" /* machine generated */
+#endif /* COLORMAP flag check */
+#endif /* flag checks */
+
+typedef struct
+{
+   /* Basic pixel information: */
+   Image*       in_image;   /* Input image */
+   const Image* out_image;  /* Output image */
+
+   /* 'background' is the value passed to the gpc_ routines, it may be NULL if
+    * it should not be used (*this* program has an error if it crashes as a
+    * result!)
+    */
+   Background        background_color;
+   const Background* background;
+
+   /* Precalculated values: */
+   int          in_opaque;   /* Value of input alpha that is opaque */
+   int          is_palette;  /* Sample values come from the palette */
+   int          accumulate;  /* Accumlate component errors (don't log) */
+   int          output_8bit; /* Output is 8-bit (else 16-bit) */
+
+   void (*in_gp)(Pixel*, png_const_voidp);
+   void (*out_gp)(Pixel*, png_const_voidp);
+
+   void (*transform)(Pixel *out, const Pixel *in, const Background *back);
+      /* A function to perform the required transform */
+
+   void (*from_linear)(Pixel *out, const Pixel *in, const Background *back);
+      /* For 'via_linear' transforms the final, from linear, step, else NULL */
+
+   png_uint_16 error[4];
+      /* Three error values for transparent, partially transparent and opaque
+       * input pixels (in turn).
+       */
+
+   png_uint_16 *error_ptr;
+      /* Where these are stored in the static array (for 'accumulate') */
+}
+Transform;
+
+/* Return a 'transform' as above for the given format conversion. */
+static void
+transform_from_formats(Transform *result, Image *in_image,
+   const Image *out_image, png_const_colorp background, int via_linear)
+{
+   png_uint_32 in_format, out_format;
+   png_uint_32 in_base, out_base;
+
+   memset(result, 0, sizeof *result);
+
+   /* Store the original images for error messages */
+   result->in_image = in_image;
+   result->out_image = out_image;
+
+   in_format = in_image->image.format;
+   out_format = out_image->image.format;
+
+   if (in_format & PNG_FORMAT_FLAG_LINEAR)
+      result->in_opaque = 65535;
+   else
+      result->in_opaque = 255;
+
+   result->output_8bit = (out_format & PNG_FORMAT_FLAG_LINEAR) == 0;
+
+   result->is_palette = 0; /* set by caller if required */
+   result->accumulate = (in_image->opts & ACCUMULATE) != 0;
+
+   /* The loaders (which need the ordering information) */
+   result->in_gp = get_pixel(in_format);
+   result->out_gp = get_pixel(out_format);
+
+   /* Remove the ordering information: */
+   in_format &= BASE_FORMATS | PNG_FORMAT_FLAG_COLORMAP;
+   in_base = in_format & BASE_FORMATS;
+   out_format &= BASE_FORMATS | PNG_FORMAT_FLAG_COLORMAP;
+   out_base = out_format & BASE_FORMATS;
+
+   if (via_linear)
+   {
+      /* Check for an error in this program: */
+      if (out_format & (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLORMAP))
+      {
+         fprintf(stderr, "internal transform via linear error 0x%x->0x%x\n",
+            in_format, out_format);
+         exit(1);
+      }
+
+      result->transform = gpc_fn[in_base][out_base | PNG_FORMAT_FLAG_LINEAR];
+      result->from_linear = gpc_fn[out_base | PNG_FORMAT_FLAG_LINEAR][out_base];
+      result->error_ptr = gpc_error_via_linear[in_format][out_format];
+   }
+
+   else if (~in_format & out_format & PNG_FORMAT_FLAG_COLORMAP)
+   {
+      /* The input is not colormapped but the output is, the errors will
+       * typically be large (only the grayscale-no-alpha case permits preserving
+       * even 8-bit values.)
+       */
+      result->transform = gpc_fn[in_base][out_base];
+      result->from_linear = NULL;
+      result->error_ptr = gpc_error_to_colormap[in_base][out_base];
+   }
+
+   else
+   {
+      /* The caller handles the colormap->pixel value conversion, so the
+       * transform function just gets a pixel value, however because libpng
+       * currently contains a different implementation for mapping a colormap if
+       * both input and output are colormapped we need different conversion
+       * functions to deal with errors in the libpng implementation.
+       */
+      if (in_format & out_format & PNG_FORMAT_FLAG_COLORMAP)
+         result->transform = gpc_fn_colormapped[in_base][out_base];
+      else
+         result->transform = gpc_fn[in_base][out_base];
+      result->from_linear = NULL;
+      result->error_ptr = gpc_error[in_format][out_format];
+   }
+
+   /* Follow the libpng simplified API rules to work out what to pass to the gpc
+    * routines as a background value, if one is not required pass NULL so that
+    * this program crashes in the even of a programming error.
+    */
+   result->background = NULL; /* default: not required */
+
+   /* Rule 1: background only need be supplied if alpha is to be removed */
+   if (in_format & ~out_format & PNG_FORMAT_FLAG_ALPHA)
+   {
+      /* The input value is 'NULL' to use the background and (otherwise) an sRGB
+       * background color (to use a solid color).  The code above uses a fixed
+       * byte value, BUFFER_INIT8, for buffer even for 16-bit output.  For
+       * linear (16-bit) output the sRGB background color is ignored; the
+       * composition is always on the background (so BUFFER_INIT8 * 257), except
+       * that for the colormap (i.e. linear colormapped output) black is used.
+       */
+      result->background = &result->background_color;
+
+      if (out_format & PNG_FORMAT_FLAG_LINEAR || via_linear)
+      {
+         if (out_format & PNG_FORMAT_FLAG_COLORMAP)
+         {
+            result->background_color.ir =
+               result->background_color.ig =
+               result->background_color.ib = 0;
+            result->background_color.dr =
+               result->background_color.dg =
+               result->background_color.db = 0;
+         }
+
+         else
+         {
+            result->background_color.ir =
+               result->background_color.ig =
+               result->background_color.ib = BUFFER_INIT8 * 257;
+            result->background_color.dr =
+               result->background_color.dg =
+               result->background_color.db = 0;
+         }
+      }
+
+      else /* sRGB output */
+      {
+         if (background != NULL)
+         {
+            if (out_format & PNG_FORMAT_FLAG_COLOR)
+            {
+               result->background_color.ir = background->red;
+               result->background_color.ig = background->green;
+               result->background_color.ib = background->blue;
+               /* TODO: sometimes libpng uses the power law conversion here, how
+                * to handle this?
+                */
+               result->background_color.dr = sRGB_to_d[background->red];
+               result->background_color.dg = sRGB_to_d[background->green];
+               result->background_color.db = sRGB_to_d[background->blue];
+            }
+
+            else /* grayscale: libpng only looks at 'g' */
+            {
+               result->background_color.ir =
+                  result->background_color.ig =
+                  result->background_color.ib = background->green;
+               /* TODO: sometimes libpng uses the power law conversion here, how
+                * to handle this?
+                */
+               result->background_color.dr =
+                  result->background_color.dg =
+                  result->background_color.db = sRGB_to_d[background->green];
+            }
+         }
+
+         else if ((out_format & PNG_FORMAT_FLAG_COLORMAP) == 0)
+         {
+            result->background_color.ir =
+               result->background_color.ig =
+               result->background_color.ib = BUFFER_INIT8;
+            /* TODO: sometimes libpng uses the power law conversion here, how
+             * to handle this?
+             */
+            result->background_color.dr =
+               result->background_color.dg =
+               result->background_color.db = sRGB_to_d[BUFFER_INIT8];
+         }
+
+         /* Else the output is colormapped and a background color must be
+          * provided; if pngstest crashes then that is a bug in this program
+          * (though libpng should png_error as well.)
+          */
+         else
+            result->background = NULL;
+      }
+   }
+
+   if (result->background == NULL)
+   {
+      result->background_color.ir =
+         result->background_color.ig =
+         result->background_color.ib = -1; /* not used */
+      result->background_color.dr =
+         result->background_color.dg =
+         result->background_color.db = 1E30; /* not used */
+   }
+
+
+   /* Copy the error values into the Transform: */
+   result->error[0] = result->error_ptr[0];
+   result->error[1] = result->error_ptr[1];
+   result->error[2] = result->error_ptr[2];
+   result->error[3] = result->error_ptr[3];
+}
+
+
+/* Compare two pixels.
+ *
+ * OLD error values:
+static int error_to_linear = 811; * by experiment *
+static int error_to_linear_grayscale = 424; * by experiment *
+static int error_to_sRGB = 6; * by experiment *
+static int error_to_sRGB_grayscale = 17; * libpng error by calculation +
+                                            2 by experiment *
+static int error_in_compose = 2; * by experiment *
+static int error_in_premultiply = 1;
+ *
+ * The following is *just* the result of a round trip from 8-bit sRGB to linear
+ * then back to 8-bit sRGB when it is done by libpng.  There are two problems:
+ *
+ * 1) libpng currently uses a 2.2 power law with no linear segment, this results
+ * in instability in the low values and even with 16-bit precision sRGB(1) ends
+ * up mapping to sRGB(0) as a result of rounding in the 16-bit representation.
+ * This gives an error of 1 in the handling of value 1 only.
+ *
+ * 2) libpng currently uses an intermediate 8-bit linear value in gamma
+ * correction of 8-bit values.  This results in many more errors, the worse of
+ * which is mapping sRGB(14) to sRGB(0).
+ *
+ * The general 'error_via_linear' is more complex because of pre-multiplication,
+ * this compounds the 8-bit errors according to the alpha value of the pixel.
+ * As a result 256 values are pre-calculated for error_via_linear.
+ */
+#if 0
+static int error_in_libpng_gamma;
+static int error_via_linear[256]; /* Indexed by 8-bit alpha */
+
+static void
+init_error_via_linear(void)
+{
+   int alpha;
+
+   error_via_linear[0] = 255; /* transparent pixel */
+
+   for (alpha=1; alpha<=255; ++alpha)
+   {
+      /* 16-bit values less than 128.5 get rounded to 8-bit 0 and so the worst
+       * case error arises with 16-bit 128.5, work out what sRGB
+       * (non-associated) value generates 128.5; any value less than this is
+       * going to map to 0, so the worst error is floor(value).
+       *
+       * Note that errors are considerably higher (more than a factor of 2)
+       * because libpng uses a simple power law for sRGB data at present.
+       *
+       * Add .1 for arithmetic errors inside libpng.
+       */
+      double v = floor(255*pow(.5/*(128.5 * 255 / 65535)*/ / alpha, 1/2.2)+.1);
+
+      error_via_linear[alpha] = (int)v;
+   }
+
+   /* This is actually 14.99, but, despite the closeness to 15, 14 seems to work
+    * ok in this case.
+    */
+   error_in_libpng_gamma = 14;
+}
+#endif
+
+static void
+print_pixel(char string[64], const Pixel *pixel, png_uint_32 format)
+{
+   switch (format & (PNG_FORMAT_FLAG_ALPHA|PNG_FORMAT_FLAG_COLOR))
+   {
+      case 0:
+         sprintf(string, "%s(%d)", format_names[format], pixel->g);
+         break;
+
+      case PNG_FORMAT_FLAG_ALPHA:
+         sprintf(string, "%s(%d,%d)", format_names[format], pixel->g,
+            pixel->a);
+         break;
+
+      case PNG_FORMAT_FLAG_COLOR:
+         sprintf(string, "%s(%d,%d,%d)", format_names[format],
+            pixel->r, pixel->g, pixel->b);
+         break;
+
+      case PNG_FORMAT_FLAG_COLOR|PNG_FORMAT_FLAG_ALPHA:
+         sprintf(string, "%s(%d,%d,%d,%d)", format_names[format],
+            pixel->r, pixel->g, pixel->b, pixel->a);
+         break;
+
+      default:
+         sprintf(string, "invalid-format");
+         break;
+   }
+}
+
+static int
+logpixel(const Transform *transform, png_uint_32 x, png_uint_32 y,
+   const Pixel *in, const Pixel *calc, const Pixel *out, const char *reason)
+{
+   const png_uint_32 in_format = transform->in_image->image.format;
+   const png_uint_32 out_format = transform->out_image->image.format;
+
+   png_uint_32 back_format = out_format & ~PNG_FORMAT_FLAG_ALPHA;
+   const char *via_linear = "";
+
+   char pixel_in[64], pixel_calc[64], pixel_out[64], pixel_loc[64];
+   char background_info[100];
+
+   print_pixel(pixel_in, in, in_format);
+   print_pixel(pixel_calc, calc, out_format);
+   print_pixel(pixel_out, out, out_format);
+
+   if (transform->is_palette)
+      sprintf(pixel_loc, "palette: %lu", (unsigned long)y);
+   else
+      sprintf(pixel_loc, "%lu,%lu", (unsigned long)x, (unsigned long)y);
+
+   if (transform->from_linear != NULL)
+   {
+      via_linear = " (via linear)";
+      /* And as a result the *read* format which did any background processing
+       * was itself linear, so the background color information is also
+       * linear.
+       */
+      back_format |= PNG_FORMAT_FLAG_LINEAR;
+   }
+
+   if (transform->background != NULL)
+   {
+      Pixel back;
+      char pixel_back[64];
+
+      back.r = transform->background->ir;
+      back.g = transform->background->ig;
+      back.b = transform->background->ib;
+      back.a = -1; /* not used */
+
+      print_pixel(pixel_back, &back, back_format);
+      sprintf(background_info, " on background %s", pixel_back);
+   }
+
+   else
+      background_info[0] = 0;
+
+   if (transform->in_image->file_name != transform->out_image->file_name)
+   {
+      char error_buffer[512];
+      sprintf(error_buffer,
+         "(%s) %s error%s:\n %s%s ->\n       %s\n  not: %s.\n"
+         "Use --preserve and examine: ", pixel_loc, reason, via_linear,
+         pixel_in, background_info, pixel_out, pixel_calc);
+      return logerror(transform->in_image, transform->in_image->file_name,
+         error_buffer, transform->out_image->file_name);
+   }
+
+   else
+   {
+      char error_buffer[512];
+      sprintf(error_buffer,
+         "(%s) %s error%s:\n %s%s ->\n       %s\n  not: %s.\n"
+         " The error happened when reading the original file with this format.",
+         pixel_loc, reason, via_linear, pixel_in, background_info, pixel_out,
+         pixel_calc);
+      return logerror(transform->in_image, transform->in_image->file_name,
+         error_buffer, "");
+   }
+}
+
+static int
+cmppixel(Transform *transform, png_const_voidp in, png_const_voidp out,
+   png_uint_32 x, png_uint_32 y/*or palette index*/)
+{
+   int maxerr;
+   png_const_charp errmsg;
+   Pixel pixel_in, pixel_calc, pixel_out;
+
+   transform->in_gp(&pixel_in, in);
+
+   if (transform->from_linear == NULL)
+      transform->transform(&pixel_calc, &pixel_in, transform->background);
+
+   else
+   {
+      transform->transform(&pixel_out, &pixel_in, transform->background);
+      transform->from_linear(&pixel_calc, &pixel_out, NULL);
+   }
+
+   transform->out_gp(&pixel_out, out);
+
+   /* Eliminate the case where the input and output values match exactly. */
+   if (pixel_calc.a == pixel_out.a && pixel_calc.r == pixel_out.r &&
+      pixel_calc.g == pixel_out.g && pixel_calc.b == pixel_out.b)
+      return 1;
+
+   /* Eliminate the case where the output pixel is transparent and the output
+    * is 8-bit - any component values are valid.  Don't check the input alpha
+    * here to also skip the 16-bit small alpha cases.
+    */
+   if (transform->output_8bit && pixel_calc.a == 0 && pixel_out.a == 0)
+      return 1;
+
+   /* Check for alpha errors first; an alpha error can damage the components too
+    * so avoid spurious checks on components if one is found.
+    */
+   errmsg = NULL;
+   {
+      int err_a = abs(pixel_calc.a-pixel_out.a);
+
+      if (err_a > transform->error[3])
+      {
+         /* If accumulating check the components too */
+         if (transform->accumulate)
+            transform->error[3] = (png_uint_16)err_a;
+
+         else
+            errmsg = "alpha";
+      }
+   }
+
+   /* Now if *either* of the output alphas are 0 but alpha is within tolerance
+    * eliminate the 8-bit component comparison.
+    */
+   if (errmsg == NULL && transform->output_8bit &&
+      (pixel_calc.a == 0 || pixel_out.a == 0))
+      return 1;
+
+   if (errmsg == NULL) /* else just signal an alpha error */
+   {
+      int err_r = abs(pixel_calc.r - pixel_out.r);
+      int err_g = abs(pixel_calc.g - pixel_out.g);
+      int err_b = abs(pixel_calc.b - pixel_out.b);
+      int limit;
+
+      if ((err_r | err_g | err_b) == 0)
+         return 1; /* exact match */
+
+      /* Mismatch on a component, check the input alpha */
+      if (pixel_in.a >= transform->in_opaque)
+      {
+         errmsg = "opaque component";
+         limit = 2; /* opaque */
+      }
+
+      else if (pixel_in.a > 0)
+      {
+         errmsg = "alpha component";
+         limit = 1; /* partially transparent */
+      }
+
+      else
+      {
+         errmsg = "transparent component (background)";
+         limit = 0; /* transparent */
+      }
+
+      maxerr = err_r;
+      if (maxerr < err_g) maxerr = err_g;
+      if (maxerr < err_b) maxerr = err_b;
+
+      if (maxerr <= transform->error[limit])
+         return 1; /* within the error limits */
+
+      /* Handle a component mis-match; log it, just return an error code, or
+       * accumulate it.
+       */
+      if (transform->accumulate)
+      {
+         transform->error[limit] = (png_uint_16)maxerr;
+         return 1; /* to cause the caller to keep going */
+      }
+   }
+
+   /* Failure to match and not accumulating, so the error must be logged. */
+   return logpixel(transform, x, y, &pixel_in, &pixel_calc, &pixel_out, errmsg);
+}
+
+static png_byte
+component_loc(png_byte loc[4], png_uint_32 format)
+{
+   /* Given a format return the number of channels and the location of
+    * each channel.
+    *
+    * The mask 'loc' contains the component offset of the channels in the
+    * following order.  Note that if 'format' is grayscale the entries 1-3 must
+    * all contain the location of the gray channel.
+    *
+    * 0: alpha
+    * 1: red or gray
+    * 2: green or gray
+    * 3: blue or gray
+    */
+   png_byte channels;
+
+   if (format & PNG_FORMAT_FLAG_COLOR)
+   {
+      channels = 3;
+
+      loc[2] = 1;
+
+#     ifdef PNG_FORMAT_BGR_SUPPORTED
+         if (format & PNG_FORMAT_FLAG_BGR)
+         {
+            loc[1] = 2;
+            loc[3] = 0;
+         }
+
+         else
+#     endif
+      {
+         loc[1] = 0;
+         loc[3] = 2;
+      }
+   }
+
+   else
+   {
+      channels = 1;
+      loc[1] = loc[2] = loc[3] = 0;
+   }
+
+   if (format & PNG_FORMAT_FLAG_ALPHA)
+   {
+#     ifdef PNG_FORMAT_AFIRST_SUPPORTED
+         if (format & PNG_FORMAT_FLAG_AFIRST)
+         {
+            loc[0] = 0;
+            ++loc[1];
+            ++loc[2];
+            ++loc[3];
+         }
+
+         else
+#     endif
+         loc[0] = channels;
+
+      ++channels;
+   }
+
+   else
+      loc[0] = 4; /* not present */
+
+   return channels;
+}
+
+/* Compare two images, the original 'a', which was written out then read back in
+ * to * give image 'b'.  The formats may have been changed.
+ */
+static int
+compare_two_images(Image *a, Image *b, int via_linear,
+   png_const_colorp background)
+{
+   ptrdiff_t stridea = a->stride;
+   ptrdiff_t strideb = b->stride;
+   png_const_bytep rowa = a->buffer+16;
+   png_const_bytep rowb = b->buffer+16;
+   const png_uint_32 width = a->image.width;
+   const png_uint_32 height = a->image.height;
+   const png_uint_32 formata = a->image.format;
+   const png_uint_32 formatb = b->image.format;
+   const unsigned int a_sample = PNG_IMAGE_SAMPLE_SIZE(formata);
+   const unsigned int b_sample = PNG_IMAGE_SAMPLE_SIZE(formatb);
+   int alpha_added, alpha_removed;
+   int bchannels;
+   int btoa[4];
+   png_uint_32 y;
+   Transform tr;
+
+   /* This should never happen: */
+   if (width != b->image.width || height != b->image.height)
+      return logerror(a, a->file_name, ": width x height changed: ",
+         b->file_name);
+
+   /* Set up the background and the transform */
+   transform_from_formats(&tr, a, b, background, via_linear);
+
+   /* Find the first row and inter-row space. */
+   if (!(formata & PNG_FORMAT_FLAG_COLORMAP) &&
+      (formata & PNG_FORMAT_FLAG_LINEAR))
+      stridea *= 2;
+
+   if (!(formatb & PNG_FORMAT_FLAG_COLORMAP) &&
+      (formatb & PNG_FORMAT_FLAG_LINEAR))
+      strideb *= 2;
+
+   if (stridea < 0) rowa += (height-1) * (-stridea);
+   if (strideb < 0) rowb += (height-1) * (-strideb);
+
+   /* First shortcut the two colormap case by comparing the image data; if it
+    * matches then we expect the colormaps to match, although this is not
+    * absolutely necessary for an image match.  If the colormaps fail to match
+    * then there is a problem in libpng.
+    */
+   if (formata & formatb & PNG_FORMAT_FLAG_COLORMAP)
+   {
+      /* Only check colormap entries that actually exist; */
+      png_const_bytep ppa, ppb;
+      int match;
+      png_byte in_use[256], amax = 0, bmax = 0;
+
+      memset(in_use, 0, sizeof in_use);
+
+      ppa = rowa;
+      ppb = rowb;
+
+      /* Do this the slow way to accumulate the 'in_use' flags, don't break out
+       * of the loop until the end; this validates the color-mapped data to
+       * ensure all pixels are valid color-map indexes.
+       */
+      for (y=0, match=1; y<height && match; ++y, ppa += stridea, ppb += strideb)
+      {
+         png_uint_32 x;
+
+         for (x=0; x<width; ++x)
+         {
+            png_byte bval = ppb[x];
+            png_byte aval = ppa[x];
+
+            if (bval > bmax)
+               bmax = bval;
+
+            if (bval != aval)
+               match = 0;
+
+            in_use[aval] = 1;
+            if (aval > amax)
+               amax = aval;
+         }
+      }
+
+      /* If the buffers match then the colormaps must too. */
+      if (match)
+      {
+         /* Do the color-maps match, entry by entry?  Only check the 'in_use'
+          * entries.  An error here should be logged as a color-map error.
+          */
+         png_const_bytep a_cmap = (png_const_bytep)a->colormap;
+         png_const_bytep b_cmap = (png_const_bytep)b->colormap;
+         int result = 1; /* match by default */
+
+         /* This is used in logpixel to get the error message correct. */
+         tr.is_palette = 1;
+
+         for (y=0; y<256; ++y, a_cmap += a_sample, b_cmap += b_sample)
+            if (in_use[y])
+         {
+            /* The colormap entries should be valid, but because libpng doesn't
+             * do any checking at present the original image may contain invalid
+             * pixel values.  These cause an error here (at present) unless
+             * accumulating errors in which case the program just ignores them.
+             */
+            if (y >= a->image.colormap_entries)
+            {
+               if ((a->opts & ACCUMULATE) == 0)
+               {
+                  char pindex[9];
+                  sprintf(pindex, "%lu[%lu]", (unsigned long)y,
+                     (unsigned long)a->image.colormap_entries);
+                  logerror(a, a->file_name, ": bad pixel index: ", pindex);
+               }
+               result = 0;
+            }
+
+            else if (y >= b->image.colormap_entries)
+            {
+               if ((b->opts & ACCUMULATE) == 0)
+                  {
+                  char pindex[9];
+                  sprintf(pindex, "%lu[%lu]", (unsigned long)y,
+                     (unsigned long)b->image.colormap_entries);
+                  logerror(b, b->file_name, ": bad pixel index: ", pindex);
+                  }
+               result = 0;
+            }
+
+            /* All the mismatches are logged here; there can only be 256! */
+            else if (!cmppixel(&tr, a_cmap, b_cmap, 0, y))
+               result = 0;
+         }
+
+         /* If reqested copy the error values back from the Transform. */
+         if (a->opts & ACCUMULATE)
+         {
+            tr.error_ptr[0] = tr.error[0];
+            tr.error_ptr[1] = tr.error[1];
+            tr.error_ptr[2] = tr.error[2];
+            tr.error_ptr[3] = tr.error[3];
+            result = 1; /* force a continue */
+         }
+
+         return result;
+      }
+
+      /* else the image buffers don't match pixel-wise so compare sample values
+       * instead, but first validate that the pixel indexes are in range (but
+       * only if not accumulating, when the error is ignored.)
+       */
+      else if ((a->opts & ACCUMULATE) == 0)
+      {
+         /* Check the original image first,
+          * TODO: deal with input images with bad pixel values?
+          */
+         if (amax >= a->image.colormap_entries)
+         {
+            char pindex[9];
+            sprintf(pindex, "%d[%lu]", amax,
+               (unsigned long)a->image.colormap_entries);
+            return logerror(a, a->file_name, ": bad pixel index: ", pindex);
+         }
+
+         else if (bmax >= b->image.colormap_entries)
+         {
+            char pindex[9];
+            sprintf(pindex, "%d[%lu]", bmax,
+               (unsigned long)b->image.colormap_entries);
+            return logerror(b, b->file_name, ": bad pixel index: ", pindex);
+         }
+      }
+   }
+
+   /* We can directly compare pixel values without the need to use the read
+    * or transform support (i.e. a memory compare) if:
+    *
+    * 1) The bit depth has not changed.
+    * 2) RGB to grayscale has not been done (the reverse is ok; we just compare
+    *    the three RGB values to the original grayscale.)
+    * 3) An alpha channel has not been removed from an 8-bit format, or the
+    *    8-bit alpha value of the pixel was 255 (opaque).
+    *
+    * If an alpha channel has been *added* then it must have the relevant opaque
+    * value (255 or 65535).
+    *
+    * The fist two the tests (in the order given above) (using the boolean
+    * equivalence !a && !b == !(a || b))
+    */
+   if (!(((formata ^ formatb) & PNG_FORMAT_FLAG_LINEAR) |
+      (formata & (formatb ^ PNG_FORMAT_FLAG_COLOR) & PNG_FORMAT_FLAG_COLOR)))
+   {
+      /* Was an alpha channel changed? */
+      const png_uint_32 alpha_changed = (formata ^ formatb) &
+         PNG_FORMAT_FLAG_ALPHA;
+
+      /* Was an alpha channel removed?  (The third test.)  If so the direct
+       * comparison is only possible if the input alpha is opaque.
+       */
+      alpha_removed = (formata & alpha_changed) != 0;
+
+      /* Was an alpha channel added? */
+      alpha_added = (formatb & alpha_changed) != 0;
+
+      /* The channels may have been moved between input and output, this finds
+       * out how, recording the result in the btoa array, which says where in
+       * 'a' to find each channel of 'b'.  If alpha was added then btoa[alpha]
+       * ends up as 4 (and is not used.)
+       */
+      {
+         int i;
+         png_byte aloc[4];
+         png_byte bloc[4];
+
+         /* The following are used only if the formats match, except that
+          * 'bchannels' is a flag for matching formats.  btoa[x] says, for each
+          * channel in b, where to find the corresponding value in a, for the
+          * bchannels.  achannels may be different for a gray to rgb transform
+          * (a will be 1 or 2, b will be 3 or 4 channels.)
+          */
+         (void)component_loc(aloc, formata);
+         bchannels = component_loc(bloc, formatb);
+
+         /* Hence the btoa array. */
+         for (i=0; i<4; ++i) if (bloc[i] < 4)
+            btoa[bloc[i]] = aloc[i]; /* may be '4' for alpha */
+
+         if (alpha_added)
+            alpha_added = bloc[0]; /* location of alpha channel in image b */
+
+         else
+            alpha_added = 4; /* Won't match an image b channel */
+
+         if (alpha_removed)
+            alpha_removed = aloc[0]; /* location of alpha channel in image a */
+
+         else
+            alpha_removed = 4;
+      }
+   }
+
+   else
+   {
+      /* Direct compare is not possible, cancel out all the corresponding local
+       * variables.
+       */
+      bchannels = 0;
+      alpha_removed = alpha_added = 4;
+      btoa[3] = btoa[2] = btoa[1] = btoa[0] = 4; /* 4 == not present */
+   }
+
+   for (y=0; y<height; ++y, rowa += stridea, rowb += strideb)
+   {
+      png_const_bytep ppa, ppb;
+      png_uint_32 x;
+
+      for (x=0, ppa=rowa, ppb=rowb; x<width; ++x)
+      {
+         png_const_bytep psa, psb;
+
+         if (formata & PNG_FORMAT_FLAG_COLORMAP)
+            psa = (png_const_bytep)a->colormap + a_sample * *ppa++;
+         else
+            psa = ppa, ppa += a_sample;
+
+         if (formatb & PNG_FORMAT_FLAG_COLORMAP)
+            psb = (png_const_bytep)b->colormap + b_sample * *ppb++;
+         else
+            psb = ppb, ppb += b_sample;
+
+         /* Do the fast test if possible. */
+         if (bchannels)
+         {
+            /* Check each 'b' channel against either the corresponding 'a'
+             * channel or the opaque alpha value, as appropriate.  If
+             * alpha_removed value is set (not 4) then also do this only if the
+             * 'a' alpha channel (alpha_removed) is opaque; only relevant for
+             * the 8-bit case.
+             */
+            if (formatb & PNG_FORMAT_FLAG_LINEAR) /* 16-bit checks */
+            {
+               png_const_uint_16p pua = aligncastconst(png_const_uint_16p, psa);
+               png_const_uint_16p pub = aligncastconst(png_const_uint_16p, psb);
+
+               switch (bchannels)
+               {
+                  case 4:
+                     if (pua[btoa[3]] != pub[3]) break;
+                  case 3:
+                     if (pua[btoa[2]] != pub[2]) break;
+                  case 2:
+                     if (pua[btoa[1]] != pub[1]) break;
+                  case 1:
+                     if (pua[btoa[0]] != pub[0]) break;
+                     if (alpha_added != 4 && pub[alpha_added] != 65535) break;
+                     continue; /* x loop */
+                  default:
+                     break; /* impossible */
+               }
+            }
+
+            else if (alpha_removed == 4 || psa[alpha_removed] == 255)
+            {
+               switch (bchannels)
+               {
+                  case 4:
+                     if (psa[btoa[3]] != psb[3]) break;
+                  case 3:
+                     if (psa[btoa[2]] != psb[2]) break;
+                  case 2:
+                     if (psa[btoa[1]] != psb[1]) break;
+                  case 1:
+                     if (psa[btoa[0]] != psb[0]) break;
+                     if (alpha_added != 4 && psb[alpha_added] != 255) break;
+                     continue; /* x loop */
+                  default:
+                     break; /* impossible */
+               }
+            }
+         }
+
+         /* If we get to here the fast match failed; do the slow match for this
+          * pixel.
+          */
+         if (!cmppixel(&tr, psa, psb, x, y) && (a->opts & KEEP_GOING) == 0)
+            return 0; /* error case */
+      }
+   }
+
+   /* If reqested copy the error values back from the Transform. */
+   if (a->opts & ACCUMULATE)
+   {
+      tr.error_ptr[0] = tr.error[0];
+      tr.error_ptr[1] = tr.error[1];
+      tr.error_ptr[2] = tr.error[2];
+      tr.error_ptr[3] = tr.error[3];
+   }
+
+   return 1;
+}
+
+/* Read the file; how the read gets done depends on which of input_file and
+ * input_memory have been set.
+ */
+static int
+read_file(Image *image, png_uint_32 format, png_const_colorp background)
+{
+   memset(&image->image, 0, sizeof image->image);
+   image->image.version = PNG_IMAGE_VERSION;
+
+   if (image->input_memory != NULL)
+   {
+      if (!png_image_begin_read_from_memory(&image->image, image->input_memory,
+         image->input_memory_size))
+         return logerror(image, "memory init: ", image->file_name, "");
+   }
+
+#  ifdef PNG_STDIO_SUPPORTED
+      else if (image->input_file != NULL)
+      {
+         if (!png_image_begin_read_from_stdio(&image->image, image->input_file))
+            return logerror(image, "stdio init: ", image->file_name, "");
+      }
+
+      else
+      {
+         if (!png_image_begin_read_from_file(&image->image, image->file_name))
+            return logerror(image, "file init: ", image->file_name, "");
+      }
+#  else
+      else
+      {
+         return logerror(image, "unsupported file/stdio init: ",
+            image->file_name, "");
+      }
+#  endif
+
+   /* This must be set after the begin_read call: */
+   if (image->opts & sRGB_16BIT)
+      image->image.flags |= PNG_IMAGE_FLAG_16BIT_sRGB;
+
+   /* Have an initialized image with all the data we need plus, maybe, an
+    * allocated file (myfile) or buffer (mybuffer) that need to be freed.
+    */
+   {
+      int result;
+      png_uint_32 image_format;
+
+      /* Print both original and output formats. */
+      image_format = image->image.format;
+
+      if (image->opts & VERBOSE)
+      {
+         printf("%s %lu x %lu %s -> %s", image->file_name,
+            (unsigned long)image->image.width,
+            (unsigned long)image->image.height,
+            format_names[image_format & FORMAT_MASK],
+            (format & FORMAT_NO_CHANGE) != 0 || image->image.format == format
+            ? "no change" : format_names[format & FORMAT_MASK]);
+
+         if (background != NULL)
+            printf(" background(%d,%d,%d)\n", background->red,
+               background->green, background->blue);
+         else
+            printf("\n");
+
+         fflush(stdout);
+      }
+
+      /* 'NO_CHANGE' combined with the color-map flag forces the base format
+       * flags to be set on read to ensure that the original representation is
+       * not lost in the pass through a colormap format.
+       */
+      if ((format & FORMAT_NO_CHANGE) != 0)
+      {
+         if ((format & PNG_FORMAT_FLAG_COLORMAP) != 0 &&
+            (image_format & PNG_FORMAT_FLAG_COLORMAP) != 0)
+            format = (image_format & ~BASE_FORMATS) | (format & BASE_FORMATS);
+
+         else
+            format = image_format;
+      }
+
+      image->image.format = format;
+
+      image->stride = PNG_IMAGE_ROW_STRIDE(image->image) + image->stride_extra;
+      allocbuffer(image);
+
+      result = png_image_finish_read(&image->image, background,
+         image->buffer+16, (png_int_32)image->stride, image->colormap);
+
+      checkbuffer(image, image->file_name);
+
+      if (result)
+         return checkopaque(image);
+
+      else
+         return logerror(image, image->file_name, ": image read failed", "");
+   }
+}
+
+/* Reads from a filename, which must be in image->file_name, but uses
+ * image->opts to choose the method.  The file is always read in its native
+ * format (the one the simplified API suggests).
+ */
+static int
+read_one_file(Image *image)
+{
+   if (!(image->opts & USE_FILE) || (image->opts & USE_STDIO))
+   {
+      /* memory or stdio. */
+      FILE *f = fopen(image->file_name, "rb");
+
+      if (f != NULL)
+      {
+         if (image->opts & USE_FILE)
+            image->input_file = f;
+
+         else /* memory */
+         {
+            if (fseek(f, 0, SEEK_END) == 0)
+            {
+               long int cb = ftell(f);
+
+               if (cb > 0)
+               {
+#ifndef __COVERITY__
+                  if ((unsigned long int)cb <= (size_t)~(size_t)0)
+#endif
+                  {
+                     png_bytep b = voidcast(png_bytep, malloc((size_t)cb));
+
+                     if (b != NULL)
+                     {
+                        rewind(f);
+
+                        if (fread(b, (size_t)cb, 1, f) == 1)
+                        {
+                           fclose(f);
+                           image->input_memory_size = cb;
+                           image->input_memory = b;
+                        }
+
+                        else
+                        {
+                           free(b);
+                           return logclose(image, f, image->file_name,
+                              ": read failed: ");
+                        }
+                     }
+
+                     else
+                        return logclose(image, f, image->file_name,
+                           ": out of memory: ");
+                  }
+
+                  else
+                     return logclose(image, f, image->file_name,
+                        ": file too big for this architecture: ");
+                     /* cb is the length of the file as a (long) and
+                      * this is greater than the maximum amount of
+                      * memory that can be requested from malloc.
+                      */
+               }
+
+               else if (cb == 0)
+                  return logclose(image, f, image->file_name,
+                     ": zero length: ");
+
+               else
+                  return logclose(image, f, image->file_name,
+                     ": tell failed: ");
+            }
+
+            else
+               return logclose(image, f, image->file_name, ": seek failed: ");
+         }
+      }
+
+      else
+         return logerror(image, image->file_name, ": open failed: ",
+            strerror(errno));
+   }
+
+   return read_file(image, FORMAT_NO_CHANGE, NULL);
+}
+
+#ifdef PNG_SIMPLIFIED_WRITE_SUPPORTED
+static int
+write_one_file(Image *output, Image *image, int convert_to_8bit)
+{
+   if (image->opts & FAST_WRITE)
+      image->image.flags |= PNG_IMAGE_FLAG_FAST;
+
+   if (image->opts & USE_STDIO)
+   {
+#ifdef PNG_SIMPLIFIED_WRITE_STDIO_SUPPORTED
+#ifndef __COVERITY__
+      FILE *f = tmpfile();
+#else
+      /* Experimental. Coverity says tmpfile() is insecure because it
+       * generates predictable names.
+       *
+       * It is possible to satisfy Coverity by using mkstemp(); however,
+       * any platform supporting mkstemp() undoubtedly has a secure tmpfile()
+       * implementation as well, and doesn't need the fix.  Note that
+       * the fix won't work on platforms that don't support mkstemp().
+       *
+       * https://www.securecoding.cert.org/confluence/display/c/
+       * FIO21-C.+Do+not+create+temporary+files+in+shared+directories
+       * says that most historic implementations of tmpfile() provide
+       * only a limited number of possible temporary file names
+       * (usually 26) before file names are recycled. That article also
+       * provides a secure solution that unfortunately depends upon mkstemp().
+       */
+      char tmpfile[] = "pngstest-XXXXXX";
+      int filedes;
+      FILE *f;
+      umask(0177);
+      filedes = mkstemp(tmpfile);
+      if (filedes < 0)
+        f = NULL;
+      else
+      {
+        f = fdopen(filedes,"w+");
+        /* Hide the filename immediately and ensure that the file does
+         * not exist after the program ends
+         */
+        (void) unlink(tmpfile);
+      }
+#endif
+
+      if (f != NULL)
+      {
+         if (png_image_write_to_stdio(&image->image, f, convert_to_8bit,
+            image->buffer+16, (png_int_32)image->stride, image->colormap))
+         {
+            if (fflush(f) == 0)
+            {
+               rewind(f);
+               initimage(output, image->opts, "tmpfile", image->stride_extra);
+               output->input_file = f;
+               if (!checkopaque(image))
+                  return 0;
+            }
+
+            else
+               return logclose(image, f, "tmpfile", ": flush: ");
+         }
+
+         else
+         {
+            fclose(f);
+            return logerror(image, "tmpfile", ": write failed", "");
+         }
+      }
+
+      else
+         return logerror(image, "tmpfile", ": open: ", strerror(errno));
+#else /* SIMPLIFIED_WRITE_STDIO */
+      return logerror(image, "tmpfile", ": open: unsupported", "");
+#endif /* SIMPLIFIED_WRITE_STDIO */
+   }
+
+   else if (image->opts & USE_FILE)
+   {
+#ifdef PNG_SIMPLIFIED_WRITE_STDIO_SUPPORTED
+      static int counter = 0;
+      char name[32];
+
+      sprintf(name, "%s%d.png", tmpf, ++counter);
+
+      if (png_image_write_to_file(&image->image, name, convert_to_8bit,
+         image->buffer+16, (png_int_32)image->stride, image->colormap))
+      {
+         initimage(output, image->opts, output->tmpfile_name,
+            image->stride_extra);
+         /* Afterwards, or freeimage will delete it! */
+         strcpy(output->tmpfile_name, name);
+
+         if (!checkopaque(image))
+            return 0;
+      }
+
+      else
+         return logerror(image, name, ": write failed", "");
+#else /* SIMPLIFIED_WRITE_STDIO */
+      return logerror(image, "stdio", ": open: unsupported", "");
+#endif /* SIMPLIFIED_WRITE_STDIO */
+   }
+
+   else /* use memory */
+   {
+      png_alloc_size_t size;
+
+      if (png_image_write_get_memory_size(image->image, size, convert_to_8bit,
+               image->buffer+16, (png_int_32)image->stride, image->colormap))
+      {
+         /* This is non-fatal but ignoring it was causing serious problems in
+          * the macro to be ignored:
+          */
+         if (size > PNG_IMAGE_PNG_SIZE_MAX(image->image))
+            return logerror(image, "memory", ": PNG_IMAGE_SIZE_MAX wrong", "");
+
+         initimage(output, image->opts, "memory", image->stride_extra);
+         output->input_memory = malloc(size);
+
+         if (output->input_memory != NULL)
+         {
+            output->input_memory_size = size;
+
+            if (png_image_write_to_memory(&image->image, output->input_memory,
+                  &output->input_memory_size, convert_to_8bit, image->buffer+16,
+                  (png_int_32)image->stride, image->colormap))
+            {
+               /* This is also non-fatal but it safes safer to error out anyway:
+                */
+               if (size != output->input_memory_size)
+                  return logerror(image, "memory", ": memory size wrong", "");
+            }
+
+            else
+               return logerror(image, "memory", ": write failed", "");
+         }
+
+         else
+            return logerror(image, "memory", ": out of memory", "");
+      }
+
+      else
+         return logerror(image, "memory", ": get size:", "");
+   }
+
+   /* 'output' has an initialized temporary image, read this back in and compare
+    * this against the original: there should be no change since the original
+    * format was written unmodified unless 'convert_to_8bit' was specified.
+    * However, if the original image was color-mapped, a simple read will zap
+    * the linear, color and maybe alpha flags, this will cause spurious failures
+    * under some circumstances.
+    */
+   if (read_file(output, image->image.format | FORMAT_NO_CHANGE, NULL))
+   {
+      png_uint_32 original_format = image->image.format;
+
+      if (convert_to_8bit)
+         original_format &= ~PNG_FORMAT_FLAG_LINEAR;
+
+      if ((output->image.format & BASE_FORMATS) !=
+         (original_format & BASE_FORMATS))
+         return logerror(image, image->file_name, ": format changed on read: ",
+            output->file_name);
+
+      return compare_two_images(image, output, 0/*via linear*/, NULL);
+   }
+
+   else
+      return logerror(output, output->tmpfile_name,
+         ": read of new file failed", "");
+}
+#endif
+
+static int
+testimage(Image *image, png_uint_32 opts, format_list *pf)
+{
+   int result;
+   Image copy;
+
+   /* Copy the original data, stealing it from 'image' */
+   checkopaque(image);
+   copy = *image;
+
+   copy.opts = opts;
+   copy.buffer = NULL;
+   copy.bufsize = 0;
+   copy.allocsize = 0;
+
+   image->input_file = NULL;
+   image->input_memory = NULL;
+   image->input_memory_size = 0;
+   image->tmpfile_name[0] = 0;
+
+   {
+      png_uint_32 counter;
+      Image output;
+
+      newimage(&output);
+
+      result = 1;
+
+      /* Use the low bit of 'counter' to indicate whether or not to do alpha
+       * removal with a background color or by composting onto the image; this
+       * step gets skipped if it isn't relevant
+       */
+      for (counter=0; counter<2*FORMAT_COUNT; ++counter)
+         if (format_isset(pf, counter >> 1))
+      {
+         png_uint_32 format = counter >> 1;
+
+         png_color background_color;
+         png_colorp background = NULL;
+
+         /* If there is a format change that removes the alpha channel then
+          * the background is relevant.  If the output is 8-bit color-mapped
+          * then a background color *must* be provided, otherwise there are
+          * two tests to do - one with a color, the other with NULL.  The
+          * NULL test happens second.
+          */
+         if ((counter & 1) == 0)
+         {
+            if ((format & PNG_FORMAT_FLAG_ALPHA) == 0 &&
+               (image->image.format & PNG_FORMAT_FLAG_ALPHA) != 0)
+            {
+               /* Alpha/transparency will be removed, the background is
+                * relevant: make it a color the first time
+                */
+               random_color(&background_color);
+               background = &background_color;
+
+               /* BUT if the output is to a color-mapped 8-bit format then
+                * the background must always be a color, so increment 'counter'
+                * to skip the NULL test.
+                */
+               if ((format & PNG_FORMAT_FLAG_COLORMAP) != 0 &&
+                  (format & PNG_FORMAT_FLAG_LINEAR) == 0)
+                  ++counter;
+            }
+
+            /* Otherwise an alpha channel is not being eliminated, just leave
+             * background NULL and skip the (counter & 1) NULL test.
+             */
+            else
+               ++counter;
+         }
+         /* else just use NULL for background */
+
+         resetimage(&copy);
+         copy.opts = opts; /* in case read_file needs to change it */
+
+         result = read_file(&copy, format, background);
+         if (!result)
+            break;
+
+         /* Make sure the file just read matches the original file. */
+         result = compare_two_images(image, &copy, 0/*via linear*/, background);
+         if (!result)
+            break;
+
+#        ifdef PNG_SIMPLIFIED_WRITE_SUPPORTED
+            /* Write the *copy* just made to a new file to make sure the write
+             * side works ok.  Check the conversion to sRGB if the copy is
+             * linear.
+             */
+            output.opts = opts;
+            result = write_one_file(&output, &copy, 0/*convert to 8bit*/);
+            if (!result)
+               break;
+
+            /* Validate against the original too; the background is needed here
+             * as well so that compare_two_images knows what color was used.
+             */
+            result = compare_two_images(image, &output, 0, background);
+            if (!result)
+               break;
+
+            if ((format & PNG_FORMAT_FLAG_LINEAR) != 0 &&
+               (format & PNG_FORMAT_FLAG_COLORMAP) == 0)
+            {
+               /* 'output' is linear, convert to the corresponding sRGB format.
+                */
+               output.opts = opts;
+               result = write_one_file(&output, &copy, 1/*convert to 8bit*/);
+               if (!result)
+                  break;
+
+               /* This may involve a conversion via linear; in the ideal world
+                * this would round-trip correctly, but libpng 1.5.7 is not the
+                * ideal world so allow a drift (error_via_linear).
+                *
+                * 'image' has an alpha channel but 'output' does not then there
+                * will a strip-alpha-channel operation (because 'output' is
+                * linear), handle this by composing on black when doing the
+                * comparison.
+                */
+               result = compare_two_images(image, &output, 1/*via_linear*/,
+                  background);
+               if (!result)
+                  break;
+            }
+#        endif /* PNG_SIMPLIFIED_WRITE_SUPPORTED */
+      }
+
+      freeimage(&output);
+   }
+
+   freeimage(&copy);
+
+   return result;
+}
+
+static int
+test_one_file(const char *file_name, format_list *formats, png_uint_32 opts,
+   int stride_extra, int log_pass)
+{
+   int result;
+   Image image;
+
+   if (!(opts & NO_RESEED))
+      reseed(); /* ensure that the random numbers don't depend on file order */
+   newimage(&image);
+   initimage(&image, opts, file_name, stride_extra);
+   result = read_one_file(&image);
+   if (result)
+      result = testimage(&image, opts, formats);
+   freeimage(&image);
+
+   /* Ensure that stderr is flushed into any log file */
+   fflush(stderr);
+
+   if (log_pass)
+   {
+      if (result)
+         printf("PASS:");
+
+      else
+         printf("FAIL:");
+
+#     ifndef PNG_SIMPLIFIED_WRITE_SUPPORTED
+         printf(" (no write)");
+#     endif
+
+      print_opts(opts);
+      printf(" %s\n", file_name);
+      /* stdout may not be line-buffered if it is piped to a file, so: */
+      fflush(stdout);
+   }
+
+   else if (!result)
+      exit(1);
+
+   return result;
+}
+
+int
+main(int argc, char **argv)
+{
+   png_uint_32 opts = FAST_WRITE | STRICT;
+   format_list formats;
+   const char *touch = NULL;
+   int log_pass = 0;
+   int redundant = 0;
+   int stride_extra = 0;
+   int retval = 0;
+   int c;
+
+#if PNG_LIBPNG_VER >= 10700
+      /* This error should not exist in 1.7 or later: */
+      opts |= GBG_ERROR;
+#endif
+
+   init_sRGB_to_d();
+#if 0
+   init_error_via_linear();
+#endif
+   format_init(&formats);
+   reseed(); /* initialize random number seeds */
+
+   for (c=1; c<argc; ++c)
+   {
+      const char *arg = argv[c];
+
+      if (strcmp(arg, "--log") == 0)
+         log_pass = 1;
+      else if (strcmp(arg, "--fresh") == 0)
+      {
+         memset(gpc_error, 0, sizeof gpc_error);
+         memset(gpc_error_via_linear, 0, sizeof gpc_error_via_linear);
+      }
+      else if (strcmp(arg, "--file") == 0)
+#        ifdef PNG_STDIO_SUPPORTED
+            opts |= USE_FILE;
+#        else
+            return SKIP; /* skipped: no support */
+#        endif
+      else if (strcmp(arg, "--memory") == 0)
+         opts &= ~USE_FILE;
+      else if (strcmp(arg, "--stdio") == 0)
+#        ifdef PNG_STDIO_SUPPORTED
+            opts |= USE_STDIO;
+#        else
+            return SKIP; /* skipped: no support */
+#        endif
+      else if (strcmp(arg, "--name") == 0)
+         opts &= ~USE_STDIO;
+      else if (strcmp(arg, "--verbose") == 0)
+         opts |= VERBOSE;
+      else if (strcmp(arg, "--quiet") == 0)
+         opts &= ~VERBOSE;
+      else if (strcmp(arg, "--preserve") == 0)
+         opts |= KEEP_TMPFILES;
+      else if (strcmp(arg, "--nopreserve") == 0)
+         opts &= ~KEEP_TMPFILES;
+      else if (strcmp(arg, "--keep-going") == 0)
+         opts |= KEEP_GOING;
+      else if (strcmp(arg, "--fast") == 0)
+         opts |= FAST_WRITE;
+      else if (strcmp(arg, "--slow") == 0)
+         opts &= ~FAST_WRITE;
+      else if (strcmp(arg, "--accumulate") == 0)
+         opts |= ACCUMULATE;
+      else if (strcmp(arg, "--redundant") == 0)
+         redundant = 1;
+      else if (strcmp(arg, "--stop") == 0)
+         opts &= ~KEEP_GOING;
+      else if (strcmp(arg, "--strict") == 0)
+         opts |= STRICT;
+      else if (strcmp(arg, "--nostrict") == 0)
+         opts &= ~STRICT;
+      else if (strcmp(arg, "--sRGB-16bit") == 0)
+         opts |= sRGB_16BIT;
+      else if (strcmp(arg, "--linear-16bit") == 0)
+         opts &= ~sRGB_16BIT;
+      else if (strcmp(arg, "--noreseed") == 0)
+         opts |= NO_RESEED;
+      else if (strcmp(arg, "--fault-gbg-warning") == 0)
+         opts |= GBG_ERROR;
+      else if (strcmp(arg, "--tmpfile") == 0)
+      {
+         if (c+1 < argc)
+         {
+            if (strlen(argv[++c]) >= sizeof tmpf)
+            {
+               fflush(stdout);
+               fprintf(stderr, "%s: %s is too long for a temp file prefix\n",
+                  argv[0], argv[c]);
+               exit(99);
+            }
+
+            /* Safe: checked above */
+            strncpy(tmpf, argv[c], sizeof (tmpf)-1);
+         }
+
+         else
+         {
+            fflush(stdout);
+            fprintf(stderr, "%s: %s requires a temporary file prefix\n",
+               argv[0], arg);
+            exit(99);
+         }
+      }
+      else if (strcmp(arg, "--touch") == 0)
+      {
+         if (c+1 < argc)
+            touch = argv[++c];
+
+         else
+         {
+            fflush(stdout);
+            fprintf(stderr, "%s: %s requires a file name argument\n",
+               argv[0], arg);
+            exit(99);
+         }
+      }
+      else if (arg[0] == '+')
+      {
+         png_uint_32 format = formatof(arg+1);
+
+         if (format > FORMAT_COUNT)
+            exit(99);
+
+         format_set(&formats, format);
+      }
+      else if (arg[0] == '-' && arg[1] != 0 && (arg[1] != '0' || arg[2] != 0))
+      {
+         fflush(stdout);
+         fprintf(stderr, "%s: unknown option: %s\n", argv[0], arg);
+         exit(99);
+      }
+      else
+      {
+         if (format_is_initial(&formats))
+            format_default(&formats, redundant);
+
+         if (arg[0] == '-')
+         {
+            const int term = (arg[1] == '0' ? 0 : '\n');
+            unsigned int ich = 0;
+
+            /* Loop reading files, use a static buffer to simplify this and just
+             * stop if the name gets to long.
+             */
+            static char buffer[4096];
+
+            do
+            {
+               int ch = getchar();
+
+               /* Don't allow '\0' in file names, and terminate with '\n' or,
+                * for -0, just '\0' (use -print0 to find to make this work!)
+                */
+               if (ch == EOF || ch == term || ch == 0)
+               {
+                  buffer[ich] = 0;
+
+                  if (ich > 0 && !test_one_file(buffer, &formats, opts,
+                     stride_extra, log_pass))
+                     retval = 1;
+
+                  if (ch == EOF)
+                     break;
+
+                  ich = 0;
+                  --ich; /* so that the increment below sets it to 0 again */
+               }
+
+               else
+                  buffer[ich] = (char)ch;
+            } while (++ich < sizeof buffer);
+
+            if (ich)
+            {
+               buffer[32] = 0;
+               buffer[4095] = 0;
+               fprintf(stderr, "%s...%s: file name too long\n", buffer,
+                  buffer+(4096-32));
+               exit(99);
+            }
+         }
+
+         else if (!test_one_file(arg, &formats, opts, stride_extra, log_pass))
+            retval = 1;
+      }
+   }
+
+   if (opts & ACCUMULATE)
+   {
+      unsigned int in;
+
+      printf("/* contrib/libtests/pngstest-errors.h\n");
+      printf(" *\n");
+      printf(" * BUILT USING:" PNG_HEADER_VERSION_STRING);
+      printf(" *\n");
+      printf(" * This code is released under the libpng license.\n");
+      printf(" * For conditions of distribution and use, see the disclaimer\n");
+      printf(" * and license in png.h\n");
+      printf(" *\n");
+      printf(" * THIS IS A MACHINE GENERATED FILE: do not edit it directly!\n");
+      printf(" * Instead run:\n");
+      printf(" *\n");
+      printf(" *    pngstest --accumulate\n");
+      printf(" *\n");
+      printf(" * on as many PNG files as possible; at least PNGSuite and\n");
+      printf(" * contrib/libtests/testpngs.\n");
+      printf(" */\n");
+
+      printf("static png_uint_16 gpc_error[16/*in*/][16/*out*/][4/*a*/] =\n");
+      printf("{\n");
+      for (in=0; in<16; ++in)
+      {
+         unsigned int out;
+         printf(" { /* input: %s */\n ", format_names[in]);
+         for (out=0; out<16; ++out)
+         {
+            unsigned int alpha;
+            printf(" {");
+            for (alpha=0; alpha<4; ++alpha)
+            {
+               printf(" %d", gpc_error[in][out][alpha]);
+               if (alpha < 3) putchar(',');
+            }
+            printf(" }");
+            if (out < 15)
+            {
+               putchar(',');
+               if (out % 4 == 3) printf("\n ");
+            }
+         }
+         printf("\n }");
+
+         if (in < 15)
+            putchar(',');
+         else
+            putchar('\n');
+      }
+      printf("};\n");
+
+      printf("static png_uint_16 gpc_error_via_linear[16][4/*out*/][4] =\n");
+      printf("{\n");
+      for (in=0; in<16; ++in)
+      {
+         unsigned int out;
+         printf(" { /* input: %s */\n ", format_names[in]);
+         for (out=0; out<4; ++out)
+         {
+            unsigned int alpha;
+            printf(" {");
+            for (alpha=0; alpha<4; ++alpha)
+            {
+               printf(" %d", gpc_error_via_linear[in][out][alpha]);
+               if (alpha < 3) putchar(',');
+            }
+            printf(" }");
+            if (out < 3)
+               putchar(',');
+         }
+         printf("\n }");
+
+         if (in < 15)
+            putchar(',');
+         else
+            putchar('\n');
+      }
+      printf("};\n");
+
+      printf("static png_uint_16 gpc_error_to_colormap[8/*i*/][8/*o*/][4] =\n");
+      printf("{\n");
+      for (in=0; in<8; ++in)
+      {
+         unsigned int out;
+         printf(" { /* input: %s */\n ", format_names[in]);
+         for (out=0; out<8; ++out)
+         {
+            unsigned int alpha;
+            printf(" {");
+            for (alpha=0; alpha<4; ++alpha)
+            {
+               printf(" %d", gpc_error_to_colormap[in][out][alpha]);
+               if (alpha < 3) putchar(',');
+            }
+            printf(" }");
+            if (out < 7)
+            {
+               putchar(',');
+               if (out % 4 == 3) printf("\n ");
+            }
+         }
+         printf("\n }");
+
+         if (in < 7)
+            putchar(',');
+         else
+            putchar('\n');
+      }
+      printf("};\n");
+      printf("/* END MACHINE GENERATED */\n");
+   }
+
+   if (retval == 0 && touch != NULL)
+   {
+      FILE *fsuccess = fopen(touch, "wt");
+
+      if (fsuccess != NULL)
+      {
+         int error = 0;
+         fprintf(fsuccess, "PNG simple API tests succeeded\n");
+         fflush(fsuccess);
+         error = ferror(fsuccess);
+
+         if (fclose(fsuccess) || error)
+         {
+            fflush(stdout);
+            fprintf(stderr, "%s: write failed\n", touch);
+            exit(99);
+         }
+      }
+
+      else
+      {
+         fflush(stdout);
+         fprintf(stderr, "%s: open failed\n", touch);
+         exit(99);
+      }
+   }
+
+   return retval;
+}
+
+#else /* !PNG_SIMPLIFIED_READ_SUPPORTED */
+int main(void)
+{
+   fprintf(stderr, "pngstest: no read support in libpng, test skipped\n");
+   /* So the test is skipped: */
+   return SKIP;
+}
+#endif /* PNG_SIMPLIFIED_READ_SUPPORTED */