samples/image_diff_png.cc - platform/external/pdfium - Gitiles

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 // This is a duplicate of chromium's src/tools/imagediff/image_diff_png.cc
 // that has been modified to build in a pdfium environment, which itself
 // was duplicated as follows:

 // This is a duplicate of ui/gfx/codec/png_codec.cc, after removing code related
 // to Skia, that we can use when running layout tests with minimal dependencies.

 #include "samples/image_diff_png.h"

 #include <stdlib.h>
 #include <string.h>

 #include <string>

 #include "third_party/base/logging.h"
 #include "third_party/libpng16/png.h"
 #include "third_party/zlib/zlib.h"

 namespace image_diff_png {

 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,
 };

 // Represents a comment in the tEXt ancillary chunk of the png.
 struct 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();
         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();
         break;
     }
   } else {
     NOTREACHED();
     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));

   if (static_cast<int>(row_num) > state->height) {
     NOTREACHED();
     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));
   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.
     if (comment.key.length() > 79)
       return;
     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) {
 #ifdef PNG_TEXT_SUPPORTED
   CommentWriter comment_writer(comments);
 #endif
   unsigned char* row_buffer = NULL;

   // 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))) {
     delete[] row_buffer;
     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
   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.
     row_buffer = new unsigned char[width * output_color_components];
     for (int y = 0; y < height; y ++) {
       converter(&input[y * row_byte_width], width, row_buffer, NULL);
       png_write_row(png_ptr, row_buffer);
     }
     delete[] row_buffer;
   }

   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();
       return false;
   }

   // Row stride should be at least as long as the length of the data.
   if (input_color_components * width < row_byte_width)
     return false;

   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 Decode(input, input_size, FORMAT_RGBA, output, width, height);
 }

 // Encode an RGBA pixel array into a PNG.
 bool EncodeRGBAPNG(const unsigned char* input,
                    int width,
                    int height,
                    int row_byte_width,
                    std::vector<unsigned char>* output) {
   return Encode(input, FORMAT_RGBA,
       width, height, row_byte_width, false,
       std::vector<Comment>(), output);
 }

 // Encode an BGRA pixel array into a PNG.
 bool EncodeBGRAPNG(const unsigned char* input,
                    int width,
                    int height,
                    int row_byte_width,
                    bool discard_transparency,
                    std::vector<unsigned char>* output) {
   return Encode(input, FORMAT_BGRA,
       width, height, row_byte_width, discard_transparency,
       std::vector<Comment>(), output);
 }

 }  // namespace image_diff_png
	// Copyright 2013 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	// This is a duplicate of chromium's src/tools/imagediff/image_diff_png.cc
	// that has been modified to build in a pdfium environment, which itself
	// was duplicated as follows:

	// This is a duplicate of ui/gfx/codec/png_codec.cc, after removing code related
	// to Skia, that we can use when running layout tests with minimal dependencies.

	#include "samples/image_diff_png.h"

	#include <stdlib.h>
	#include <string.h>

	#include <string>

	#include "third_party/base/logging.h"
	#include "third_party/libpng16/png.h"
	#include "third_party/zlib/zlib.h"

	namespace image_diff_png {

	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,
	};

	// Represents a comment in the tEXt ancillary chunk of the png.
	struct 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();
	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();
	break;
	}
	} else {
	NOTREACHED();
	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));

	if (static_cast<int>(row_num) > state->height) {
	NOTREACHED();
	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));
	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.
	if (comment.key.length() > 79)
	return;
	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) {
	#ifdef PNG_TEXT_SUPPORTED
	CommentWriter comment_writer(comments);
	#endif
	unsigned char* row_buffer = NULL;

	// 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))) {
	delete[] row_buffer;
	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
	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.
	row_buffer = new unsigned char[width * output_color_components];
	for (int y = 0; y < height; y ++) {
	converter(&input[y * row_byte_width], width, row_buffer, NULL);
	png_write_row(png_ptr, row_buffer);
	}
	delete[] row_buffer;
	}

	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();
	return false;
	}

	// Row stride should be at least as long as the length of the data.
	if (input_color_components * width < row_byte_width)
	return false;

	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 Decode(input, input_size, FORMAT_RGBA, output, width, height);
	}

	// Encode an RGBA pixel array into a PNG.
	bool EncodeRGBAPNG(const unsigned char* input,
	int width,
	int height,
	int row_byte_width,
	std::vector<unsigned char>* output) {
	return Encode(input, FORMAT_RGBA,
	width, height, row_byte_width, false,
	std::vector<Comment>(), output);
	}

	// Encode an BGRA pixel array into a PNG.
	bool EncodeBGRAPNG(const unsigned char* input,
	int width,
	int height,
	int row_byte_width,
	bool discard_transparency,
	std::vector<unsigned char>* output) {
	return Encode(input, FORMAT_BGRA,
	width, height, row_byte_width, discard_transparency,
	std::vector<Comment>(), output);
	}

	} // namespace image_diff_png