Extract an independent webkit_support_gfx target.

webkit/support/webkit_support_gfx.cc

Index: webkit/support/webkit_support_gfx.cc
diff --git a/webkit/support/webkit_support_gfx.cc b/webkit/support/webkit_support_gfx.cc
index ecf788df95a0455f56dc5aa38922a73220980687..c21688d617b82d233e4dc569f8ebbcad0015be57 100644
--- a/webkit/support/webkit_support_gfx.cc
+++ b/webkit/support/webkit_support_gfx.cc
@@ -2,19 +2,632 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "build/build_config.h"
 #include "webkit/support/webkit_support_gfx.h"
 
-#include "ui/gfx/codec/png_codec.h"
-#include "ui/gfx/size.h"
+#include <stdlib.h>
+#include <string.h>
+
+extern "C" {
+#if defined(USE_SYSTEM_LIBPNG)
+#include <png.h>
+#else
+#include "third_party/libpng/png.h"
+#endif
+}
 
 namespace webkit_support {
 
+// Define macro here to make webkit_support_gfx independent of target base.
+#define NOTREACHED(msg) exit(1)
+#define DCHECK(condition) \
+  if (!(condition)) fprintf(stderr, "DCHECK failed: " #condition ".")
+
+// The new webkit_support_gfx, used by DumpRenderTree and ImageDiff,
+// doesn't depend on most other parts of chromium. This could make building
+// the individual target of ImageDiff fast. It's implemented by duplicating
+// most code in ui/gfx/codec/png_codec.cc, after removing code related to
+// Skia.
+namespace {
+
+enum ColorFormat {
+  // 3 bytes per pixel (packed), in RGB order regardless of endianness.
+  // This is the native JPEG format.
+  FORMAT_RGB,
+
+  // 4 bytes per pixel, in RGBA order in memory regardless of endianness.
+  FORMAT_RGBA,
+
+  // 4 bytes per pixel, in BGRA order in memory regardless of endianness.
+  // This is the default Windows DIB order.
+  FORMAT_BGRA,
+
+  // 4 bytes per pixel, in pre-multiplied kARGB_8888_Config format. For use
+  // with directly writing to a skia bitmap.
+  FORMAT_SkBitmap
+};
+
+// Represents a comment in the tEXt ancillary chunk of the png.
+struct Comment {
+  Comment(const std::string& k, const std::string& t)
+    : key(k), text(t) {
+  }
+
+  ~Comment() {
+  };
+
+  std::string key;
+  std::string text;
+};
+
+// Converts BGRA->RGBA and RGBA->BGRA.
+void ConvertBetweenBGRAandRGBA(const unsigned char* input, int pixel_width,
+                               unsigned char* output, bool* is_opaque) {
+  for (int x = 0; x < pixel_width; x++) {
+    const unsigned char* pixel_in = &input[x * 4];
+    unsigned char* pixel_out = &output[x * 4];
+    pixel_out[0] = pixel_in[2];
+    pixel_out[1] = pixel_in[1];
+    pixel_out[2] = pixel_in[0];
+    pixel_out[3] = pixel_in[3];
+  }
+}
+
+void ConvertRGBAtoRGB(const unsigned char* rgba, int pixel_width,
+                      unsigned char* rgb, bool* is_opaque) {
+  for (int x = 0; x < pixel_width; x++) {
+    const unsigned char* pixel_in = &rgba[x * 4];
+    unsigned char* pixel_out = &rgb[x * 3];
+    pixel_out[0] = pixel_in[0];
+    pixel_out[1] = pixel_in[1];
+    pixel_out[2] = pixel_in[2];
+  }
+}
+
+}  // namespace
+
+// Decoder --------------------------------------------------------------------
+//
+// This code is based on WebKit libpng interface (PNGImageDecoder), which is
+// in turn based on the Mozilla png decoder.
+
+namespace {
+
+// Gamma constants: We assume we're on Windows which uses a gamma of 2.2.
+const double kMaxGamma = 21474.83;  // Maximum gamma accepted by png library.
+const double kDefaultGamma = 2.2;
+const double kInverseGamma = 1.0 / kDefaultGamma;
+
+class PngDecoderState {
+ public:
+  // Output is a vector<unsigned char>.
+  PngDecoderState(ColorFormat ofmt, std::vector<unsigned char>* o)
+      : output_format(ofmt),
+        output_channels(0),
+        is_opaque(true),
+        output(o),
+        row_converter(NULL),
+        width(0),
+        height(0),
+        done(false) {
+  }
+
+  ColorFormat output_format;
+  int output_channels;
+
+  // Used during the reading of an SkBitmap. Defaults to true until we see a
+  // pixel with anything other than an alpha of 255.
+  bool is_opaque;
+
+  // An intermediary buffer for decode output.
+  std::vector<unsigned char>* output;
+
+  // Called to convert a row from the library to the correct output format.
+  // When NULL, no conversion is necessary.
+  void (*row_converter)(const unsigned char* in, int w, unsigned char* out,
+                        bool* is_opaque);
+
+  // Size of the image, set in the info callback.
+  int width;
+  int height;
+
+  // Set to true when we've found the end of the data.
+  bool done;
+};
+
+void ConvertRGBtoRGBA(const unsigned char* rgb, int pixel_width,
+                      unsigned char* rgba, bool* is_opaque) {
+  for (int x = 0; x < pixel_width; x++) {
+    const unsigned char* pixel_in = &rgb[x * 3];
+    unsigned char* pixel_out = &rgba[x * 4];
+    pixel_out[0] = pixel_in[0];
+    pixel_out[1] = pixel_in[1];
+    pixel_out[2] = pixel_in[2];
+    pixel_out[3] = 0xff;
+  }
+}
+
+void ConvertRGBtoBGRA(const unsigned char* rgb, int pixel_width,
+                      unsigned char* bgra, bool* is_opaque) {
+  for (int x = 0; x < pixel_width; x++) {
+    const unsigned char* pixel_in = &rgb[x * 3];
+    unsigned char* pixel_out = &bgra[x * 4];
+    pixel_out[0] = pixel_in[2];
+    pixel_out[1] = pixel_in[1];
+    pixel_out[2] = pixel_in[0];
+    pixel_out[3] = 0xff;
+  }
+}
+
+// Called when the png header has been read. This code is based on the WebKit
+// PNGImageDecoder
+void DecodeInfoCallback(png_struct* png_ptr, png_info* info_ptr) {
+  PngDecoderState* state = static_cast<PngDecoderState*>(
+      png_get_progressive_ptr(png_ptr));
+
+  int bit_depth, color_type, interlace_type, compression_type;
+  int filter_type, channels;
+  png_uint_32 w, h;
+  png_get_IHDR(png_ptr, info_ptr, &w, &h, &bit_depth, &color_type,
+               &interlace_type, &compression_type, &filter_type);
+
+  // Bounds check. When the image is unreasonably big, we'll error out and
+  // end up back at the setjmp call when we set up decoding.  "Unreasonably big"
+  // means "big enough that w * h * 32bpp might overflow an int"; we choose this
+  // threshold to match WebKit and because a number of places in code assume
+  // that an image's size (in bytes) fits in a (signed) int.
+  unsigned long long total_size =
+      static_cast<unsigned long long>(w) * static_cast<unsigned long long>(h);
+  if (total_size > ((1 << 29) - 1))
+    longjmp(png_jmpbuf(png_ptr), 1);
+  state->width = static_cast<int>(w);
+  state->height = static_cast<int>(h);
+
+  // Expand to ensure we use 24-bit for RGB and 32-bit for RGBA.
+  if (color_type == PNG_COLOR_TYPE_PALETTE ||
+      (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8))
+    png_set_expand(png_ptr);
+
+  // Transparency for paletted images.
+  if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
+    png_set_expand(png_ptr);
+
+  // Convert 16-bit to 8-bit.
+  if (bit_depth == 16)
+    png_set_strip_16(png_ptr);
+
+  // Expand grayscale to RGB.
+  if (color_type == PNG_COLOR_TYPE_GRAY ||
+      color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
+    png_set_gray_to_rgb(png_ptr);
+
+  // Deal with gamma and keep it under our control.
+  double gamma;
+  if (png_get_gAMA(png_ptr, info_ptr, &gamma)) {
+    if (gamma <= 0.0 || gamma > kMaxGamma) {
+      gamma = kInverseGamma;
+      png_set_gAMA(png_ptr, info_ptr, gamma);
+    }
+    png_set_gamma(png_ptr, kDefaultGamma, gamma);
+  } else {
+    png_set_gamma(png_ptr, kDefaultGamma, kInverseGamma);
+  }
+
+  // Tell libpng to send us rows for interlaced pngs.
+  if (interlace_type == PNG_INTERLACE_ADAM7)
+    png_set_interlace_handling(png_ptr);
+
+  // Update our info now
+  png_read_update_info(png_ptr, info_ptr);
+  channels = png_get_channels(png_ptr, info_ptr);
+
+  // Pick our row format converter necessary for this data.
+  if (channels == 3) {
+    switch (state->output_format) {
+      case FORMAT_RGB:
+        state->row_converter = NULL;  // no conversion necessary
+        state->output_channels = 3;
+        break;
+      case FORMAT_RGBA:
+        state->row_converter = &ConvertRGBtoRGBA;
+        state->output_channels = 4;
+        break;
+      case FORMAT_BGRA:
+        state->row_converter = &ConvertRGBtoBGRA;
+        state->output_channels = 4;
+        break;
+      default:
+        NOTREACHED("Unknown output format");
+        break;
+    }
+  } else if (channels == 4) {
+    switch (state->output_format) {
+      case FORMAT_RGB:
+        state->row_converter = &ConvertRGBAtoRGB;
+        state->output_channels = 3;
+        break;
+      case FORMAT_RGBA:
+        state->row_converter = NULL;  // no conversion necessary
+        state->output_channels = 4;
+        break;
+      case FORMAT_BGRA:
+        state->row_converter = &ConvertBetweenBGRAandRGBA;
+        state->output_channels = 4;
+        break;
+      default:
+        NOTREACHED("Unknown output format");
+        break;
+    }
+  } else {
+    NOTREACHED("Unknown input channels");
+    longjmp(png_jmpbuf(png_ptr), 1);
+  }
+
+  state->output->resize(
+      state->width * state->output_channels * state->height);
+}
+
+void DecodeRowCallback(png_struct* png_ptr, png_byte* new_row,
+                       png_uint_32 row_num, int pass) {
+  PngDecoderState* state = static_cast<PngDecoderState*>(
+      png_get_progressive_ptr(png_ptr));
+
+  DCHECK(pass == 0);
+  if (static_cast<int>(row_num) > state->height) {
+    NOTREACHED("Invalid row");
+    return;
+  }
+
+  unsigned char* base = NULL;
+  base = &state->output->front();
+
+  unsigned char* dest = &base[state->width * state->output_channels * row_num];
+  if (state->row_converter)
+    state->row_converter(new_row, state->width, dest, &state->is_opaque);
+  else
+    memcpy(dest, new_row, state->width * state->output_channels);
+}
+
+void DecodeEndCallback(png_struct* png_ptr, png_info* info) {
+  PngDecoderState* state = static_cast<PngDecoderState*>(
+      png_get_progressive_ptr(png_ptr));
+
+  // Mark the image as complete, this will tell the Decode function that we
+  // have successfully found the end of the data.
+  state->done = true;
+}
+
+// Automatically destroys the given read structs on destruction to make
+// cleanup and error handling code cleaner.
+class PngReadStructDestroyer {
+ public:
+  PngReadStructDestroyer(png_struct** ps, png_info** pi) : ps_(ps), pi_(pi) {
+  }
+  ~PngReadStructDestroyer() {
+    png_destroy_read_struct(ps_, pi_, NULL);
+  }
+ private:
+  png_struct** ps_;
+  png_info** pi_;
+};
+
+bool BuildPNGStruct(const unsigned char* input, size_t input_size,
+                    png_struct** png_ptr, png_info** info_ptr) {
+  if (input_size < 8)
+    return false;  // Input data too small to be a png
+
+  // Have libpng check the signature, it likes the first 8 bytes.
+  if (png_sig_cmp(const_cast<unsigned char*>(input), 0, 8) != 0)
+    return false;
+
+  *png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
+  if (!*png_ptr)
+    return false;
+
+  *info_ptr = png_create_info_struct(*png_ptr);
+  if (!*info_ptr) {
+    png_destroy_read_struct(png_ptr, NULL, NULL);
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace
+
+// static
+bool Decode(const unsigned char* input, size_t input_size,
+                      ColorFormat format, std::vector<unsigned char>* output,
+                      int* w, int* h) {
+  png_struct* png_ptr = NULL;
+  png_info* info_ptr = NULL;
+  if (!BuildPNGStruct(input, input_size, &png_ptr, &info_ptr))
+    return false;
+
+  PngReadStructDestroyer destroyer(&png_ptr, &info_ptr);
+  if (setjmp(png_jmpbuf(png_ptr))) {
+    // The destroyer will ensure that the structures are cleaned up in this
+    // case, even though we may get here as a jump from random parts of the
+    // PNG library called below.
+    return false;
+  }
+
+  PngDecoderState state(format, output);
+
+  png_set_progressive_read_fn(png_ptr, &state, &DecodeInfoCallback,
+                              &DecodeRowCallback, &DecodeEndCallback);
+  png_process_data(png_ptr,
+                   info_ptr,
+                   const_cast<unsigned char*>(input),
+                   input_size);
+
+  if (!state.done) {
+    // Fed it all the data but the library didn't think we got all the data, so
+    // this file must be truncated.
+    output->clear();
+    return false;
+  }
+
+  *w = state.width;
+  *h = state.height;
+  return true;
+}
+
+// Encoder --------------------------------------------------------------------
+//
+// This section of the code is based on nsPNGEncoder.cpp in Mozilla
+// (Copyright 2005 Google Inc.)
+
+namespace {
+
+// Passed around as the io_ptr in the png structs so our callbacks know where
+// to write data.
+struct PngEncoderState {
+  explicit PngEncoderState(std::vector<unsigned char>* o) : out(o) {}
+  std::vector<unsigned char>* out;
+};
+
+// Called by libpng to flush its internal buffer to ours.
+void EncoderWriteCallback(png_structp png, png_bytep data, png_size_t size) {
+  PngEncoderState* state = static_cast<PngEncoderState*>(png_get_io_ptr(png));
+  DCHECK(state->out);
+
+  size_t old_size = state->out->size();
+  state->out->resize(old_size + size);
+  memcpy(&(*state->out)[old_size], data, size);
+}
+
+void FakeFlushCallback(png_structp png) {
+  // We don't need to perform any flushing since we aren't doing real IO, but
+  // we're required to provide this function by libpng.
+}
+
+void ConvertBGRAtoRGB(const unsigned char* bgra, int pixel_width,
+                      unsigned char* rgb, bool* is_opaque) {
+  for (int x = 0; x < pixel_width; x++) {
+    const unsigned char* pixel_in = &bgra[x * 4];
+    unsigned char* pixel_out = &rgb[x * 3];
+    pixel_out[0] = pixel_in[2];
+    pixel_out[1] = pixel_in[1];
+    pixel_out[2] = pixel_in[0];
+  }
+}
+
+#ifdef PNG_TEXT_SUPPORTED
+
+inline char* strdup(const char* str) {
+#if defined(OS_WIN)
+  return _strdup(str);
+#else
+  return ::strdup(str);
+#endif
+}
+
+class CommentWriter {
+ public:
+  explicit CommentWriter(const std::vector<Comment>& comments)
+      : comments_(comments),
+        png_text_(new png_text[comments.size()]) {
+    for (size_t i = 0; i < comments.size(); ++i)
+      AddComment(i, comments[i]);
+  }
+
+  ~CommentWriter() {
+    for (size_t i = 0; i < comments_.size(); ++i) {
+      free(png_text_[i].key);
+      free(png_text_[i].text);
+    }
+    delete [] png_text_;
+  }
+
+  bool HasComments() {
+    return !comments_.empty();
+  }
+
+  png_text* get_png_text() {
+    return png_text_;
+  }
+
+  int size() {
+    return static_cast<int>(comments_.size());
+  }
+
+ private:
+  void AddComment(size_t pos, const Comment& comment) {
+    png_text_[pos].compression = PNG_TEXT_COMPRESSION_NONE;
+    // A PNG comment's key can only be 79 characters long.
+    DCHECK(comment.key.length() < 79);
+    png_text_[pos].key = strdup(comment.key.substr(0, 78).c_str());
+    png_text_[pos].text = strdup(comment.text.c_str());
+    png_text_[pos].text_length = comment.text.length();
+#ifdef PNG_iTXt_SUPPORTED
+    png_text_[pos].itxt_length = 0;
+    png_text_[pos].lang = 0;
+    png_text_[pos].lang_key = 0;
+#endif
+  }
+
+  const std::vector<Comment> comments_;
+  png_text* png_text_;
+};
+#endif  // PNG_TEXT_SUPPORTED
+
+// The type of functions usable for converting between pixel formats.
+typedef void (*FormatConverter)(const unsigned char* in, int w,
+                                unsigned char* out, bool* is_opaque);
+
+// libpng uses a wacky setjmp-based API, which makes the compiler nervous.
+// We constrain all of the calls we make to libpng where the setjmp() is in
+// place to this function.
+// Returns true on success.
+bool DoLibpngWrite(png_struct* png_ptr, png_info* info_ptr,
+                   PngEncoderState* state,
+                   int width, int height, int row_byte_width,
+                   const unsigned char* input, int compression_level,
+                   int png_output_color_type, int output_color_components,
+                   FormatConverter converter,
+                   const std::vector<Comment>& comments) {
+  // Make sure to not declare any locals here -- locals in the presence
+  // of setjmp() in C++ code makes gcc complain.
+
+  if (setjmp(png_jmpbuf(png_ptr)))
+    return false;
+
+  png_set_compression_level(png_ptr, compression_level);
+
+  // Set our callback for libpng to give us the data.
+  png_set_write_fn(png_ptr, state, EncoderWriteCallback, FakeFlushCallback);
+
+  png_set_IHDR(png_ptr, info_ptr, width, height, 8, png_output_color_type,
+               PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT,
+               PNG_FILTER_TYPE_DEFAULT);
+
+#ifdef PNG_TEXT_SUPPORTED
+  CommentWriter comment_writer(comments);
+  if (comment_writer.HasComments()) {
+    png_set_text(png_ptr, info_ptr, comment_writer.get_png_text(),
+                 comment_writer.size());
+  }
+#endif
+
+  png_write_info(png_ptr, info_ptr);
+
+  if (!converter) {
+    // No conversion needed, give the data directly to libpng.
+    for (int y = 0; y < height; y ++) {
+      png_write_row(png_ptr,
+                    const_cast<unsigned char*>(&input[y * row_byte_width]));
+    }
+  } else {
+    // Needs conversion using a separate buffer.
+    unsigned char* row = new unsigned char[width * output_color_components];
+    for (int y = 0; y < height; y ++) {
+      converter(&input[y * row_byte_width], width, row, NULL);
+      png_write_row(png_ptr, row);
+    }
+    delete[] row;
+  }
+
+  png_write_end(png_ptr, info_ptr);
+  return true;
+}
+
+}  // namespace
+
+// static
+bool EncodeWithCompressionLevel(const unsigned char* input, ColorFormat format,
+                                const int width, const int height,
+                                int row_byte_width,
+                                bool discard_transparency,
+                                const std::vector<Comment>& comments,
+                                int compression_level,
+                                std::vector<unsigned char>* output) {
+  // Run to convert an input row into the output row format, NULL means no
+  // conversion is necessary.
+  FormatConverter converter = NULL;
+
+  int input_color_components, output_color_components;
+  int png_output_color_type;
+  switch (format) {
+    case FORMAT_RGB:
+      input_color_components = 3;
+      output_color_components = 3;
+      png_output_color_type = PNG_COLOR_TYPE_RGB;
+      discard_transparency = false;
+      break;
+
+    case FORMAT_RGBA:
+      input_color_components = 4;
+      if (discard_transparency) {
+        output_color_components = 3;
+        png_output_color_type = PNG_COLOR_TYPE_RGB;
+        converter = ConvertRGBAtoRGB;
+      } else {
+        output_color_components = 4;
+        png_output_color_type = PNG_COLOR_TYPE_RGB_ALPHA;
+        converter = NULL;
+      }
+      break;
+
+    case FORMAT_BGRA:
+      input_color_components = 4;
+      if (discard_transparency) {
+        output_color_components = 3;
+        png_output_color_type = PNG_COLOR_TYPE_RGB;
+        converter = ConvertBGRAtoRGB;
+      } else {
+        output_color_components = 4;
+        png_output_color_type = PNG_COLOR_TYPE_RGB_ALPHA;
+        converter = ConvertBetweenBGRAandRGBA;
+      }
+      break;
+
+    default:
+      NOTREACHED("Unknown pixel format");
+      return false;
+  }
+
+  // Row stride should be at least as long as the length of the data.
+  DCHECK(input_color_components * width <= row_byte_width);
+
+  png_struct* png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
+                                                NULL, NULL, NULL);
+  if (!png_ptr)
+    return false;
+  png_info* info_ptr = png_create_info_struct(png_ptr);
+  if (!info_ptr) {
+    png_destroy_write_struct(&png_ptr, NULL);
+    return false;
+  }
+
+  PngEncoderState state(output);
+  bool success = DoLibpngWrite(png_ptr, info_ptr, &state,
+                               width, height, row_byte_width,
+                               input, compression_level, png_output_color_type,
+                               output_color_components, converter, comments);
+  png_destroy_write_struct(&png_ptr, &info_ptr);
+
+  return success;
+}
+
+// static
+bool Encode(const unsigned char* input, ColorFormat format,
+            const int width, const int height, int row_byte_width,
+            bool discard_transparency,
+            const std::vector<Comment>& comments,
+            std::vector<unsigned char>* output) {
+  return EncodeWithCompressionLevel(input, format, width, height,
+                                    row_byte_width,
+                                    discard_transparency,
+                                    comments, Z_DEFAULT_COMPRESSION,
+                                    output);
+}
+
 // Decode a PNG into an RGBA pixel array.
 bool DecodePNG(const unsigned char* input, size_t input_size,
                std::vector<unsigned char>* output,
                int* width, int* height) {
-  return gfx::PNGCodec::Decode(input, input_size, gfx::PNGCodec::FORMAT_RGBA,
-                               output, width, height);
+  return Decode(input, input_size, FORMAT_RGBA, output, width, height);
 }
 
 // Encode an RGBA pixel array into a PNG.
@@ -23,9 +636,9 @@ bool EncodeRGBAPNG(const unsigned char* input,
                    int height,
                    int row_byte_width,
                    std::vector<unsigned char>* output) {
-  return gfx::PNGCodec::Encode(input, gfx::PNGCodec::FORMAT_RGBA,
-      gfx::Size(width, height), row_byte_width, false,
-      std::vector<gfx::PNGCodec::Comment>(), output);
+  return Encode(input, FORMAT_RGBA,
+      width, height, row_byte_width, false,
+      std::vector<Comment>(), output);
 }
 
 // Encode an BGRA pixel array into a PNG.
@@ -35,9 +648,9 @@ bool EncodeBGRAPNG(const unsigned char* input,
                    int row_byte_width,
                    bool discard_transparency,
                    std::vector<unsigned char>* output) {
-  return gfx::PNGCodec::Encode(input, gfx::PNGCodec::FORMAT_BGRA,
-      gfx::Size(width, height), row_byte_width, discard_transparency,
-      std::vector<gfx::PNGCodec::Comment>(), output);
+  return Encode(input, FORMAT_BGRA,
+      width, height, row_byte_width, discard_transparency,
+      std::vector<Comment>(), output);
 }
 
 bool EncodeBGRAPNGWithChecksum(const unsigned char* input,
@@ -47,10 +660,10 @@ bool EncodeBGRAPNGWithChecksum(const unsigned char* input,
                                bool discard_transparency,
                                const std::string& checksum,
                                std::vector<unsigned char>* output) {
-  std::vector<gfx::PNGCodec::Comment> comments;
-  comments.push_back(gfx::PNGCodec::Comment("checksum", checksum));
-  return gfx::PNGCodec::Encode(input, gfx::PNGCodec::FORMAT_BGRA,
-      gfx::Size(width, height), row_byte_width, discard_transparency,
+  std::vector<Comment> comments;
+  comments.push_back(Comment("checksum", checksum));
+  return Encode(input, FORMAT_BGRA,
+      width, height, row_byte_width, discard_transparency,
       comments, output);
 }