XFA: Support PNG format in pdfium_test and add image diffing

Lays the groundwork for pixel-diffing tests in pdfium.

This is a port of chromium's tools/image_diff/image_diff_png.cc onto the
top of the fxcodec-provided version of libpng. pdfium_test is modified
to support the new format, and the tools/image_diff.cc image comparison
utility is provided, stripped of its base/ dependencies.

Unfortunately, this can't be back-ported to origin/main, since the
underlying PNG support isn't present inside pdfium.

BUG=https://code.google.com/p/pdfium/issues/detail?id=62
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/870203005

6 files changed, 1171 insertions, 9 deletions

diff --git a/samples/BUILD.gn b/samples/BUILD.gn
index e690c23622..7f2f41fa82 100644
--- a/samples/BUILD.gn
+++ b/samples/BUILD.gn
@@ -10,9 +10,21 @@ group("samples") {
 
 executable("pdfium_test") {
   sources = [
+    "image_diff_png.cc",
     "pdfium_test.cc",
   ]
   deps = [
     "//third_party/pdfium",
   ]
 }
+
+executable("pdfium_diff") {
+  sources = [
+    "image_diff.cc",
+    "image_diff_png.cc",
+    "image_diff_png.h",
+  ]
+  deps = [
+    "//third_party/pdfium",
+  ]
+}
\ No newline at end of file
diff --git a/samples/image_diff.cc b/samples/image_diff.cc
new file mode 100644
index 0000000000..3f82b66527
--- /dev/null
+++ b/samples/image_diff.cc
@@ -0,0 +1,398 @@
+// Copyright (c) 2011 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 file input format is based loosely on
+// Tools/DumpRenderTree/ImageDiff.m
+
+// The exact format of this tool's output to stdout is important, to match
+// what the run-webkit-tests script expects.
+
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <map>
+#include <string>
+#include <vector>
+
+#include "../third_party/base/logging.h"
+#include "../third_party/base/numerics/safe_conversions.h"
+#include "image_diff_png.h"
+
+#if defined(OS_WIN)
+#include "windows.h"
+#endif
+
+// Return codes used by this utility.
+static const int kStatusSame = 0;
+static const int kStatusDifferent = 1;
+static const int kStatusError = 2;
+
+// Color codes.
+static const uint32_t RGBA_RED = 0x000000ff;
+static const uint32_t RGBA_ALPHA = 0xff000000;
+
+class Image {
+ public:
+  Image() : w_(0), h_(0) {
+  }
+
+  Image(const Image& image)
+      : w_(image.w_),
+        h_(image.h_),
+        data_(image.data_) {
+  }
+
+  bool has_image() const {
+    return w_ > 0 && h_ > 0;
+  }
+
+  int w() const {
+    return w_;
+  }
+
+  int h() const {
+    return h_;
+  }
+
+  const unsigned char* data() const {
+    return &data_.front();
+  }
+
+  // Creates the image from the given filename on disk, and returns true on
+  // success.
+  bool CreateFromFilename(const std::string& path) {
+    FILE* f = fopen(path.c_str(), "rb");
+    if (!f)
+      return false;
+
+    std::vector<unsigned char> compressed;
+    const int buf_size = 1024;
+    unsigned char buf[buf_size];
+    size_t num_read = 0;
+    while ((num_read = fread(buf, 1, buf_size, f)) > 0) {
+      compressed.insert(compressed.end(), buf, buf + num_read);
+    }
+
+    fclose(f);
+
+    if (!image_diff_png::DecodePNG(&compressed[0], compressed.size(),
+                                   &data_, &w_, &h_)) {
+      Clear();
+      return false;
+    }
+    return true;
+  }
+
+  void Clear() {
+    w_ = h_ = 0;
+    data_.clear();
+  }
+
+  // Returns the RGBA value of the pixel at the given location
+  uint32_t pixel_at(int x, int y) const {
+    if (x >= 0 && x < w_ && y >= 0 && y < h_)
+      return *reinterpret_cast<const uint32_t*>(&(data_[(y * w_ + x) * 4]));
+    return 0;
+  }
+
+  void set_pixel_at(int x, int y, uint32_t color) const {
+    if (x >= 0 && x < w_ && y >= 0 && y < h_) {
+      void* addr = &const_cast<unsigned char*>(&data_.front())[(y * w_ + x) * 4];
+      *reinterpret_cast<uint32_t*>(addr) = color;
+    }
+  }
+
+ private:
+  // pixel dimensions of the image
+  int w_, h_;
+
+  std::vector<unsigned char> data_;
+};
+
+float PercentageDifferent(const Image& baseline, const Image& actual) {
+  int w = std::min(baseline.w(), actual.w());
+  int h = std::min(baseline.h(), actual.h());
+
+  // Compute pixels different in the overlap.
+  int pixels_different = 0;
+  for (int y = 0; y < h; y++) {
+    for (int x = 0; x < w; x++) {
+      if (baseline.pixel_at(x, y) != actual.pixel_at(x, y))
+        pixels_different++;
+    }
+  }
+
+  // Count pixels that are a difference in size as also being different.
+  int max_w = std::max(baseline.w(), actual.w());
+  int max_h = std::max(baseline.h(), actual.h());
+  // These pixels are off the right side, not including the lower right corner.
+  pixels_different += (max_w - w) * h;
+  // These pixels are along the bottom, including the lower right corner.
+  pixels_different += (max_h - h) * max_w;
+
+  // Like the WebKit ImageDiff tool, we define percentage different in terms
+  // of the size of the 'actual' bitmap.
+  float total_pixels = static_cast<float>(actual.w()) *
+                       static_cast<float>(actual.h());
+  if (total_pixels == 0) {
+    // When the bitmap is empty, they are 100% different.
+    return 100.0f;
+  }
+  return 100.0f * pixels_different / total_pixels;
+}
+
+// FIXME: Replace with unordered_map when available.
+typedef std::map<uint32_t, int32_t> RgbaToCountMap;
+
+float HistogramPercentageDifferent(const Image& baseline, const Image& actual) {
+  // TODO(johnme): Consider using a joint histogram instead, as described in
+  // "Comparing Images Using Joint Histograms" by Pass & Zabih
+  // http://www.cs.cornell.edu/~rdz/papers/pz-jms99.pdf
+
+  int w = std::min(baseline.w(), actual.w());
+  int h = std::min(baseline.h(), actual.h());
+
+  // Count occurences of each RGBA pixel value of baseline in the overlap.
+  RgbaToCountMap baseline_histogram;
+  for (int y = 0; y < h; y++) {
+    for (int x = 0; x < w; x++) {
+      // hash_map operator[] inserts a 0 (default constructor) if key not found.
+      baseline_histogram[baseline.pixel_at(x, y)]++;
+    }
+  }
+
+  // Compute pixels different in the histogram of the overlap.
+  int pixels_different = 0;
+  for (int y = 0; y < h; y++) {
+    for (int x = 0; x < w; x++) {
+      uint32_t actual_rgba = actual.pixel_at(x, y);
+      RgbaToCountMap::iterator it = baseline_histogram.find(actual_rgba);
+      if (it != baseline_histogram.end() && it->second > 0)
+        it->second--;
+      else
+        pixels_different++;
+    }
+  }
+
+  // Count pixels that are a difference in size as also being different.
+  int max_w = std::max(baseline.w(), actual.w());
+  int max_h = std::max(baseline.h(), actual.h());
+  // These pixels are off the right side, not including the lower right corner.
+  pixels_different += (max_w - w) * h;
+  // These pixels are along the bottom, including the lower right corner.
+  pixels_different += (max_h - h) * max_w;
+
+  // Like the WebKit ImageDiff tool, we define percentage different in terms
+  // of the size of the 'actual' bitmap.
+  float total_pixels = static_cast<float>(actual.w()) *
+                       static_cast<float>(actual.h());
+  if (total_pixels == 0) {
+    // When the bitmap is empty, they are 100% different.
+    return 100.0f;
+  }
+  return 100.0f * pixels_different / total_pixels;
+}
+
+void PrintHelp() {
+  fprintf(stderr,
+    "Usage:\n"
+    "  image_diff [--histogram] <compare file> <reference file>\n"
+    "    Compares two files on disk, returning 0 when they are the same;\n"
+    "    passing \"--histogram\" additionally calculates a diff of the\n"
+    "    RGBA value histograms (which is resistant to shifts in layout)\n"
+    "  image_diff --diff <compare file> <reference file> <output file>\n"
+    "    Compares two files on disk, outputs an image that visualizes the\n"
+    "    difference to <output file>\n");
+}
+
+int CompareImages(const std::string& file1,
+                  const std::string& file2,
+                  bool compare_histograms) {
+  Image actual_image;
+  Image baseline_image;
+
+  if (!actual_image.CreateFromFilename(file1)) {
+    fprintf(stderr, "image_diff: Unable to open file \"%s\"\n", file1.c_str());
+    return kStatusError;
+  }
+  if (!baseline_image.CreateFromFilename(file2)) {
+    fprintf(stderr, "image_diff: Unable to open file \"%s\"\n", file2.c_str());
+    return kStatusError;
+  }
+
+  if (compare_histograms) {
+    float percent = HistogramPercentageDifferent(actual_image, baseline_image);
+    const char* passed = percent > 0.0 ? "failed" : "passed";
+    printf("histogram diff: %01.2f%% %s\n", percent, passed);
+  }
+
+  const char* diff_name = compare_histograms ? "exact diff" : "diff";
+  float percent = PercentageDifferent(actual_image, baseline_image);
+  const char* passed = percent > 0.0 ? "failed" : "passed";
+  printf("%s: %01.2f%% %s\n", diff_name, percent, passed);
+  if (percent > 0.0) {
+    // failure: The WebKit version also writes the difference image to
+    // stdout, which seems excessive for our needs.
+    return kStatusDifferent;
+  }
+  // success
+  return kStatusSame;
+
+/* Untested mode that acts like WebKit's image comparator. I wrote this but
+   decided it's too complicated. We may use it in the future if it looks useful
+
+  char buffer[2048];
+  while (fgets(buffer, sizeof(buffer), stdin)) {
+
+    if (strncmp("Content-length: ", buffer, 16) == 0) {
+      char* context;
+      strtok_s(buffer, " ", &context);
+      int image_size = strtol(strtok_s(NULL, " ", &context), NULL, 10);
+
+      bool success = false;
+      if (image_size > 0 && actual_image.has_image() == 0) {
+        if (!actual_image.CreateFromStdin(image_size)) {
+          fputs("Error, input image can't be decoded.\n", stderr);
+          return 1;
+        }
+      } else if (image_size > 0 && baseline_image.has_image() == 0) {
+        if (!baseline_image.CreateFromStdin(image_size)) {
+          fputs("Error, baseline image can't be decoded.\n", stderr);
+          return 1;
+        }
+      } else {
+        fputs("Error, image size must be specified.\n", stderr);
+        return 1;
+      }
+    }
+
+    if (actual_image.has_image() && baseline_image.has_image()) {
+      float percent = PercentageDifferent(actual_image, baseline_image);
+      if (percent > 0.0) {
+        // failure: The WebKit version also writes the difference image to
+        // stdout, which seems excessive for our needs.
+        printf("diff: %01.2f%% failed\n", percent);
+      } else {
+        // success
+        printf("diff: %01.2f%% passed\n", percent);
+      }
+      actual_image.Clear();
+      baseline_image.Clear();
+    }
+
+    fflush(stdout);
+  }
+*/
+}
+
+bool CreateImageDiff(const Image& image1, const Image& image2, Image* out) {
+  int w = std::min(image1.w(), image2.w());
+  int h = std::min(image1.h(), image2.h());
+  *out = Image(image1);
+  bool same = (image1.w() == image2.w()) && (image1.h() == image2.h());
+
+  // TODO(estade): do something with the extra pixels if the image sizes
+  // are different.
+  for (int y = 0; y < h; y++) {
+    for (int x = 0; x < w; x++) {
+      uint32_t base_pixel = image1.pixel_at(x, y);
+      if (base_pixel != image2.pixel_at(x, y)) {
+        // Set differing pixels red.
+        out->set_pixel_at(x, y, RGBA_RED | RGBA_ALPHA);
+        same = false;
+      } else {
+        // Set same pixels as faded.
+        uint32_t alpha = base_pixel & RGBA_ALPHA;
+        uint32_t new_pixel = base_pixel - ((alpha / 2) & RGBA_ALPHA);
+        out->set_pixel_at(x, y, new_pixel);
+      }
+    }
+  }
+
+  return same;
+}
+
+int DiffImages(const std::string& file1,
+               const std::string& file2,
+               const std::string& out_file) {
+  Image actual_image;
+  Image baseline_image;
+
+  if (!actual_image.CreateFromFilename(file1)) {
+    fprintf(stderr, "image_diff: Unable to open file \"%s\"\n", file1.c_str());
+    return kStatusError;
+  }
+  if (!baseline_image.CreateFromFilename(file2)) {
+    fprintf(stderr, "image_diff: Unable to open file \"%s\"\n", file2.c_str());
+    return kStatusError;
+  }
+
+  Image diff_image;
+  bool same = CreateImageDiff(baseline_image, actual_image, &diff_image);
+  if (same)
+    return kStatusSame;
+
+  std::vector<unsigned char> png_encoding;
+  image_diff_png::EncodeRGBAPNG(
+      diff_image.data(), diff_image.w(), diff_image.h(),
+      diff_image.w() * 4, &png_encoding);
+
+  FILE *f = fopen(out_file.c_str(), "wb");
+  if (!f)
+    return kStatusError;
+
+  size_t size = png_encoding.size();
+  char *ptr = reinterpret_cast<char*>(&png_encoding.front());
+  if (fwrite(ptr, 1, size, f) != size)
+    return kStatusError;
+
+  return kStatusDifferent;
+}
+
+int main(int argc, const char* argv[]) {
+  bool histograms = false;
+  bool produce_diff_image = false;
+  std::string filename1;
+  std::string filename2;
+  std::string diff_filename;
+
+  int i;
+  for (i = 1; i < argc; ++i) {
+    const char* arg = argv[i];
+    if (strstr(arg, "--") != arg)
+      break;
+    if (strcmp(arg, "--histogram") == 0) {
+      histograms = true;
+    } else if (strcmp(arg, "--diff") == 0) {
+      produce_diff_image = true;
+    }
+  }
+  if (i < argc) {
+    filename1 = argv[i];
+    ++i;
+  }
+  if (i < argc) {
+    filename2 = argv[i];
+    ++i;
+  }
+  if (i < argc) {
+    diff_filename = argv[i];
+    ++i;
+  }
+
+  if (produce_diff_image) {
+    if (!diff_filename.empty()) {
+      return DiffImages(filename1, filename2, diff_filename);
+    }
+  } else if (!filename2.empty()) {
+    return CompareImages(filename1, filename2, histograms);
+  }
+
+  PrintHelp();
+  return kStatusError;
+}
diff --git a/samples/image_diff_png.cc b/samples/image_diff_png.cc
new file mode 100644
index 0000000000..2ccc3e5c91
--- /dev/null
+++ b/samples/image_diff_png.cc
@@ -0,0 +1,644 @@
+// 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 "image_diff_png.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+#include <string>
+
+#include "../third_party/base/logging.h"
+#include "../core/src/fxcodec/fx_lpng/include/fx_png.h"
+#include "../core/src/fxcodec/fx_zlib/include/fx_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);
+}
+
+}  // image_diff_png
diff --git a/samples/image_diff_png.h b/samples/image_diff_png.h
new file mode 100644
index 0000000000..ea77e9a98f
--- /dev/null
+++ b/samples/image_diff_png.h
@@ -0,0 +1,34 @@
+// 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.
+
+#ifndef TOOLS_IMAGEDIFF_IMAGE_DIFF_PNG_H_
+#define TOOLS_IMAGEDIFF_IMAGE_DIFF_PNG_H_
+
+#include <vector>
+
+namespace image_diff_png {
+
+// 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);
+
+// 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);
+
+// 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);
+
+}  // namespace image_diff_png
+
+#endif  // TOOLS_IMAGEDIFF_IMAGE_DIFF_PNG_H_
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index f7911c36b9..fc4bbe4aa4 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -21,6 +21,7 @@
 #include "../fpdfsdk/include/fpdfview.h"
 #include "../core/include/fxcrt/fx_system.h"
 #include "v8/include/v8.h"
+#include "image_diff_png.h"
 
 #ifdef _WIN32
 #define snprintf _snprintf
@@ -32,6 +33,7 @@
 enum OutputFormat {
   OUTPUT_NONE,
   OUTPUT_PPM,
+  OUTPUT_PNG,
 #ifdef _WIN32
   OUTPUT_BMP,
   OUTPUT_EMF,
@@ -113,14 +115,21 @@ static bool GetExternalData(const Options& options,
 }
 #endif  // V8_USE_EXTERNAL_STARTUP_DATA
 
+static bool CheckDimensions(int stride, int width, int height) {
+  if (stride < 0 || width < 0 || height < 0)
+    return false;
+  if (height > 0 && width > INT_MAX / height)
+    return false;
+  return true;
+}
+
 static void WritePpm(const char* pdf_name, int num, const void* buffer_void,
                      int stride, int width, int height) {
   const char* buffer = reinterpret_cast<const char*>(buffer_void);
 
-  if (stride < 0 || width < 0 || height < 0)
-    return;
-  if (height > 0 && width > INT_MAX / height)
+  if (!CheckDimensions(stride, width, height))
     return;
+
   int out_len = width * height;
   if (out_len > INT_MAX / 3)
     return;
@@ -154,13 +163,48 @@ static void WritePpm(const char* pdf_name, int num, const void* buffer_void,
   fclose(fp);
 }
 
+static void WritePng(const char* pdf_name, int num, const void* buffer_void,
+                     int stride, int width, int height) {
+  if (!CheckDimensions(stride, width, height))
+    return;
+
+  std::vector<unsigned char> png_encoding;
+  const unsigned char* buffer = static_cast<const unsigned char*>(buffer_void);
+  if (!image_diff_png::EncodeBGRAPNG(
+          buffer, width, height, stride, false, &png_encoding)) {
+    fprintf(stderr, "Failed to convert bitmap to PNG\n");
+    return;
+  }
+
+  char filename[256];
+  int chars_formatted = snprintf(
+      filename, sizeof(filename), "%s.%d.png", pdf_name, num);
+  if (chars_formatted < 0 ||
+      static_cast<size_t>(chars_formatted) >= sizeof(filename)) {
+    fprintf(stderr, "Filname %s is too long\n", filename);
+    return;
+  }
+
+  FILE* fp = fopen(filename, "wb");
+  if (!fp) {
+    fprintf(stderr, "Failed to open %s for output\n", filename);
+    return;
+  }
+
+  size_t bytes_written = fwrite(
+      &png_encoding.front(), 1, png_encoding.size(), fp);
+  if (bytes_written != png_encoding.size())
+    fprintf(stderr, "Failed to write to  %s\n", filename);
+
+  (void) fclose(fp);
+}
+
 #ifdef _WIN32
 static void WriteBmp(const char* pdf_name, int num, const void* buffer,
                      int stride, int width, int height) {
-  if (stride < 0 || width < 0 || height < 0)
-    return;
-  if (height > 0 && width > INT_MAX / height)
+  if (!CheckDimensions(stride, width, height))
     return;
+
   int out_len = stride * height;
   if (out_len > INT_MAX / 3)
     return;
@@ -199,9 +243,9 @@ void WriteEmf(FPDF_PAGE page, const char* pdf_name, int num) {
   snprintf(filename, sizeof(filename), "%s.%d.emf", pdf_name, num);
 
   HDC dc = CreateEnhMetaFileA(NULL, filename, NULL, NULL);
-  
-  HRGN rgn = CreateRectRgn(0, 0, width, height); 
-  SelectClipRgn(dc, rgn); 
+
+  HRGN rgn = CreateRectRgn(0, 0, width, height);
+  SelectClipRgn(dc, rgn);
   DeleteObject(rgn);
 
   SelectObject(dc, GetStockObject(NULL_PEN));
@@ -293,6 +337,12 @@ bool ParseCommandLine(const std::vector<std::string>& args,
         return false;
       }
       options->output_format = OUTPUT_PPM;
+    } else if (cur_arg == "--png") {
+      if (options->output_format != OUTPUT_NONE) {
+        fprintf(stderr, "Duplicate or conflicting --png argument\n");
+        return false;
+      }
+      options->output_format = OUTPUT_PNG;
     }
 #ifdef _WIN32
     else if (cur_arg == "--emf") {
@@ -476,6 +526,11 @@ void RenderPdf(const std::string& name, const char* pBuf, size_t len,
       case OUTPUT_PPM:
         WritePpm(name.c_str(), i, buffer, stride, width, height);
         break;
+
+      case OUTPUT_PNG:
+        WritePng(name.c_str(), i, buffer, stride, width, height);
+        break;
+
       default:
         break;
     }
@@ -505,6 +560,7 @@ int main(int argc, const char* argv[]) {
     printf("Usage: pdfium_test [OPTION] [FILE]...\n");
     printf("--bin-dir=<path> - override path to v8 external data\n");
     printf("--scale=<number> - scale output size by number (e.g. 0.5)\n");
+    printf("--png - write page images <pdf-name>.<page-number>.png\n");
     printf("--ppm - write page images <pdf-name>.<page-number>.ppm\n");
 #ifdef _WIN32
     printf("--bmp - write page images <pdf-name>.<page-number>.bmp\n");
diff --git a/samples/samples.gyp b/samples/samples.gyp
index 0c8ee93661..9d316ec005 100644
--- a/samples/samples.gyp
+++ b/samples/samples.gyp
@@ -15,6 +15,24 @@
       'target_name': 'pdfium_test',
       'sources': [
         'pdfium_test.cc',
+        'image_diff_png.cc',
+      ],
+    },
+    {
+      'target_name': 'pdfium_diff',
+      'type': 'executable',
+      'variables': { 'enable_wexit_time_destructors': 1, },
+      'dependencies': [
+        '../pdfium.gyp:fxcodec',
+        '../third_party/third_party.gyp:safemath',
+      ],
+      'include_dirs': [
+        '../../',
+      ],
+      'sources': [
+        'image_diff.cc',
+        'image_diff_png.h',
+        'image_diff_png.cc',
       ],
     },
   ],