Complete (but inefficient) implementation of the image retargetting method.

ui/gfx/content_analysis_unittest.cc

Index: ui/gfx/content_analysis_unittest.cc
diff --git a/ui/gfx/content_analysis_unittest.cc b/ui/gfx/content_analysis_unittest.cc
new file mode 100644
index 0000000000000000000000000000000000000000..bd904b433d6181c2301445cb580f3310aaaecfba
--- /dev/null
+++ b/ui/gfx/content_analysis_unittest.cc
@@ -0,0 +1,523 @@
+// Copyright (c) 2012 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 "ui/gfx/content_analysis.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cstdlib>
+#include <fstream>
+#include <iostream>
+#include <limits>
+#include <numeric>
+#include <vector>
+
+#include "base/file_util.h"
+#include "base/files/file_path.h"
+#include "base/memory/scoped_ptr.h"
+#include "testing/gtest/include/gtest/gtest.h"
+#include "third_party/skia/include/core/SkBitmap.h"
+#include "third_party/skia/include/core/SkColor.h"
+#include "ui/gfx/canvas.h"
+#include "ui/gfx/color_analysis.h"
+#include "ui/gfx/color_utils.h"
+#include "ui/gfx/image/image.h"
+#include "ui/gfx/rect.h"
+#include "ui/gfx/size.h"
+
+namespace {
+
+#ifndef M_PI
+#define M_PI 3.14159265358979323846
+#endif
+
+void DumpBitmapToFile(const SkBitmap& bitmap) {

[Alexei Svitkine (slow), 2013/04/09 18:02:23]

If you're not using these in your tests, please remove them and the associated
std header files you're including.

[motek., 2013/04/11 16:18:38]

Done.

+  SkAutoLockPixels bitmap_lock(bitmap);
+  scoped_refptr<base::RefCountedMemory> png_bytes =
+      gfx::Image::CreateFrom1xBitmap(bitmap).As1xPNGBytes();
+  base::FilePath file_path;
+  if (file_util::CreateTemporaryFile(&file_path)) {
+    file_util::WriteFile(file_path,
+                         reinterpret_cast<const char*>(png_bytes->front()),
+                         png_bytes->size());
+    DLOG(INFO) << "Image content writen out to " << file_path.value();
+  } else {
+    NOTREACHED();
+  }
+}
+
+void DumpBitmapBytesToFile(const SkBitmap& bitmap) {
+  SkAutoLockPixels bitmap_lock(bitmap);
+  base::FilePath file_path;
+  if (file_util::CreateTemporaryFile(&file_path)) {
+    file_util::WriteFile(file_path,
+                         static_cast<const char*>(bitmap.getPixels()),
+                         bitmap.getSafeSize());
+    DLOG(INFO) << "Image content writen out to " << file_path.value();
+  } else {
+    NOTREACHED();
+  }
+}
+
+template<class InputIterator>
+void DumpVectorToFile(InputIterator first, InputIterator last) {
+  base::FilePath file_path;
+  if (file_util::CreateTemporaryFile(&file_path)) {
+    std::ofstream outfile;
+    outfile.open(file_path.value().c_str());
+    for (; first < last; ++first) {
+      outfile << *first;
+      outfile << "\n";
+    }
+    outfile.close();
+    DLOG(INFO) << "Vector content writen out to " << file_path.value();
+  }
+}
+
+unsigned long ImagePixelSum(const SkBitmap& bitmap, const gfx::Rect& rect) {
+  // Get the sum of pixel values in the rectangle. Applicable only to
+  // monochrome bitmaps.
+  DCHECK_EQ(SkBitmap::kA8_Config, bitmap.config());
+  unsigned long total = 0;
+  for (int r = rect.y(); r < rect.bottom(); ++r) {
+    const uint8* row_data = static_cast<const uint8*>(
+        bitmap.getPixels()) + r * bitmap.rowBytes();
+    for (int c = rect.x(); c < rect.right(); ++c)
+      total += row_data[c];
+  }
+
+  return total;
+}
+
+bool CompareImageFragments(const SkBitmap& bitmap_left,
+                           const SkBitmap& bitmap_right,
+                           const gfx::Size& comparison_area,
+                           const gfx::Point& origin_left,
+                           const gfx::Point& origin_right) {
+  SkAutoLockPixels left_lock(bitmap_left);
+  SkAutoLockPixels right_lock(bitmap_right);
+  for (int r = 0; r < comparison_area.height(); ++r) {
+    for (int c = 0; c < comparison_area.width(); ++c) {
+      SkColor clr_left = bitmap_left.getColor(origin_left.x() + c,
+                                              origin_left.y() + r);
+      SkColor clr_right = bitmap_right.getColor(origin_right.x() + c,
+                                                origin_right.y() + r);
+      if (clr_left != clr_right)
+        return false;
+    }
+  }
+
+  return true;
+}
+
+}

[Alexei Svitkine (slow), 2013/04/09 18:02:23]

Add "  // namespace"

[motek., 2013/04/11 16:18:38]

Done.

+
+class ContentAnalysisTest : public testing::Test {
+};
+
+TEST_F(ContentAnalysisTest, ApplyGradientMagnitudeOnImpulse) {
+  gfx::Canvas canvas(gfx::Size(800, 600), ui::SCALE_FACTOR_100P, true);
+
+  // The image consists of vertical non-overlapping stripes 100 pixels wide.
+  canvas.FillRect(gfx::Rect(0, 0, 800, 600), SkColorSetARGB(0, 10, 10, 10));
+  canvas.FillRect(gfx::Rect(400, 300, 1, 1), SkColorSetRGB(255, 255, 255));
+
+  SkBitmap source =
+      skia::GetTopDevice(*canvas.sk_canvas())->accessBitmap(false);
+
+  SkBitmap reduced_color;
+  reduced_color.setConfig(
+      SkBitmap::kA8_Config, source.width(), source.height());
+  reduced_color.allocPixels();
+
+  gfx::Vector3dF transform(0.299f, 0.587f, 0.114f);
+  EXPECT_TRUE(color_utils::ApplyColorReduction(
+      source, transform, true, &reduced_color));
+
+  float sigma = 2.5f;
+  color_utils::ApplyGaussianGradientMagnitudeFilter(&reduced_color, sigma);
+
+  // Expect everything to be within 8 * sigma.
+  int tail_length = static_cast<int>(8.0f * sigma + 0.5f);
+  gfx::Rect echo_rect(399 - tail_length, 299 - tail_length,
+                      2 * tail_length + 1, 2 * tail_length + 1);
+  unsigned long data_sum = ImagePixelSum(reduced_color, echo_rect);
+  unsigned long all_sum = ImagePixelSum(reduced_color, gfx::Rect(800, 600));
+  EXPECT_GT(data_sum, 0U);
+  EXPECT_EQ(data_sum, all_sum);
+
+  sigma = 5.0f;
+  color_utils::ApplyGaussianGradientMagnitudeFilter(&reduced_color, sigma);
+
+  // Expect everything to be within 8 * sigma.
+  tail_length = static_cast<int>(8.0f * sigma + 0.5f);
+  echo_rect = gfx::Rect(399 - tail_length, 299 - tail_length,
+                        2 * tail_length + 1, 2 * tail_length + 1);
+  data_sum = ImagePixelSum(reduced_color, echo_rect);
+  all_sum = ImagePixelSum(reduced_color, gfx::Rect(800, 600));
+  EXPECT_GT(data_sum, 0U);
+  EXPECT_EQ(data_sum, all_sum);
+}
+
+TEST_F(ContentAnalysisTest, ApplyGradientMagnitudeOnFrame) {
+  gfx::Canvas canvas(gfx::Size(800, 600), ui::SCALE_FACTOR_100P, true);
+
+  // The image consists of a single white block in the centre.
+  gfx::Rect draw_rect(300, 200, 200, 200);
+  canvas.FillRect(gfx::Rect(0, 0, 800, 600), SkColorSetARGB(0, 0, 0, 0));
+  canvas.DrawRect(draw_rect, SkColorSetRGB(255, 255, 255));
+
+  SkBitmap source =
+      skia::GetTopDevice(*canvas.sk_canvas())->accessBitmap(false);
+
+  SkBitmap reduced_color;
+  reduced_color.setConfig(
+      SkBitmap::kA8_Config, source.width(), source.height());
+  reduced_color.allocPixels();
+
+  gfx::Vector3dF transform(0.299f, 0.587f, 0.114f);
+  EXPECT_TRUE(color_utils::ApplyColorReduction(
+      source, transform, true, &reduced_color));
+
+  float sigma = 2.5f;
+  color_utils::ApplyGaussianGradientMagnitudeFilter(&reduced_color, sigma);
+
+  int tail_length = static_cast<int>(8.0f * sigma + 0.5f);
+  gfx::Rect outer_rect(draw_rect.x() - tail_length,
+                       draw_rect.y() - tail_length,
+                       draw_rect.width() + 2 * tail_length,
+                       draw_rect.height() + 2 * tail_length);
+  gfx::Rect inner_rect(draw_rect.x() + tail_length,
+                       draw_rect.y() + tail_length,
+                       draw_rect.width() - 2 * tail_length,
+                       draw_rect.height() - 2 * tail_length);
+  unsigned long data_sum = ImagePixelSum(reduced_color, outer_rect);
+  unsigned long all_sum = ImagePixelSum(reduced_color, gfx::Rect(800, 600));
+  EXPECT_GT(data_sum, 0U);
+  EXPECT_EQ(data_sum, all_sum);
+  EXPECT_EQ(ImagePixelSum(reduced_color, inner_rect), 0U);
+}
+
+TEST_F(ContentAnalysisTest, ExtractImageProfileInformation) {
+  gfx::Canvas canvas(gfx::Size(800, 600), ui::SCALE_FACTOR_100P, true);
+
+  // The image consists of a white frame drawn in the centre.
+  gfx::Rect draw_rect(100, 100, 200, 100);
+  gfx::Rect image_rect(0, 0, 800, 600);
+  canvas.FillRect(image_rect, SkColorSetARGB(0, 0, 0, 0));
+  canvas.DrawRect(draw_rect, SkColorSetRGB(255, 255, 255));
+
+  SkBitmap source =
+      skia::GetTopDevice(*canvas.sk_canvas())->accessBitmap(false);
+  SkBitmap reduced_color;
+  reduced_color.setConfig(
+      SkBitmap::kA8_Config, source.width(), source.height());
+  reduced_color.allocPixels();
+
+  gfx::Vector3dF transform(1, 0, 0);
+  EXPECT_TRUE(color_utils::ApplyColorReduction(
+      source, transform, true, &reduced_color));
+  std::vector<float> column_profile;
+  std::vector<float> row_profile;
+  color_utils::ExtractImageProfileInformation(reduced_color,
+                                              image_rect,
+                                              gfx::Size(),
+                                              false,
+                                              &row_profile,
+                                              &column_profile);
+  EXPECT_EQ(0, std::accumulate(column_profile.begin(),
+                               column_profile.begin() + draw_rect.x() - 1,
+                               0));
+  EXPECT_EQ(column_profile[draw_rect.x()], 255U * (draw_rect.height() + 1));
+  EXPECT_EQ(2 * 255 * (draw_rect.width() - 2),
+            std::accumulate(column_profile.begin() + draw_rect.x() + 1,
+                            column_profile.begin() + draw_rect.right() - 1,
+                            0));
+
+  EXPECT_EQ(0, std::accumulate(row_profile.begin(),
+                               row_profile.begin() + draw_rect.y() - 1,
+                               0));
+  EXPECT_EQ(row_profile[draw_rect.y()], 255U * (draw_rect.width() + 1));
+  EXPECT_EQ(2 * 255 * (draw_rect.height() - 2),
+            std::accumulate(row_profile.begin() + draw_rect.y() + 1,
+                            row_profile.begin() + draw_rect.bottom() - 1,
+                            0));
+
+  gfx::Rect test_rect(150, 80, 400, 100);
+  color_utils::ExtractImageProfileInformation(reduced_color,
+                                              test_rect,
+                                              gfx::Size(),
+                                              false,
+                                              &row_profile,
+                                              &column_profile);
+
+  // Some overlap with the drawn rectagle. If you work it out on a piece of
+  // paper, sums should be as follows.
+  EXPECT_EQ(255 * (test_rect.bottom() - draw_rect.y()) +
+            255 * (draw_rect.right() - test_rect.x()),
+            std::accumulate(row_profile.begin(), row_profile.end(), 0));
+  EXPECT_EQ(255 * (test_rect.bottom() - draw_rect.y()) +
+            255 * (draw_rect.right() - test_rect.x()),
+            std::accumulate(column_profile.begin(), column_profile.end(), 0));
+}
+
+TEST_F(ContentAnalysisTest, ExtractImageProfileInformationWithClosing) {
+  gfx::Canvas canvas(gfx::Size(800, 600), ui::SCALE_FACTOR_100P, true);
+
+  // The image consists of a two white frames drawn side by side, with a
+  // single-pixel vertical gap in between.
+  gfx::Rect image_rect(0, 0, 800, 600);
+  canvas.FillRect(image_rect, SkColorSetARGB(0, 0, 0, 0));
+  canvas.DrawRect(gfx::Rect(300, 250, 99, 100), SkColorSetRGB(255, 255, 255));
+  canvas.DrawRect(gfx::Rect(401, 250, 99, 100), SkColorSetRGB(255, 255, 255));
+
+  SkBitmap source =
+      skia::GetTopDevice(*canvas.sk_canvas())->accessBitmap(false);
+  SkBitmap reduced_color;
+  reduced_color.setConfig(
+      SkBitmap::kA8_Config, source.width(), source.height());
+  reduced_color.allocPixels();
+
+  gfx::Vector3dF transform(1, 0, 0);
+  EXPECT_TRUE(color_utils::ApplyColorReduction(
+      source, transform, true, &reduced_color));
+  std::vector<float> column_profile;
+  std::vector<float> row_profile;
+
+  color_utils::ExtractImageProfileInformation(reduced_color,
+                                              image_rect,
+                                              gfx::Size(),
+                                              true,
+                                              &row_profile,
+                                              &column_profile);
+  // Column profiles should have two spikes in the middle, with a single
+  // 0-valued value between them.
+  EXPECT_GT(column_profile[398], 0.0f);
+  EXPECT_GT(column_profile[399], column_profile[398]);
+  EXPECT_GT(column_profile[402], 0.0f);
+  EXPECT_GT(column_profile[401], column_profile[402]);
+  EXPECT_EQ(column_profile[401], column_profile[399]);
+  EXPECT_EQ(column_profile[402], column_profile[398]);
+  EXPECT_EQ(column_profile[400], 0.0f);
+  EXPECT_EQ(column_profile[299], 0.0f);
+  EXPECT_EQ(column_profile[502], 0.0f);
+
+  // Now the same with closing applied. The space in the middle will be closed.
+  color_utils::ExtractImageProfileInformation(reduced_color,
+                                              image_rect,
+                                              gfx::Size(200, 100),
+                                              true,
+                                              &row_profile,
+                                              &column_profile);
+  EXPECT_GT(column_profile[398], 0);
+  EXPECT_GT(column_profile[400], 0);
+  EXPECT_GT(column_profile[402], 0);
+  EXPECT_EQ(column_profile[299], 0);
+  EXPECT_EQ(column_profile[502], 0);
+  EXPECT_EQ(column_profile[399], column_profile[401]);
+  EXPECT_EQ(column_profile[398], column_profile[402]);
+}
+
+TEST_F(ContentAnalysisTest, AutoSegmentPeaks) {
+  std::vector<float> profile_info;
+
+  EXPECT_EQ(color_utils::AutoSegmentPeaks(profile_info),
+            std::numeric_limits<float>::max());
+  profile_info.resize(1000, 1.0f);
+  EXPECT_EQ(color_utils::AutoSegmentPeaks(profile_info), 1.0f);
+  std::srand(42);
+  std::generate(profile_info.begin(), profile_info.end(), std::rand);
+  float threshold = color_utils::AutoSegmentPeaks(profile_info);
+  EXPECT_GT(threshold, 0);  // Not much to expect.
+
+  // There should be roughly 50% above and below the threshold.
+  // Random is not really random thanks to srand, so we can sort-of compare.
+  int above_count = std::count_if(
+      profile_info.begin(),
+      profile_info.end(),
+      std::bind2nd(std::greater<float>(), threshold));
+  EXPECT_GT(above_count, 475);  // Not much to expect.
+  EXPECT_LT(above_count, 525);
+
+  for (unsigned i = 0; i < profile_info.size(); ++i) {
+    float y = std::sin(M_PI * i / 250.0f);
+    profile_info[i] = y > 0 ? y : 0;
+  }
+  threshold = color_utils::AutoSegmentPeaks(profile_info);
+
+  above_count = std::count_if(
+      profile_info.begin(),
+      profile_info.end(),
+      std::bind2nd(std::greater<float>(), threshold));
+  EXPECT_LT(above_count, 500);  // Negative y expected to fall below threshold.
+
+  // Expect two peaks around between 0 and 250 and 500 and 750.
+  std::vector<bool> thresholded_values(profile_info.size(), false);
+  std::transform(profile_info.begin(),
+                 profile_info.end(),
+                 thresholded_values.begin(),
+                 std::bind2nd(std::greater<float>(), threshold));
+  EXPECT_TRUE(thresholded_values[125]);
+  EXPECT_TRUE(thresholded_values[625]);
+  int transitions = 0;
+  for (unsigned i = 1; i < thresholded_values.size(); ++i) {
+    if (thresholded_values[i] != thresholded_values[i-1])
+      transitions++;
+  }
+  EXPECT_EQ(transitions, 4);  // We have two contiguous peaks. Good going!
+}
+
+TEST_F(ContentAnalysisTest, ComputeDecimatedImage) {
+  gfx::Size image_size(1600, 1200);
+  gfx::Canvas canvas(image_size, ui::SCALE_FACTOR_100P, true);
+
+  // Make some content we will later want to keep.
+  canvas.FillRect(gfx::Rect(100, 200, 100, 100), SkColorSetARGB(0, 125, 0, 0));
+  canvas.FillRect(gfx::Rect(300, 200, 100, 100), SkColorSetARGB(0, 0, 200, 0));
+  canvas.FillRect(gfx::Rect(500, 200, 100, 100), SkColorSetARGB(0, 0, 0, 225));
+  canvas.FillRect(gfx::Rect(100, 400, 600, 100),
+                  SkColorSetARGB(0, 125, 200, 225));
+
+  std::vector<bool> rows(image_size.height(), false);
+  std::fill_n(rows.begin() + 200, 100, true);
+  std::fill_n(rows.begin() + 400, 100, true);
+
+  std::vector<bool> columns(image_size.width(), false);
+  std::fill_n(columns.begin() + 100, 100, true);
+  std::fill_n(columns.begin() + 300, 100, true);
+  std::fill_n(columns.begin() + 500, 100, true);
+
+  SkBitmap source =
+      skia::GetTopDevice(*canvas.sk_canvas())->accessBitmap(false);
+  scoped_ptr<SkBitmap> result(
+      color_utils::ComputeDecimatedImage(source, rows, columns));
+  EXPECT_FALSE(result.get() == NULL);
+  EXPECT_EQ(300, result->width());
+  EXPECT_EQ(200, result->height());
+
+  // The call should have removed all empty spaces.
+  ASSERT_TRUE(CompareImageFragments(source,
+                                    *result,
+                                    gfx::Size(100, 100),
+                                    gfx::Point(100, 200),
+                                    gfx::Point(0, 0)));
+  ASSERT_TRUE(CompareImageFragments(source,
+                                    *result,
+                                    gfx::Size(100, 100),
+                                    gfx::Point(300, 200),
+                                    gfx::Point(100, 0)));
+  ASSERT_TRUE(CompareImageFragments(source,
+                                    *result,
+                                    gfx::Size(100, 100),
+                                    gfx::Point(500, 200),
+                                    gfx::Point(200, 0)));
+  ASSERT_TRUE(CompareImageFragments(source,
+                                    *result,
+                                    gfx::Size(100, 100),
+                                    gfx::Point(100, 400),
+                                    gfx::Point(0, 0)));
+}
+
+TEST_F(ContentAnalysisTest, CreateRetargettedThumbnailImage) {
+  gfx::Size image_size(1200, 1300);
+  gfx::Canvas canvas(image_size, ui::SCALE_FACTOR_100P, true);
+
+  // The following will create a 'fake image' consisting of color blocks placed
+  // on a neutral background. The entire layout is supposed to mimic a
+  // screenshot of a web page.
+  // The tested function is supposed to locate the interesing areas in the
+  // middle.
+  const int margin_horizontal = 60;
+  const int margin_vertical = 20;
+  canvas.FillRect(gfx::Rect(image_size), SkColorSetRGB(200, 210, 210));
+  const gfx::Rect header_rect(margin_horizontal,
+                              margin_vertical,
+                              image_size.width() - 2 * margin_horizontal,
+                              100);
+  const gfx::Rect footer_rect(margin_horizontal,
+                              image_size.height() - margin_vertical - 100,
+                              image_size.width() - 2 * margin_horizontal,
+                              100);
+  const gfx::Rect body_rect(margin_horizontal,
+                            header_rect.bottom() + margin_vertical,
+                            image_size.width() - 2 * margin_horizontal,
+                            footer_rect.y() - header_rect.bottom() -
+                            2 * margin_vertical);
+  canvas.FillRect(header_rect, SkColorSetRGB(200, 40, 10));
+  canvas.FillRect(footer_rect, SkColorSetRGB(10, 40, 180));
+  canvas.FillRect(body_rect, SkColorSetRGB(150, 180, 40));
+
+  // 'Fine print' at the bottom.
+  const int fine_print = 8;
+  const SkColor print_color = SkColorSetRGB(45, 30, 30);
+  for (int y = footer_rect.y() + fine_print;
+       y < footer_rect.bottom() - fine_print;
+       y += 2 * fine_print) {
+    for (int x = footer_rect.x() + fine_print;
+         x < footer_rect.right() - fine_print;
+         x += 2 * fine_print) {
+      canvas.DrawRect(gfx::Rect(x, y, fine_print, fine_print),  print_color);
+    }
+  }
+
+  // Blocky content at the top.
+  const int block_size = header_rect.height() - margin_vertical;
+  for (int x = header_rect.x() + margin_horizontal;
+       x < header_rect.right() - block_size;
+       x += block_size + margin_horizontal) {
+    const int half_block = block_size / 2 - 5;
+    const SkColor block_color = SkColorSetRGB(255, 255, 255);
+    const int y = header_rect.y() + margin_vertical / 2;
+    int second_col = x + half_block + 10;
+    int second_row = y + half_block + 10;
+    canvas.FillRect(gfx::Rect(x, y, half_block, block_size), block_color);
+    canvas.FillRect(gfx::Rect(second_col,  y, half_block, half_block),
+                    block_color);
+    canvas.FillRect(gfx::Rect(second_col, second_row, half_block, half_block),
+                    block_color);
+  }
+
+  // Now the main body. Mostly text with some 'pictures'.
+  for (int y = body_rect.y() + fine_print;
+       y < body_rect.bottom() - fine_print;
+       y += 2 * fine_print) {
+    for (int x = body_rect.x() + fine_print;
+         x < body_rect.right() - fine_print;
+         x += 2 * fine_print) {
+      canvas.DrawRect(gfx::Rect(x, y, fine_print, fine_print),  print_color);
+    }
+  }
+
+  for (int line = 0; line < 3; ++line) {
+    int alignment = line % 2;
+    const int y = body_rect.y() +
+        body_rect.height() / 3 * line + margin_vertical;
+    const int x = body_rect.x() +
+        alignment * body_rect.width() / 2 + margin_vertical;
+    gfx::Rect pict_rect(x, y,
+                        body_rect.width() / 2 - 2 * margin_vertical,
+                        body_rect.height() / 3 - 2 * margin_vertical);
+    canvas.FillRect(pict_rect, SkColorSetRGB(255, 255, 255));
+    canvas.DrawRect(pict_rect, SkColorSetRGB(0, 0, 0));
+  }
+
+  SkBitmap source =
+      skia::GetTopDevice(*canvas.sk_canvas())->accessBitmap(false);
+
+  scoped_ptr<SkBitmap> result(color_utils::CreateRetargettedThumbnailImage(
+      source, gfx::Size(424, 264), 2.5));
+  EXPECT_FALSE(result.get() == NULL);
+
+  // Given the nature of computation We can't really assert much here about the
+  // image itself. We know it should have been computed, should be smaller than
+  // the original and it must not be zero.
+  EXPECT_LT(result->width(), image_size.width());
+  EXPECT_LT(result->height(), image_size.height());
+
+  int histogram[256];
+  color_utils::BuildLumaHistogram(*result, histogram);
+  int non_zero_color_count = std::count_if(
+      histogram, histogram + 256, std::bind2nd(std::greater<int>(), 0));
+  EXPECT_GT(non_zero_color_count, 4);
+}