Integration of most changes from the GoogleTV project around the convolver/sc...

skia/ext/image_operations_unittest.cc

Index: skia/ext/image_operations_unittest.cc
===================================================================
--- skia/ext/image_operations_unittest.cc	(revision 72357)
+++ skia/ext/image_operations_unittest.cc	(working copy)
@@ -1,7 +1,15 @@
-// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// 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.
 
+#include <algorithm>
+#include <iomanip>
+#include <vector>
+
+#include "base/basictypes.h"
+#include "base/file_util.h"
+#include "base/string_util.h"
+#include "gfx/codec/png_codec.h"
 #include "skia/ext/image_operations.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/skia/include/core/SkBitmap.h"
@@ -32,6 +40,64 @@
                         static_cast<unsigned char>(accum[0] / count));
 }
 
+// Computes the average pixel (/color) value for the given colors.
+SkColor AveragePixel(const SkColor colors[], size_t color_count) {
+  float accum[4] = { 0.0f, 0.0f, 0.0f, 0.0f };
+  for (size_t i = 0; i < color_count; ++i) {
+    const SkColor cur = colors[i];
+    accum[0] += static_cast<float>(SkColorGetA(cur));
+    accum[1] += static_cast<float>(SkColorGetR(cur));
+    accum[2] += static_cast<float>(SkColorGetG(cur));
+    accum[3] += static_cast<float>(SkColorGetB(cur));
+  }
+  const SkColor average_color =
+      SkColorSetARGB(static_cast<uint8_t>(accum[0] / color_count),
+                     static_cast<uint8_t>(accum[1] / color_count),
+                     static_cast<uint8_t>(accum[2] / color_count),
+                     static_cast<uint8_t>(accum[3] / color_count));
+  return average_color;
+}
+
+void PrintPixel(const SkBitmap& bmp,
+                int x_min, int x_max,
+                int y_min, int y_max) {
+  char str[128];
+
+  for (int y = y_min; y <= y_max; ++y) {
+    for (int x = x_min; x <= x_max; ++x) {
+      const uint32_t cur = *bmp.getAddr32(x, y);
+      base::snprintf(str, sizeof(str), "bmp[%d,%d] = %08X", x, y, cur);
+      ADD_FAILURE() << str;
+    }
+  }
+}
+
+// Returns the euclidian distance between two RGBA colors interpreted
+// as 4-components vectors.
+//
+// Notes:
+// - This is a really poor definition of color distance. Yet it
+//   is "good enough" for our uses here.
+// - More realistic measures like the various Delta E formulas defined
+//   by CIE are way more complex and themselves require the RGBA to
+//   to transformed into CIELAB (typically via sRGB first).
+// - The static_cast<int> below are needed to avoid interpreting "negative"
+//   differences as huge positive values.
+float ColorsEuclidianDistance(const SkColor a, const SkColor b) {
+  int b_int_diff = static_cast<int>(SkColorGetB(a) - SkColorGetB(b));
+  int g_int_diff = static_cast<int>(SkColorGetG(a) - SkColorGetG(b));
+  int r_int_diff = static_cast<int>(SkColorGetR(a) - SkColorGetR(b));
+  int a_int_diff = static_cast<int>(SkColorGetA(a) - SkColorGetA(b));
+
+  float b_float_diff = static_cast<float>(b_int_diff);
+  float g_float_diff = static_cast<float>(g_int_diff);
+  float r_float_diff = static_cast<float>(r_int_diff);
+  float a_float_diff = static_cast<float>(a_int_diff);
+
+  return sqrtf((b_float_diff * b_float_diff) + (g_float_diff * g_float_diff) +
+               (r_float_diff * r_float_diff) + (a_float_diff * a_float_diff));
+}
+
 // Returns true if each channel of the given two colors are "close." This is
 // used for comparing colors where rounding errors may cause off-by-one.
 bool ColorsClose(uint32_t a, uint32_t b) {
@@ -45,18 +111,277 @@
   bmp->setConfig(SkBitmap::kARGB_8888_Config, w, h);
   bmp->allocPixels();
 
-  unsigned char* src_data =
-      reinterpret_cast<unsigned char*>(bmp->getAddr32(0, 0));
-  for (int i = 0; i < w * h; i++) {
-    src_data[i * 4 + 0] = static_cast<unsigned char>(i % 255);
-    src_data[i * 4 + 1] = static_cast<unsigned char>(i % 255);
-    src_data[i * 4 + 2] = static_cast<unsigned char>(i % 255);
-    src_data[i * 4 + 3] = static_cast<unsigned char>(i % 255);
+  for (int y = 0; y < h; ++y) {
+    for (int x = 0; x < w; ++x) {
+      const uint8_t component = static_cast<uint8_t>(y * w + x);
+      const SkColor pixel = SkColorSetARGB(component, component,
+                                           component, component);
+      *bmp->getAddr32(x, y) = pixel;
+    }
   }
 }
 
+// Draws a horizontal and vertical grid into the w x h bitmap passed in.
+// Each line in the grid is drawn with a width of "grid_width" pixels,
+// and those lines repeat every "grid_pitch" pixels. The top left pixel (0, 0)
+// is considered to be part of a grid line.
+// The pixels that fall on a line are colored with "grid_color", while those
+// outside of the lines are colored in "background_color".
+// Note that grid_with can be greather than or equal to grid_pitch, in which
+// case the resulting bitmap will be a solid color "grid_color".
+void DrawGridToBitmap(int w, int h,
+                      SkColor background_color, SkColor grid_color,
+                      int grid_pitch, int grid_width,
+                      SkBitmap* bmp) {
+  ASSERT_GT(grid_pitch, 0);
+  ASSERT_GT(grid_width, 0);
+  ASSERT_NE(background_color, grid_color);
+
+  bmp->setConfig(SkBitmap::kARGB_8888_Config, w, h);
+  bmp->allocPixels();
+
+  for (int y = 0; y < h; ++y) {
+    bool y_on_grid = ((y % grid_pitch) < grid_width);
+
+    for (int x = 0; x < w; ++x) {
+      bool on_grid = (y_on_grid || ((x % grid_pitch) < grid_width));
+
+      *bmp->getAddr32(x, y) = (on_grid ? grid_color : background_color);
+    }
+  }
+}
+
+// Draws a checkerboard pattern into the w x h bitmap passed in.
+// Each rectangle is rect_w in width, rect_h in height.
+// The colors alternate between color1 and color2, color1 being used
+// in the rectangle at the top left corner.
+void DrawCheckerToBitmap(int w, int h,
+                         SkColor color1, SkColor color2,
+                         int rect_w, int rect_h,
+                         SkBitmap* bmp) {
+  ASSERT_GT(rect_w, 0);
+  ASSERT_GT(rect_h, 0);
+  ASSERT_NE(color1, color2);
+
+  bmp->setConfig(SkBitmap::kARGB_8888_Config, w, h);
+  bmp->allocPixels();
+
+  for (int y = 0; y < h; ++y) {
+    bool y_bit = (((y / rect_h) & 0x1) == 0);
+
+    for (int x = 0; x < w; ++x) {
+      bool x_bit = (((x / rect_w) & 0x1) == 0);
+
+      bool use_color2 = (x_bit != y_bit);  // xor
+
+      *bmp->getAddr32(x, y) = (use_color2 ? color2 : color1);
+    }
+  }
+}
+
+// DEBUG_BITMAP_GENERATION (0 or 1) controls whether the routines
+// to save the test bitmaps are present. By default the test just fails
+// without reading/writing files but it is then convenient to have
+// a simple way to make the failing tests write out the input/output images
+// to check them visually.
+#define DEBUG_BITMAP_GENERATION (0)
+
+#if DEBUG_BITMAP_GENERATION
+void SaveBitmapToPNG(const SkBitmap& bmp, const char* path) {
+  SkAutoLockPixels lock(bmp);
+  std::vector<unsigned char> png;
+  gfx::PNGCodec::ColorFormat color_format = gfx::PNGCodec::FORMAT_RGBA;
+  if (!gfx::PNGCodec::Encode(
+          reinterpret_cast<const unsigned char*>(bmp.getPixels()),
+          color_format, bmp.width(), bmp.height(),
+          static_cast<int>(bmp.rowBytes()),
+          false, &png)) {
+    FAIL() << "Failed to encode image";
+  }
+
+  const FilePath fpath(path);
+  const int num_written =
+      file_util::WriteFile(fpath, reinterpret_cast<const char*>(&png[0]),
+                           png.size());
+  if (num_written != static_cast<int>(png.size())) {
+    FAIL() << "Failed to write dest \"" << path << '"';
+  }
+}
+#endif  // #if DEBUG_BITMAP_GENERATION
+
+void CheckResampleToSame(skia::ImageOperations::ResizeMethod method) {
+  // Make our source bitmap.
+  const int src_w = 16, src_h = 34;
+  SkBitmap src;
+  FillDataToBitmap(src_w, src_h, &src);
+
+  // Do a resize of the full bitmap to the same size. The lanczos filter is good
+  // enough that we should get exactly the same image for output.
+  SkBitmap results = skia::ImageOperations::Resize(src, method, src_w, src_h);
+  ASSERT_EQ(src_w, results.width());
+  ASSERT_EQ(src_h, results.height());
+
+  SkAutoLockPixels src_lock(src);
+  SkAutoLockPixels results_lock(results);
+  for (int y = 0; y < src_h; y++) {
+    for (int x = 0; x < src_w; x++) {
+      EXPECT_EQ(*src.getAddr32(x, y), *results.getAddr32(x, y));
+    }
+  }
+}
+
+// Types defined outside of the ResizeShouldAverageColors test to allow
+// use of the arraysize() macro.
+//
+// 'max_color_distance_override' is used in a max() call together with
+// the value of 'max_color_distance' defined in a TestedPixel instance.
+// Hence a value of 0.0 in 'max_color_distance_override' means
+// "use the pixel-specific value" and larger values can be used to allow
+// worse computation errors than provided in a TestedPixel instance.
+struct TestedResizeMethod {
+  skia::ImageOperations::ResizeMethod method;
+  const char* name;
+  float max_color_distance_override;
+};
+
+struct TestedPixel {
+  int         x;
+  int         y;
+  float       max_color_distance;
+  const char* name;
+};
+
+// Helper function used by the test "ResizeShouldAverageColors" below.
+// Note that ASSERT_EQ does a "return;" on failure, hence we can't have
+// a "bool" return value to reflect success. Hence "all_pixels_pass"
+void CheckResizeMethodShouldAverageGrid(
+    const SkBitmap& src,
+    const TestedResizeMethod& tested_method,
+    int dest_w, int dest_h, SkColor average_color,
+    bool* method_passed) {
+  *method_passed = false;
+
+  // TODO(evannier): The math inside image_operations.cc is incorrect is off
+  // by half a pixel. As a result, the calculated distances become extremely
+  // large. Once the fix is in to correct this half pixel issue, most of these
+  // values can become a lot tighter.
+  const TestedPixel tested_pixels[] = {
+    // Corners
+    { 0,          0,           59.0f, "Top left corner"  },
+    { 0,          dest_h - 1,  2.3f, "Bottom left corner" },
+    { dest_w - 1, 0,           7.1f, "Top right corner" },
+    { dest_w - 1, dest_h - 1,  2.3f, "Bottom right corner" },
+    // Middle points of each side
+    { dest_w / 2, 0,           1.0f, "Top middle" },
+    { dest_w / 2, dest_h - 1,  1.0f, "Bottom middle" },
+    { 0,          dest_h / 2,  1.0f, "Left middle" },
+    { dest_w - 1, dest_h / 2,  1.0f, "Right middle" },
+    // Center
+    { dest_w / 2, dest_h / 2,  1.0f, "Center" }
+  };
+
+  // Resize the src
+  const skia::ImageOperations::ResizeMethod method = tested_method.method;
+
+  SkBitmap dest = skia::ImageOperations::Resize(src, method, dest_w, dest_h);
+  ASSERT_EQ(dest_w, dest.width());
+  ASSERT_EQ(dest_h, dest.height());
+
+  // Check that pixels match the expected average.
+  float max_observed_distance = 0.0f;
+  bool all_pixels_ok = true;
+
+  SkAutoLockPixels dest_lock(dest);
+
+  for (size_t pixel_index = 0;
+       pixel_index < arraysize(tested_pixels);
+       ++pixel_index) {
+    const TestedPixel& tested_pixel = tested_pixels[pixel_index];
+
+    const int   x = tested_pixel.x;
+    const int   y = tested_pixel.y;
+    const float max_allowed_distance =
+        std::max(tested_pixel.max_color_distance,
+                 tested_method.max_color_distance_override);
+
+    const SkColor actual_color = *dest.getAddr32(x, y);
+
+    // Check that the pixels away from the border region are very close
+    // to the expected average color
+    float distance = ColorsEuclidianDistance(average_color, actual_color);
+
+    EXPECT_LE(distance, max_allowed_distance)
+        << "Resizing method: " << tested_method.name
+        << ", pixel tested: " << tested_pixel.name
+        << "(" << x << ", " << y << ")"
+        << std::hex << std::showbase
+        << ", expected (avg) hex: " <<  average_color
+        << ", actual hex: " << actual_color;
+
+    if (distance > max_allowed_distance) {
+      all_pixels_ok = false;
+    }
+    if (distance > max_observed_distance) {
+      max_observed_distance = distance;
+    }
+  }
+
+  if (!all_pixels_ok) {
+    ADD_FAILURE() << "Maximum observed color distance for method "
+                  << tested_method.name << ": " << max_observed_distance;
+
+#if DEBUG_BITMAP_GENERATION
+    char path[128];
+    base::snprintf(path, sizeof(path),
+                   "/tmp/ResizeShouldAverageColors_%s_dest.png",
+                   tested_method.name);
+    SaveBitmapToPNG(dest, path);
+#endif  // #if DEBUG_BITMAP_GENERATION
+  }
+
+  *method_passed = all_pixels_ok;
+}
+
+
 }  // namespace
 
+// Helper tests that saves bitmaps to PNGs in /tmp/ to visually check
+// that the bitmap generation functions work as expected.
+// Those tests are not enabled by default as verification is done
+// manually/visually, however it is convenient to leave the functions
+// in place.
+#if 0 && DEBUG_BITMAP_GENERATION
+TEST(ImageOperations, GenerateGradientBitmap) {
+  // Make our source bitmap.
+  const int src_w = 640, src_h = 480;
+  SkBitmap src;
+  FillDataToBitmap(src_w, src_h, &src);
+
+  SaveBitmapToPNG(src, "/tmp/gradient_640x480.png");
+}
+
+TEST(ImageOperations, GenerateGridBitmap) {
+  const int src_w = 640, src_h = 480, src_grid_pitch = 10, src_grid_width = 4;
+  const SkColor grid_color = SK_ColorRED, background_color = SK_ColorBLUE;
+  SkBitmap src;
+  DrawGridToBitmap(src_w, src_h,
+                   background_color, grid_color,
+                   src_grid_pitch, src_grid_width,
+                   &src);
+
+  SaveBitmapToPNG(src, "/tmp/grid_640x408_10_4_red_blue.png");
+}
+
+TEST(ImageOperations, GenerateCheckerBitmap) {
+  const int src_w = 640, src_h = 480, rect_w = 10, rect_h = 4;
+  const SkColor color1 = SK_ColorRED, color2 = SK_ColorBLUE;
+  SkBitmap src;
+  DrawCheckerToBitmap(src_w, src_h, color1, color2, rect_w, rect_h, &src);
+
+  SaveBitmapToPNG(src, "/tmp/checker_640x408_10_4_red_blue.png");
+}
+#endif  // #if ... && DEBUG_BITMAP_GENERATION
+
 // Makes the bitmap 50% the size as the original using a box filter. This is
 // an easy operation that we can check the results for manually.
 TEST(ImageOperations, Halve) {
@@ -75,15 +400,37 @@
   SkAutoLockPixels lock(actual_results);
   for (int y = 0; y < actual_results.height(); y++) {
     for (int x = 0; x < actual_results.width(); x++) {
+      // Note that those expressions take into account the "half-pixel"
+      // offset that comes into play due to considering the coordinates
+      // of the center of the pixels. So x * 2 is a simplification
+      // of ((x+0.5) * 2 - 1) and (x * 2 + 1) is really (x + 0.5) * 2.
+      // TODO(evannier): for now these stay broken because of the half pixel
+      // issue mentioned inside image_operations.cc. The code should read:
+      // int first_x = x * 2;
+      // int last_x = std::min(src_w - 1, x * 2 + 1);
+
+      // int first_y = y * 2;
+      // int last_y = std::min(src_h - 1, y * 2 + 1);
       int first_x = std::max(0, x * 2 - 1);
       int last_x = std::min(src_w - 1, x * 2);
 
       int first_y = std::max(0, y * 2 - 1);
       int last_y = std::min(src_h - 1, y * 2);
 
-      uint32_t expected_color = AveragePixel(src,
-                                             first_x, last_x, first_y, last_y);
-      EXPECT_TRUE(ColorsClose(expected_color, *actual_results.getAddr32(x, y)));
+      const uint32_t expected_color = AveragePixel(src,
+                                                   first_x, last_x,
+                                                   first_y, last_y);
+      const uint32_t actual_color = *actual_results.getAddr32(x, y);
+      const bool close = ColorsClose(expected_color, actual_color);
+      EXPECT_TRUE(close);
+      if (!close) {
+        char str[128];
+        base::snprintf(str, sizeof(str),
+                       "exp[%d,%d] = %08X, actual[%d,%d] = %08X",
+                       x, y, expected_color, x, y, actual_color);
+        ADD_FAILURE() << str;
+        PrintPixel(src, first_x, last_x, first_y, last_y);
+      }
     }
   }
 }
@@ -122,25 +469,175 @@
   }
 }
 
-// Resamples an iamge to the same image, it should give almost the same result.
-TEST(ImageOperations, ResampleToSame) {
+// Resamples an image to the same image, it should give the same result.
+TEST(ImageOperations, ResampleToSameHamming1) {
+  CheckResampleToSame(skia::ImageOperations::RESIZE_HAMMING1);
+}
+
+TEST(ImageOperations, ResampleToSameLanczos2) {
+  CheckResampleToSame(skia::ImageOperations::RESIZE_LANCZOS2);
+}
+
+TEST(ImageOperations, ResampleToSameLanczos3) {
+  CheckResampleToSame(skia::ImageOperations::RESIZE_LANCZOS3);
+}
+
+// Check that all Good/Better/Best, Box, Lanczos2 and Lanczos3 generate purple
+// when resizing a 4x8 red/blue checker pattern by 1/16x1/16.
+TEST(ImageOperations, ResizeShouldAverageColors) {
   // Make our source bitmap.
-  int src_w = 16, src_h = 34;
+  const int src_w = 640, src_h = 480, checker_rect_w = 4, checker_rect_h = 8;
+  const SkColor checker_color1 = SK_ColorRED, checker_color2 = SK_ColorBLUE;
+
+  const int dest_w = src_w / (4 * checker_rect_w);
+  const int dest_h = src_h / (2 * checker_rect_h);
+
+  // Compute the expected (average) color
+  const SkColor colors[] = { checker_color1, checker_color2 };
+  const SkColor average_color = AveragePixel(colors, arraysize(colors));
+
+  // RESIZE_SUBPIXEL is only supported on Linux/non-GTV platforms.
+  static const TestedResizeMethod tested_methods[] = {
+    { skia::ImageOperations::RESIZE_GOOD,     "GOOD",     0.0f },
+    { skia::ImageOperations::RESIZE_BETTER,   "BETTER",   0.0f },
+    { skia::ImageOperations::RESIZE_BEST,     "BEST",     0.0f },
+    { skia::ImageOperations::RESIZE_BOX,      "BOX",      0.0f },
+    { skia::ImageOperations::RESIZE_HAMMING1, "HAMMING1", 0.0f },
+    { skia::ImageOperations::RESIZE_LANCZOS2, "LANCZOS2", 0.0f },
+    { skia::ImageOperations::RESIZE_LANCZOS3, "LANCZOS3", 0.0f },
+#if defined(OS_LINUX) && !defined(GTV)
+    // SUBPIXEL has slightly worse performance than the other filters:
+    //   6.324  Bottom left/right corners
+    //   5.099  Top left/right corners
+    //   2.828  Bottom middle
+    //   1.414  Top/Left/Right middle, center
+    //
+    // This is expected since, in order to judge RESIZE_SUBPIXEL accurately,
+    // we'd need to compute distances for each sub-pixel, and potentially
+    // tweak the test parameters so that expectations were realistic when
+    // looking at sub-pixels in isolation.
+    //
+    // Rather than going to these lengths, we added the "max_distance_override"
+    // field in TestedResizeMethod, intended for RESIZE_SUBPIXEL. It allows
+    // us to to enable its testing without having to lower the success criteria
+    // for the other methods. This procedure is  distateful but defining
+    // a distance limit for each tested pixel for each method was judged to add
+    // unneeded complexity.
+    { skia::ImageOperations::RESIZE_SUBPIXEL, "SUBPIXEL", 6.4f },
+#endif
+  };
+
+  // Create our source bitmap.
   SkBitmap src;
-  FillDataToBitmap(src_w, src_h, &src);
+  DrawCheckerToBitmap(src_w, src_h,
+                      checker_color1, checker_color2,
+                      checker_rect_w, checker_rect_h,
+                      &src);
 
-  // Do a resize of the full bitmap to the same size. The lanczos filter is good
-  // enough that we should get exactly the same image for output.
-  SkBitmap results = skia::ImageOperations::Resize(
-      src, skia::ImageOperations::RESIZE_LANCZOS3, src_w, src_h);
-  ASSERT_EQ(src_w, results.width());
-  ASSERT_EQ(src_h, results.height());
+  // For each method, downscale by 16 in each dimension,
+  // and check each tested pixel against the expected average color.
+  bool all_methods_ok = true;
 
-  SkAutoLockPixels src_lock(src);
-  SkAutoLockPixels results_lock(results);
-  for (int y = 0; y < src_h; y++) {
-    for (int x = 0; x < src_w; x++) {
-      EXPECT_EQ(*src.getAddr32(x, y), *results.getAddr32(x, y));
+  for (size_t method_index = 0;
+       method_index < arraysize(tested_methods);
+       ++method_index) {
+    bool pass = true;
+    CheckResizeMethodShouldAverageGrid(src,
+                                       tested_methods[method_index],
+                                       dest_w, dest_h, average_color,
+                                       &pass);
+    if (!pass) {
+      all_methods_ok = false;
     }
   }
+
+#if DEBUG_BITMAP_GENERATION
+  if (!all_methods_ok) {
+    SaveBitmapToPNG(src, "/tmp/ResizeShouldAverageColors_src.png");
+  }
+#endif  // #if DEBUG_BITMAP_GENERATION
 }
+
+
+// Check that Lanczos2 and Lanczos3 thumbnails produce similar results
+TEST(ImageOperations, CompareLanczosMethods) {
+  const int src_w = 640, src_h = 480, src_grid_pitch = 8, src_grid_width = 4;
+
+  const int dest_w = src_w / 4;
+  const int dest_h = src_h / 4;
+
+  // 5.0f is the maximum distance we see in this test given the current
+  // parameters. The value is very ad-hoc and the parameters of the scaling
+  // were picked to produce a small value. So this test is very much about
+  // revealing egregious regression rather than doing a good job at checking
+  // the math behind the filters.
+  // TODO(evannier): because of the half pixel error mentioned inside
+  // image_operations.cc, this distance is much larger than it should be.
+  // This should read:
+  // const float max_color_distance = 5.0f;
+  const float max_color_distance = 12.1f;
+
+  // Make our source bitmap.
+  SkColor grid_color = SK_ColorRED, background_color = SK_ColorBLUE;
+  SkBitmap src;
+  DrawGridToBitmap(src_w, src_h,
+                   background_color, grid_color,
+                   src_grid_pitch, src_grid_width,
+                   &src);
+
+  // Resize the src using both methods.
+  SkBitmap dest_l2 = skia::ImageOperations::Resize(
+      src,
+      skia::ImageOperations::RESIZE_LANCZOS2,
+      dest_w, dest_h);
+  ASSERT_EQ(dest_w, dest_l2.width());
+  ASSERT_EQ(dest_h, dest_l2.height());
+
+  SkBitmap dest_l3 = skia::ImageOperations::Resize(
+      src,
+      skia::ImageOperations::RESIZE_LANCZOS3,
+      dest_w, dest_h);
+  ASSERT_EQ(dest_w, dest_l3.width());
+  ASSERT_EQ(dest_h, dest_l3.height());
+
+  // Compare the pixels produced by both methods.
+  float max_observed_distance = 0.0f;
+  bool all_pixels_ok = true;
+
+  SkAutoLockPixels l2_lock(dest_l2);
+  SkAutoLockPixels l3_lock(dest_l3);
+  for (int y = 0; y < dest_h; ++y) {
+    for (int x = 0; x < dest_w; ++x) {
+      const SkColor color_lanczos2 = *dest_l2.getAddr32(x, y);
+      const SkColor color_lanczos3 = *dest_l3.getAddr32(x, y);
+
+      float distance = ColorsEuclidianDistance(color_lanczos2, color_lanczos3);
+
+      EXPECT_LE(distance, max_color_distance)
+          << "pixel tested: (" << x << ", " << y
+          << std::hex << std::showbase
+          << "), lanczos2 hex: " << color_lanczos2
+          << ", lanczos3 hex: " << color_lanczos3
+          << std::setprecision(2)
+          << ", distance: " << distance;
+
+      if (distance > max_color_distance) {
+        all_pixels_ok = false;
+      }
+      if (distance > max_observed_distance) {
+        max_observed_distance = distance;
+      }
+    }
+  }
+
+  if (!all_pixels_ok) {
+    ADD_FAILURE() << "Maximum observed color distance: "
+                  << max_observed_distance;
+
+#if DEBUG_BITMAP_GENERATION
+    SaveBitmapToPNG(src, "/tmp/CompareLanczosMethods_source.png");
+    SaveBitmapToPNG(dest_l2, "/tmp/CompareLanczosMethods_lanczos2.png");
+    SaveBitmapToPNG(dest_l3, "/tmp/CompareLanczosMethods_lanczos3.png");
+#endif  // #if DEBUG_BITMAP_GENERATION
+  }
+}