text: // 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 <stdint.h>
  #include <stdio.h>
  #include <string.h>
  
  #include <algorithm>
  #include <iostream>
  #include <map>
  #include <string>
  #include <vector>
  
  #include "image_diff_png.h"
  #include "third_party/base/logging.h"
  #include "third_party/base/numerics/safe_conversions.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 size_t kBufSize = 1024;
      unsigned char buf[kBufSize];
      size_t num_read = 0;
      while ((num_read = fread(buf, 1, kBufSize, f)) > 0) {
        compressed.insert(compressed.end(), buf, buf + num_read);
      }
  
      fclose(f);
  
      if (!image_diff_png::DecodePNG(compressed.data(), 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 (!pixel_in_bounds(x, y))
        return 0;
      return *reinterpret_cast<const uint32_t*>(&(data_[pixel_address(x, y)]));
    }
  
    void set_pixel_at(int x, int y, uint32_t color) {
      if (!pixel_in_bounds(x, y))
        return;
  
      void* addr = &data_[pixel_address(x, y)];
      *reinterpret_cast<uint32_t*>(addr) = color;
    }
  
   private:
    bool pixel_in_bounds(int x, int y) const {
      return x >= 0 && x < w_ && y >= 0 && y < h_;
    }
  
    size_t pixel_address(int x, int y) const { return (y * w_ + x) * 4; }
  
    // Pixel dimensions of the image.
    int w_;
    int h_;
  
    std::vector<unsigned char> data_;
  };
  
  float CalculateDifferencePercentage(const Image& actual, int pixels_different) {
    // 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 CountImageSizeMismatchAsPixelDifference(const Image& baseline,
                                               const Image& actual,
                                               int* pixels_different) {
    int w = std::min(baseline.w(), actual.w());
    int h = std::min(baseline.h(), actual.h());
  
    // 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;
  }
  
  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;
      }
    }
  
    CountImageSizeMismatchAsPixelDifference(baseline, actual, &pixels_different);
    return CalculateDifferencePercentage(actual, pixels_different);
  }
  
  // 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;
      }
    }
  
    CountImageSizeMismatchAsPixelDifference(baseline, actual, &pixels_different);
    return CalculateDifferencePercentage(actual, pixels_different);
  }
  
  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* const diff_name = compare_histograms ? "exact diff" : "diff";
    float percent = PercentageDifferent(actual_image, baseline_image);
    const char* const 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;
  }
  
  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++];
    if (i < argc)
      filename2 = argv[i++];
    if (i < argc)
      diff_filename = argv[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;
  }
  