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

This CL contains:
1. Convolution of arbitrary channel with arbitrary kernel (convolver*).
2. Gaussian gradient magnitude implementation.
3. Image profile (X and Y projections) computations.
4. Otsu-like thresholding of profiles.
5. Image decimation.
6. The main routine which binds it all together.

Note: 1 and 2 do work, but remain main sources of suckiness due to performance
problems. I actively work on this. Still, I'd love to get the current state in
to establish a baseline and for viewing pleasure of those who are interested.

BUG=155269

Review URL: https://codereview.chromium.org/13947013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194565 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/thumbnails/content_analysis.cc

+// Copyright (c) 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.
+
+#include "chrome/browser/thumbnails/content_analysis.h"
+
+#include <algorithm>
+#include <cmath>
+#include <deque>
+#include <functional>
+#include <limits>
+#include <numeric>
+#include <vector>
+
+#include "base/logging.h"
+#include "skia/ext/convolver.h"
+#include "third_party/skia/include/core/SkBitmap.h"
+#include "third_party/skia/include/core/SkSize.h"
+#include "ui/gfx/color_analysis.h"
+
+namespace {
+
+template<class InputIterator, class OutputIterator, class Compare>
+void SlidingWindowMinMax(InputIterator first,
+                         InputIterator last,
+                         OutputIterator output,
+                         int window_size,
+                         Compare cmp) {
+  typedef std::deque<
+      std::pair<typename std::iterator_traits<InputIterator>::value_type, int> >
+          deque_type;
+  deque_type slider;
+  int front_tail_length = window_size / 2;
+  int i = 0;
+  DCHECK_LT(front_tail_length, last - first);
+  // This min-max filter functions the way image filters do. The min/max we
+  // compute is placed in the center of the window. Thus, first we need to
+  // 'pre-load' the window with the slider with right-tail of the filter.
+  for (; first < last && i < front_tail_length; ++i, ++first)
+    slider.push_back(std::make_pair(*first, i));
+
+  for (; first < last; ++i, ++first, ++output) {
+    while (!slider.empty() && !cmp(slider.back().first, *first))
+      slider.pop_back();
+    slider.push_back(std::make_pair(*first, i));
+
+    while (slider.front().second <= i - window_size)
+      slider.pop_front();
+    *output = slider.front().first;
+  }
+
+  // Now at the tail-end we will simply need to use whatever value is left of
+  // the filter to compute the remaining front_tail_length taps in the output.
+
+  // If input shorter than window, remainder length needs to be adjusted.
+  front_tail_length = std::min(front_tail_length, i);
+  for (; front_tail_length >= 0; --front_tail_length, ++i) {
+    while (slider.front().second <= i - window_size)
+      slider.pop_front();
+    *output = slider.front().first;
+  }
+}
+
+}  // namespace
+
+namespace thumbnailing_utils {
+
+void ApplyGaussianGradientMagnitudeFilter(SkBitmap* input_bitmap,
+                                          float kernel_sigma) {
+  // The purpose of this function is to highlight salient
+  // (attention-attracting?) features of the image for use in image
+  // retargetting.
+  SkAutoLockPixels source_lock(*input_bitmap);
+  DCHECK(input_bitmap);
+  DCHECK(input_bitmap->getPixels());
+  DCHECK_EQ(SkBitmap::kA8_Config, input_bitmap->config());
+
+  const int tail_length = static_cast<int>(4.0f * kernel_sigma + 0.5f);
+  const int kernel_size = tail_length * 2 + 1;
+  const float sigmasq = kernel_sigma * kernel_sigma;
+  std::vector<float> smoother_weights(kernel_size, 0.0);
+  float kernel_sum = 1.0f;
+
+  smoother_weights[tail_length] = 1.0f;
+
+  for (int ii = 1; ii <= tail_length; ++ii) {
+    float v = std::exp(-0.5f * ii * ii / sigmasq);
+    smoother_weights[tail_length + ii] = v;
+    smoother_weights[tail_length - ii] = v;
+    kernel_sum += 2.0f * v;
+  }
+
+  for (int i = 0; i < kernel_size; ++i)
+    smoother_weights[i] /= kernel_sum;
+
+  std::vector<float> gradient_weights(kernel_size, 0.0);
+  gradient_weights[tail_length] = 0.0;
+  for (int ii = 1; ii <= tail_length; ++ii) {
+    float v = sigmasq * smoother_weights[tail_length + ii] / ii;
+    gradient_weights[tail_length + ii] = v;
+    gradient_weights[tail_length - ii] = -v;
+  }
+
+  skia::ConvolutionFilter1D smoothing_filter;
+  skia::ConvolutionFilter1D gradient_filter;
+  smoothing_filter.AddFilter(0, &smoother_weights[0], smoother_weights.size());
+  gradient_filter.AddFilter(0, &gradient_weights[0], gradient_weights.size());
+
+  // To perform computations we will need one intermediate buffer. It can
+  // very well be just another bitmap.
+  const SkISize image_size = SkISize::Make(input_bitmap->width(),
+                                           input_bitmap->height());
+  SkBitmap intermediate;
+  intermediate.setConfig(
+      input_bitmap->config(), image_size.width(), image_size.height());
+  intermediate.allocPixels();
+
+  skia::SingleChannelConvolveX1D(
+      input_bitmap->getAddr8(0, 0),
+      static_cast<int>(input_bitmap->rowBytes()),
+      0, input_bitmap->bytesPerPixel(),
+      smoothing_filter,
+      image_size,
+      intermediate.getAddr8(0, 0),
+      static_cast<int>(intermediate.rowBytes()),
+      0, intermediate.bytesPerPixel(), false);
+  skia::SingleChannelConvolveY1D(
+      intermediate.getAddr8(0, 0),
+      static_cast<int>(intermediate.rowBytes()),
+      0, intermediate.bytesPerPixel(),
+      smoothing_filter,
+      image_size,
+      input_bitmap->getAddr8(0, 0),
+      static_cast<int>(input_bitmap->rowBytes()),
+      0, input_bitmap->bytesPerPixel(), false);
+
+  // Now the gradient operator (we will need two buffers).
+  SkBitmap intermediate2;
+  intermediate2.setConfig(
+      input_bitmap->config(), image_size.width(), image_size.height());
+  intermediate2.allocPixels();
+
+  skia::SingleChannelConvolveX1D(
+      input_bitmap->getAddr8(0, 0),
+      static_cast<int>(input_bitmap->rowBytes()),
+      0, input_bitmap->bytesPerPixel(),
+      gradient_filter,
+      image_size,
+      intermediate.getAddr8(0, 0),
+      static_cast<int>(intermediate.rowBytes()),
+      0, intermediate.bytesPerPixel(), true);
+  skia::SingleChannelConvolveY1D(
+      input_bitmap->getAddr8(0, 0),
+      static_cast<int>(input_bitmap->rowBytes()),
+      0, input_bitmap->bytesPerPixel(),
+      gradient_filter,
+      image_size,
+      intermediate2.getAddr8(0, 0),
+      static_cast<int>(intermediate2.rowBytes()),
+      0, intermediate2.bytesPerPixel(), true);
+
+  unsigned grad_max = 0;
+  for (int r = 0; r < image_size.height(); ++r) {
+    const uint8* grad_x_row = intermediate.getAddr8(0, r);
+    const uint8* grad_y_row = intermediate2.getAddr8(0, r);
+    for (int c = 0; c < image_size.width(); ++c) {
+      unsigned grad_x = grad_x_row[c];
+      unsigned grad_y = grad_y_row[c];
+      grad_max = std::max(grad_max, grad_x * grad_x + grad_y * grad_y);
+    }
+  }
+
+  int bit_shift = 0;
+  if (grad_max > 255)
+    bit_shift = static_cast<int>(
+        std::log10(static_cast<float>(grad_max)) / std::log10(2.0f)) - 7;
+  for (int r = 0; r < image_size.height(); ++r) {
+    const uint8* grad_x_row =intermediate.getAddr8(0, r);
+    const uint8* grad_y_row = intermediate2.getAddr8(0, r);
+    uint8* target_row = input_bitmap->getAddr8(0, r);
+    for (int c = 0; c < image_size.width(); ++c) {
+      unsigned grad_x = grad_x_row[c];
+      unsigned grad_y = grad_y_row[c];
+      target_row[c] = (grad_x * grad_x + grad_y * grad_y) >> bit_shift;
+    }
+  }
+}
+
+void ExtractImageProfileInformation(const SkBitmap& input_bitmap,
+                                    const gfx::Rect& area,
+                                    const gfx::Size& target_size,
+                                    bool apply_log,
+                                    std::vector<float>* rows,
+                                    std::vector<float>* columns) {
+  SkAutoLockPixels source_lock(input_bitmap);
+  DCHECK(rows);
+  DCHECK(columns);
+  DCHECK(input_bitmap.getPixels());
+  DCHECK_EQ(SkBitmap::kA8_Config, input_bitmap.config());
+  DCHECK_GE(area.x(), 0);
+  DCHECK_GE(area.y(), 0);
+  DCHECK_LE(area.right(), input_bitmap.width());
+  DCHECK_LE(area.bottom(), input_bitmap.height());
+
+  // Make sure rows and columns are allocated and initialized to 0.
+  rows->clear();
+  columns->clear();
+  rows->resize(area.height(), 0);
+  columns->resize(area.width(), 0);
+
+  for (int r = 0; r < area.height(); ++r) {
+    // Points to the first byte of the row in the rectangle.
+    const uint8* image_row = input_bitmap.getAddr8(area.x(), r + area.y());
+    unsigned row_sum = 0;
+    for (int c = 0; c < area.width(); ++c, ++image_row) {
+      row_sum += *image_row;
+      (*columns)[c] += *image_row;
+    }
+    (*rows)[r] = row_sum;
+  }
+
+  if (apply_log) {
+    // Generally for processing we will need to take logarithm of this data.
+    // The option not to apply it is left principally as a test seam.
+    std::vector<float>::iterator it;
+    for (it = columns->begin(); it < columns->end(); ++it)
+      *it = std::log(1.0f + *it);
+
+    for (it = rows->begin(); it < rows->end(); ++it)
+      *it = std::log(1.0f + *it);
+  }
+
+  if (!target_size.IsEmpty()) {
+    // If the target size is given, profiles should be further processed through
+    // morphological closing. The idea is to close valleys smaller than what
+    // can be seen after scaling down to avoid deforming noticable features
+    // when profiles are used.
+    // Morphological closing is defined as dilation followed by errosion. In
+    // normal-speak: sliding-window maximum followed by minimum.
+    int column_window_size = 1 + 2 *
+        static_cast<int>(0.5f * area.width() / target_size.width() + 0.5f);
+    int row_window_size = 1 + 2 *
+        static_cast<int>(0.5f * area.height() / target_size.height() + 0.5f);
+
+    // Dilate and erode each profile with the given window size.
+    if (column_window_size >= 3) {
+      SlidingWindowMinMax(columns->begin(),
+                          columns->end(),
+                          columns->begin(),
+                          column_window_size,
+                          std::greater<float>());
+      SlidingWindowMinMax(columns->begin(),
+                          columns->end(),
+                          columns->begin(),
+                          column_window_size,
+                          std::less<float>());
+    }
+
+    if (row_window_size >= 3) {
+      SlidingWindowMinMax(rows->begin(),
+                          rows->end(),
+                          rows->begin(),
+                          row_window_size,
+                          std::greater<float>());
+      SlidingWindowMinMax(rows->begin(),
+                          rows->end(),
+                          rows->begin(),
+                          row_window_size,
+                          std::less<float>());
+    }
+  }
+}
+
+float AutoSegmentPeaks(const std::vector<float>& input) {
+  // This is a thresholding operation based on Otsu's method.
+  std::vector<int> histogram(256, 0);
+  std::vector<float>::const_iterator it;
+
+  float value_min = std::numeric_limits<float>::max();
+  float value_max = std::numeric_limits<float>::min();
+
+  for (it = input.begin(); it < input.end(); ++it) {
+    value_min = std::min(value_min, *it);
+    value_max = std::max(value_max, *it);
+  }
+
+  if (value_max - value_min <= std::numeric_limits<float>::epsilon() * 100) {
+    // Scaling won't work and there is nothing really to segment anyway.
+    return value_min;
+  }
+
+  float value_span = value_max - value_min;
+  for (it = input.begin(); it < input.end(); ++it) {
+    float scaled_value = (*it - value_min) / value_span * 255;
+    histogram[static_cast<int>(scaled_value)] += 1;
+  }
+
+  // Otsu's method seeks to maximize variance between two classes of pixels
+  // correspondng to valleys and peaks of the profile.
+  double w1 = histogram[0];  // Total weight of the first class.
+  double t1 = 0.5 * w1;
+  double w2 = 0;
+  double t2 = 0;
+  for (size_t i = 1; i < histogram.size(); ++i) {
+    w2 += histogram[i];
+    t2 += (0.5 + i) * histogram[i];
+  }
+
+  size_t max_index = 0;
+  double m1 = t1 / w1;
+  double m2 = t2 / w2;
+  double max_variance_score = w1 * w2 * (m1 - m2) * (m1 - m2);
+  // Iterate through all possible ways of splitting the histogram.
+  for (size_t i = 1; i < histogram.size() - 1; i++) {
+    double bin_volume = (0.5 + i) * histogram[i];
+    w1 += histogram[i];
+    w2 -= histogram[i];
+    t2 -= bin_volume;
+    t1 += bin_volume;
+    m1 = t1 / w1;
+    m2 = t2 / w2;
+    double variance_score = w1 * w2 * (m1 - m2) * (m1 - m2);
+    if (variance_score >= max_variance_score) {
+      max_variance_score = variance_score;
+      max_index = i;
+    }
+  }
+
+  // max_index refers to the bin *after* which we need to split. The sought
+  // threshold is the centre of this bin, scaled back to the original range.
+  return value_span * (max_index + 0.5f) / 255.0f + value_min;
+}
+
+SkBitmap ComputeDecimatedImage(const SkBitmap& bitmap,
+                               const std::vector<bool>& rows,
+                               const std::vector<bool>& columns) {
+  SkAutoLockPixels source_lock(bitmap);
+  DCHECK(bitmap.getPixels());
+  DCHECK_GT(bitmap.bytesPerPixel(), 0);
+  DCHECK_EQ(bitmap.width(), static_cast<int>(columns.size()));
+  DCHECK_EQ(bitmap.height(), static_cast<int>(rows.size()));
+
+  unsigned target_row_count = std::count(rows.begin(), rows.end(), true);
+  unsigned target_column_count = std::count(
+      columns.begin(), columns.end(), true);
+
+  if (target_row_count == 0 || target_column_count == 0)
+    return SkBitmap();  // Not quite an error, so no DCHECK. Just return empty.
+
+  if (target_row_count == rows.size() && target_column_count == columns.size())
+    return SkBitmap();  // Equivalent of the situation above (empty target).
+
+  // Allocate the target image.
+  SkBitmap target;
+  target.setConfig(bitmap.config(), target_column_count, target_row_count);
+  target.allocPixels();
+
+  int target_row = 0;
+  for (int r = 0; r < bitmap.height(); ++r) {
+    if (!rows[r])
+      continue;  // We can just skip this one.
+    uint8* src_row =
+        static_cast<uint8*>(bitmap.getPixels()) + r * bitmap.rowBytes();
+    uint8* insertion_target = static_cast<uint8*>(target.getPixels()) +
+        target_row * target.rowBytes();
+    int left_copy_pixel = -1;
+    for (int c = 0; c < bitmap.width(); ++c) {
+      if (left_copy_pixel < 0 && columns[c]) {
+        left_copy_pixel = c;  // Next time we will start copying from here.
+      } else if (left_copy_pixel >= 0 && !columns[c]) {
+        // This closes a fragment we want to copy. We do it now.
+        size_t bytes_to_copy = (c - left_copy_pixel) * bitmap.bytesPerPixel();
+        memcpy(insertion_target,
+               src_row + left_copy_pixel * bitmap.bytesPerPixel(),
+               bytes_to_copy);
+        left_copy_pixel = -1;
+        insertion_target += bytes_to_copy;
+      }
+    }
+    // We can still have the tail end to process here.
+    if (left_copy_pixel >= 0) {
+      size_t bytes_to_copy =
+          (bitmap.width() - left_copy_pixel) * bitmap.bytesPerPixel();
+      memcpy(insertion_target,
+             src_row + left_copy_pixel * bitmap.bytesPerPixel(),
+             bytes_to_copy);
+    }
+    target_row++;
+  }
+
+  return target;
+}
+
+SkBitmap CreateRetargettedThumbnailImage(
+    const SkBitmap& source_bitmap,
+    const gfx::Size& target_size,
+    float kernel_sigma) {
+  // First thing we need for this method is to color-reduce the source_bitmap.
+  SkBitmap reduced_color;
+  reduced_color.setConfig(
+      SkBitmap::kA8_Config, source_bitmap.width(), source_bitmap.height());
+  reduced_color.allocPixels();
+
+  if (!color_utils::ComputePrincipalComponentImage(source_bitmap,
+                                                   &reduced_color)) {
+    // CCIR601 luminance conversion vector.
+    gfx::Vector3dF transform(0.299f, 0.587f, 0.114f);
+    if (!color_utils::ApplyColorReduction(
+            source_bitmap, transform, true, &reduced_color)) {
+      DLOG(WARNING) << "Failed to compute luminance image from a screenshot. "
+                    << "Cannot compute retargetted thumbnail.";
+      return SkBitmap();
+    }
+    DLOG(WARNING) << "Could not compute principal color image for a thumbnail. "
+                  << "Using luminance instead.";
+  }
+
+  // Turn 'color-reduced' image into the 'energy' image.
+  ApplyGaussianGradientMagnitudeFilter(&reduced_color, kernel_sigma);
+
+  // Extract vertical and horizontal projection of image features.
+  std::vector<float> row_profile;
+  std::vector<float> column_profile;
+  ExtractImageProfileInformation(reduced_color,
+                                 gfx::Rect(reduced_color.width(),
+                                           reduced_color.height()),
+                                 target_size,
+                                 true,
+                                 &row_profile,
+                                 &column_profile);
+  float threshold_rows = AutoSegmentPeaks(row_profile);
+  float threshold_columns = AutoSegmentPeaks(column_profile);
+
+  // Apply thresholding.
+  std::vector<bool> included_rows(row_profile.size(), false);
+  std::transform(row_profile.begin(),
+                 row_profile.end(),
+                 included_rows.begin(),
+                 std::bind2nd(std::greater<float>(), threshold_rows));
+
+  std::vector<bool> included_columns(column_profile.size(), false);
+  std::transform(column_profile.begin(),
+                 column_profile.end(),
+                 included_columns.begin(),
+                 std::bind2nd(std::greater<float>(), threshold_columns));
+
+  // Use the original image and computed inclusion vectors to create a resized
+  // image.
+  return ComputeDecimatedImage(source_bitmap, included_rows, included_columns);
+}
+
+}  // thumbnailing_utils

chrome/browser/thumbnails/content_analysis.h

+// Copyright (c) 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 CHROME_BROWSER_THUMBNAILS_CONTENT_ANALYSIS_H_
+#define CHROME_BROWSER_THUMBNAILS_CONTENT_ANALYSIS_H_
+
+#include <vector>
+
+#include "base/basictypes.h"
+#include "third_party/skia/include/core/SkColor.h"
+#include "ui/base/ui_export.h"
+#include "ui/gfx/rect.h"
+#include "ui/gfx/size.h"
+
+class SkBitmap;
+
+namespace thumbnailing_utils {
+
+// Compute in-place gaussian gradient magnitude of |input_bitmap| with sigma
+// |kernel_sigma|. |input_bitmap| is requried to be of SkBitmap::kA8_Config
+// type. The routine computes first-order gaussian derivative on a
+// gaussian-smoothed image. Beware, this is fairly slow since kernel size is
+// 4 * kernel_sigma + 1.
+void ApplyGaussianGradientMagnitudeFilter(SkBitmap* input_bitmap,
+                                          float kernel_sigma);
+
+// Accumulates vertical and horizontal sum of pixel values from a subsection of
+// |input_bitmap| defined by |image_area|. The image is required to be of
+// SkBitmap::kA8_Config type.
+// If non-empty |target_size| is given, the routine will use it to process the
+// profiles with closing operator sized to eliminate gaps which would be smaller
+// than 1 pixel after rescaling to |target_size|.
+// If |apply_log| is true, logarithm of counts are used for morhology (and
+// returned).
+void ExtractImageProfileInformation(const SkBitmap& input_bitmap,
+                                    const gfx::Rect& image_area,
+                                    const gfx::Size& target_size,
+                                    bool apply_log,
+                                    std::vector<float>* rows,
+                                    std::vector<float>* columns);
+
+// Compute a threshold value separating background (low) from signal (high)
+// areas in the |input| profile.
+float AutoSegmentPeaks(const std::vector<float>& input);
+
+// Shrinks the source |bitmap| by removing rows and columns where |rows| and
+// |columns| are false, respectively. The function returns a new bitmap if the
+// shrinking can be performed and an empty instance otherwise.
+SkBitmap ComputeDecimatedImage(const SkBitmap& bitmap,
+                               const std::vector<bool>& rows,
+                               const std::vector<bool>& columns);
+
+// Creates a new bitmap which contains only 'interesting' areas of
+// |source_bitmap|. The |target_size| is used to estimate some computation
+// parameters, but the resulting bitmap will not necessarily be of that size.
+// |kernel_sigma| defines the degree of image smoothing in gradient computation.
+// For a natural-sized (not shrunk) screenshot at 96 DPI and regular font size
+// 5.0 was determined to be a good value.
+SkBitmap CreateRetargettedThumbnailImage(const SkBitmap& source_bitmap,
+                                         const gfx::Size& target_size,
+                                         float kernel_sigma);
+
+}  // namespace thumbnailing_utils
+
+#endif  // CHROME_BROWSER_THUMBNAILS_CONTENT_ANALYSIS_H_

chrome/browser/thumbnails/content_analysis_unittest.cc

+// 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 "chrome/browser/thumbnails/content_analysis.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cstdlib>
+#include <limits>
+#include <numeric>
+#include <vector>
+
+#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
+
+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 color_left = bitmap_left.getColor(origin_left.x() + c,
+                                                origin_left.y() + r);
+      SkColor color_right = bitmap_right.getColor(origin_right.x() + c,
+                                                  origin_right.y() + r);
+      if (color_left != color_right)
+        return false;
+    }
+  }
+
+  return true;
+}
+
+}  // namespace
+
+namespace thumbnailing_utils {
+
+class ThumbnailContentAnalysisTest : public testing::Test {
+};
+
+TEST_F(ThumbnailContentAnalysisTest, 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;
+  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;
+  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(ThumbnailContentAnalysisTest, 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;
+  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(ThumbnailContentAnalysisTest, 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;
+  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);
+  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(ThumbnailContentAnalysisTest,
+       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;
+
+  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.
+  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(ThumbnailContentAnalysisTest, AutoSegmentPeaks) {
+  std::vector<float> profile_info;
+
+  EXPECT_EQ(AutoSegmentPeaks(profile_info), std::numeric_limits<float>::max());
+  profile_info.resize(1000, 1.0f);
+  EXPECT_EQ(AutoSegmentPeaks(profile_info), 1.0f);
+  std::srand(42);
+  std::generate(profile_info.begin(), profile_info.end(), std::rand);
+  float threshold = 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, 450);  // Not much to expect.
+  EXPECT_LT(above_count, 550);
+
+  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 = 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(ThumbnailContentAnalysisTest, 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);
+  SkBitmap result = ComputeDecimatedImage(source, rows, columns);
+  EXPECT_FALSE(result.empty());
+  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(ThumbnailContentAnalysisTest, 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);
+
+  SkBitmap result = CreateRetargettedThumbnailImage(
+      source, gfx::Size(424, 264), 2.5);
+  EXPECT_FALSE(result.empty());
+
+  // 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);
+}
+
+}  // namespace thumbnailing_utils

chrome/chrome_browser.gypi

@@ -2042,6 +2042,8 @@
         'browser/themes/theme_syncable_service.h',
         'browser/three_d_api_observer.cc',
         'browser/three_d_api_observer.h',
+        'browser/thumbnails/content_analysis.cc',
+        'browser/thumbnails/content_analysis.h',
         'browser/thumbnails/simple_thumbnail_crop.cc',
         'browser/thumbnails/simple_thumbnail_crop.h',
         'browser/thumbnails/render_widget_snapshot_taker.cc',

chrome/chrome_tests_unit.gypi

@@ -1191,6 +1191,7 @@
         'browser/themes/theme_properties_unittest.cc',
         'browser/themes/theme_service_unittest.cc',
         'browser/themes/theme_syncable_service_unittest.cc',
+        'browser/thumbnails/content_analysis_unittest.cc',
         'browser/thumbnails/render_widget_snapshot_taker_unittest.cc',
         'browser/thumbnails/simple_thumbnail_crop_unittest.cc',
         'browser/thumbnails/thumbnail_service_unittest.cc',

skia/ext/convolver.cc

@@ -4,8 +4,10 @@
 
 #include <algorithm>
 
+#include "base/logging.h"
 #include "skia/ext/convolver.h"
 #include "skia/ext/convolver_SSE2.h"
+#include "third_party/skia/include/core/SkSize.h"
 #include "third_party/skia/include/core/SkTypes.h"
 
 namespace skia {
@@ -22,6 +24,17 @@ inline unsigned char ClampTo8(int a) {
   return 255;
 }
 
+// Takes the value produced by accumulating element-wise product of image with
+// a kernel and brings it back into range.
+// All of the filter scaling factors are in fixed point with kShiftBits bits of
+// fractional part.
+inline unsigned char BringBackTo8(int a, bool take_absolute) {
+  a >>= ConvolutionFilter1D::kShiftBits;
+  if (take_absolute)
+    a = std::abs(a);
+  return ClampTo8(a);
+}
+
 // Stores a list of rows in a circular buffer. The usage is you write into it
 // by calling AdvanceRow. It will keep track of which row in the buffer it
 // should use next, and the total number of rows added.
@@ -271,6 +284,7 @@ void ConvolutionFilter1D::AddFilter(int filter_offset,
   // cases it is beneficial to only store the central factors.
   // For a scaling to 1/4th in each dimension using a Lanczos-2 filter on
   // a 1080p image this optimization gives a ~10% speed improvement.
+  int filter_size = filter_length;
   int first_non_zero = 0;
   while (first_non_zero < filter_length && filter_values[first_non_zero] == 0)
     first_non_zero++;
@@ -298,12 +312,27 @@ void ConvolutionFilter1D::AddFilter(int filter_offset,
   instance.data_location = (static_cast<int>(filter_values_.size()) -
                             filter_length);
   instance.offset = filter_offset;
-  instance.length = filter_length;
+  instance.trimmed_length = filter_length;
+  instance.length = filter_size;
   filters_.push_back(instance);
 
   max_filter_ = std::max(max_filter_, filter_length);
 }
 
+const ConvolutionFilter1D::Fixed* ConvolutionFilter1D::GetSingleFilter(
+    int* specified_filter_length,
+    int* filter_offset,
+    int* filter_length) const {
+  const FilterInstance& filter = filters_[0];
+  *filter_offset = filter.offset;
+  *filter_length = filter.trimmed_length;
+  *specified_filter_length = filter.length;
+  if (filter.trimmed_length == 0)
+    return NULL;
+
+  return &filter_values_[filter.data_location];
+}
+
 typedef void (*ConvolveVertically_pointer)(
     const ConvolutionFilter1D::Fixed* filter_values,
     int filter_length,
@@ -478,4 +507,169 @@ void BGRAConvolve2D(const unsigned char* source_data,
   }
 }
 
+void SingleChannelConvolveX1D(const unsigned char* source_data,
+                              int source_byte_row_stride,
+                              int input_channel_index,
+                              int input_channel_count,
+                              const ConvolutionFilter1D& filter,
+                              const SkISize& image_size,
+                              unsigned char* output,
+                              int output_byte_row_stride,
+                              int output_channel_index,
+                              int output_channel_count,
+                              bool absolute_values) {
+  int filter_offset, filter_length, filter_size;
+  // Very much unlike BGRAConvolve2D, here we expect to have the same filter
+  // for all pixels.
+  const ConvolutionFilter1D::Fixed* filter_values =
+      filter.GetSingleFilter(&filter_size, &filter_offset, &filter_length);
+
+  if (filter_values == NULL || image_size.width() < filter_size) {
+    NOTREACHED();
+    return;
+  }
+
+  int centrepoint = filter_length / 2;
+  if (filter_size - filter_offset != 2 * filter_offset) {
+    // This means the original filter was not symmetrical AND
+    // got clipped from one side more than from the other.
+    centrepoint = filter_size / 2 - filter_offset;
+  }
+
+  const unsigned char* source_data_row = source_data;
+  unsigned char* output_row = output;
+
+  for (int r = 0; r < image_size.height(); ++r) {
+    unsigned char* target_byte = output_row + output_channel_index;
+    // Process the lead part, padding image to the left with the first pixel.
+    int c = 0;
+    for (; c < centrepoint; ++c, target_byte += output_channel_count) {
+      int accval = 0;
+      int i = 0;
+      int pixel_byte_index = input_channel_index;
+      for (; i < centrepoint - c; ++i)  // Padding part.
+        accval += filter_values[i] * source_data_row[pixel_byte_index];
+
+      for (; i < filter_length; ++i, pixel_byte_index += input_channel_count)
+        accval += filter_values[i] * source_data_row[pixel_byte_index];
+
+      *target_byte = BringBackTo8(accval, absolute_values);
+    }
+
+    // Now for the main event.
+    for (; c < image_size.width() - centrepoint;
+         ++c, target_byte += output_channel_count) {
+      int accval = 0;
+      int pixel_byte_index = (c - centrepoint) * input_channel_count +
+          input_channel_index;
+
+      for (int i = 0; i < filter_length;
+           ++i, pixel_byte_index += input_channel_count) {
+        accval += filter_values[i] * source_data_row[pixel_byte_index];
+      }
+
+      *target_byte = BringBackTo8(accval, absolute_values);
+    }
+
+    for (; c < image_size.width(); ++c, target_byte += output_channel_count) {
+      int accval = 0;
+      int overlap_taps = image_size.width() - c + centrepoint;
+      int pixel_byte_index = (c - centrepoint) * input_channel_count +
+          input_channel_index;
+      int i = 0;
+      for (; i < overlap_taps - 1; ++i, pixel_byte_index += input_channel_count)
+        accval += filter_values[i] * source_data_row[pixel_byte_index];
+
+      for (; i < filter_length; ++i)
+        accval += filter_values[i] * source_data_row[pixel_byte_index];
+
+      *target_byte = BringBackTo8(accval, absolute_values);
+    }
+
+    source_data_row += source_byte_row_stride;
+    output_row += output_byte_row_stride;
+  }
+}
+
+void SingleChannelConvolveY1D(const unsigned char* source_data,
+                              int source_byte_row_stride,
+                              int input_channel_index,
+                              int input_channel_count,
+                              const ConvolutionFilter1D& filter,
+                              const SkISize& image_size,
+                              unsigned char* output,
+                              int output_byte_row_stride,
+                              int output_channel_index,
+                              int output_channel_count,
+                              bool absolute_values) {
+  int filter_offset, filter_length, filter_size;
+  // Very much unlike BGRAConvolve2D, here we expect to have the same filter
+  // for all pixels.
+  const ConvolutionFilter1D::Fixed* filter_values =
+      filter.GetSingleFilter(&filter_size, &filter_offset, &filter_length);
+
+  if (filter_values == NULL || image_size.height() < filter_size) {
+    NOTREACHED();
+    return;
+  }
+
+  int centrepoint = filter_length / 2;
+  if (filter_size - filter_offset != 2 * filter_offset) {
+    // This means the original filter was not symmetrical AND
+    // got clipped from one side more than from the other.
+    centrepoint = filter_size / 2 - filter_offset;
+  }
+
+  for (int c = 0; c < image_size.width(); ++c) {
+    unsigned char* target_byte = output + c * output_channel_count +
+        output_channel_index;
+    int r = 0;
+
+    for (; r < centrepoint; ++r, target_byte += output_byte_row_stride) {
+      int accval = 0;
+      int i = 0;
+      int pixel_byte_index = c * input_channel_count + input_channel_index;
+
+      for (; i < centrepoint - r; ++i)  // Padding part.
+        accval += filter_values[i] * source_data[pixel_byte_index];
+
+      for (; i < filter_length; ++i, pixel_byte_index += source_byte_row_stride)
+        accval += filter_values[i] * source_data[pixel_byte_index];
+
+      *target_byte = BringBackTo8(accval, absolute_values);
+    }
+
+    for (; r < image_size.height() - centrepoint;
+         ++r, target_byte += output_byte_row_stride) {
+      int accval = 0;
+      int pixel_byte_index = (r - centrepoint) * source_byte_row_stride +
+          c * input_channel_count + input_channel_index;
+      for (int i = 0; i < filter_length;
+           ++i, pixel_byte_index += source_byte_row_stride) {
+        accval += filter_values[i] * source_data[pixel_byte_index];
+      }
+
+      *target_byte = BringBackTo8(accval, absolute_values);
+    }
+
+    for (; r < image_size.height();
+         ++r, target_byte += output_byte_row_stride) {
+      int accval = 0;
+      int overlap_taps = image_size.height() - r + centrepoint;
+      int pixel_byte_index = (r - centrepoint) * source_byte_row_stride +
+          c * input_channel_count + input_channel_index;
+      int i = 0;
+      for (; i < overlap_taps - 1;
+           ++i, pixel_byte_index += source_byte_row_stride) {
+        accval += filter_values[i] * source_data[pixel_byte_index];
+      }
+
+      for (; i < filter_length; ++i)
+        accval += filter_values[i] * source_data[pixel_byte_index];
+
+      *target_byte = BringBackTo8(accval, absolute_values);
+    }
+  }
+}
+
 }  // namespace skia

skia/ext/convolver.h

@@ -10,6 +10,7 @@
 
 #include "base/basictypes.h"
 #include "base/cpu.h"
+#include "third_party/skia/include/core/SkSize.h"
 #include "third_party/skia/include/core/SkTypes.h"
 
 // We can build SSE2 optimized versions for all x86 CPUs
@@ -99,13 +100,24 @@ class ConvolutionFilter1D {
                                      int* filter_length) const {
     const FilterInstance& filter = filters_[value_offset];
     *filter_offset = filter.offset;
-    *filter_length = filter.length;
-    if (filter.length == 0) {
+    *filter_length = filter.trimmed_length;
+    if (filter.trimmed_length == 0) {
       return NULL;
     }
     return &filter_values_[filter.data_location];
   }
 
+  // Retrieves the filter for the offset 0, presumed to be the one and only.
+  // The offset and length of the filter values are put into the corresponding
+  // out arguments (see AddFilter). Note that |filter_legth| and
+  // |specified_filter_length| may be different if leading/trailing zeros of the
+  // original floating point form were clipped.
+  // There will be |filter_length| values in the return array.
+  // Returns NULL if the filter is 0-length (for instance when all floating
+  // point values passed to AddFilter were clipped to 0).
+  const Fixed* GetSingleFilter(int* specified_filter_length,
+                               int* filter_offset,
+                               int* filter_length) const;
 
   inline void PaddingForSIMD() {
     // Padding |padding_count| of more dummy coefficients after the coefficients
@@ -128,6 +140,11 @@ class ConvolutionFilter1D {
     int offset;
 
     // Number of values in this filter instance.
+    int trimmed_length;
+
+    // Filter length as specified. Note that this may be different from
+    // 'trimmed_length' if leading/trailing zeros of the original floating
+    // point form were clipped differently on each tail.
     int length;
   };
 
@@ -169,6 +186,39 @@ SK_API void BGRAConvolve2D(const unsigned char* source_data,
                            int output_byte_row_stride,
                            unsigned char* output,
                            bool use_simd_if_possible);
+
+// Does a 1D convolution of the given source image along the X dimension on
+// a single channel of the bitmap.
+//
+// The function uses the same convolution kernel for each pixel. That kernel
+// must be added to |filter| at offset 0. This is a most straightforward
+// implementation of convolution, intended chiefly for development purposes.
+SK_API void SingleChannelConvolveX1D(const unsigned char* source_data,
+                                     int source_byte_row_stride,
+                                     int input_channel_index,
+                                     int input_channel_count,
+                                     const ConvolutionFilter1D& filter,
+                                     const SkISize& image_size,
+                                     unsigned char* output,
+                                     int output_byte_row_stride,
+                                     int output_channel_index,
+                                     int output_channel_count,
+                                     bool absolute_values);
+
+// Does a 1D convolution of the given source image along the Y dimension on
+// a single channel of the bitmap.
+SK_API void SingleChannelConvolveY1D(const unsigned char* source_data,
+                                     int source_byte_row_stride,
+                                     int input_channel_index,
+                                     int input_channel_count,
+                                     const ConvolutionFilter1D& filter,
+                                     const SkISize& image_size,
+                                     unsigned char* output,
+                                     int output_byte_row_stride,
+                                     int output_channel_index,
+                                     int output_channel_count,
+                                     bool absolute_values);
+
 }  // namespace skia
 
 #endif  // SKIA_EXT_CONVOLVER_H_

skia/ext/convolver_unittest.cc

@@ -324,4 +324,153 @@ TEST(Convolver, SIMDVerification) {
   }
 }
 
+TEST(Convolver, SeparableSingleConvolution) {
+  static const int kImgWidth = 1024;
+  static const int kImgHeight = 1024;
+  static const int kChannelCount = 3;
+  static const int kStrideSlack = 22;
+  ConvolutionFilter1D filter;
+  const float box[5] = { 0.2f, 0.2f, 0.2f, 0.2f, 0.2f };
+  filter.AddFilter(0, box, 5);
+
+  // Allocate a source image and set to 0.
+  const int src_row_stride = kImgWidth * kChannelCount + kStrideSlack;
+  int src_byte_count = src_row_stride * kImgHeight;
+  std::vector<unsigned char> input;
+  const int signal_x = kImgWidth / 2;
+  const int signal_y = kImgHeight / 2;
+  input.resize(src_byte_count, 0);
+  // The image has a single impulse pixel in channel 1, smack in the middle.
+  const int non_zero_pixel_index =
+      signal_y * src_row_stride + signal_x * kChannelCount + 1;
+  input[non_zero_pixel_index] = 255;
+
+  // Destination will be a single channel image with stide matching width.
+  const int dest_row_stride = kImgWidth;
+  const int dest_byte_count = dest_row_stride * kImgHeight;
+  std::vector<unsigned char> output;
+  output.resize(dest_byte_count);
+
+  // Apply convolution in X.
+  SingleChannelConvolveX1D(&input[0], src_row_stride, 1, kChannelCount,
+                           filter, SkISize::Make(kImgWidth, kImgHeight),
+                           &output[0], dest_row_stride, 0, 1, false);
+  for (int x = signal_x - 2; x <= signal_x + 2; ++x)
+    EXPECT_GT(output[signal_y * dest_row_stride + x], 0);
+
+  EXPECT_EQ(output[signal_y * dest_row_stride + signal_x - 3], 0);
+  EXPECT_EQ(output[signal_y * dest_row_stride + signal_x + 3], 0);
+
+  // Apply convolution in Y.
+  SingleChannelConvolveY1D(&input[0], src_row_stride, 1, kChannelCount,
+                           filter, SkISize::Make(kImgWidth, kImgHeight),
+                           &output[0], dest_row_stride, 0, 1, false);
+  for (int y = signal_y - 2; y <= signal_y + 2; ++y)
+    EXPECT_GT(output[y * dest_row_stride + signal_x], 0);
+
+  EXPECT_EQ(output[(signal_y - 3) * dest_row_stride + signal_x], 0);
+  EXPECT_EQ(output[(signal_y + 3) * dest_row_stride + signal_x], 0);
+
+  EXPECT_EQ(output[signal_y * dest_row_stride + signal_x - 1], 0);
+  EXPECT_EQ(output[signal_y * dest_row_stride + signal_x + 1], 0);
+
+  // The main point of calling this is to invoke the routine on input without
+  // padding.
+  std::vector<unsigned char> output2;
+  output2.resize(dest_byte_count);
+  SingleChannelConvolveX1D(&output[0], dest_row_stride, 0, 1,
+                           filter, SkISize::Make(kImgWidth, kImgHeight),
+                           &output2[0], dest_row_stride, 0, 1, false);
+  // This should be a result of 2D convolution.
+  for (int x = signal_x - 2; x <= signal_x + 2; ++x) {
+    for (int y = signal_y - 2; y <= signal_y + 2; ++y)
+      EXPECT_GT(output2[y * dest_row_stride + x], 0);
+  }
+  EXPECT_EQ(output2[0], 0);
+  EXPECT_EQ(output2[dest_row_stride - 1], 0);
+  EXPECT_EQ(output2[dest_byte_count - 1], 0);
+}
+
+TEST(Convolver, SeparableSingleConvolutionEdges) {
+  // The purpose of this test is to check if the implementation treats correctly
+  // edges of the image.
+  static const int kImgWidth = 600;
+  static const int kImgHeight = 800;
+  static const int kChannelCount = 3;
+  static const int kStrideSlack = 22;
+  static const int kChannel = 1;
+  ConvolutionFilter1D filter;
+  const float box[5] = { 0.2f, 0.2f, 0.2f, 0.2f, 0.2f };
+  filter.AddFilter(0, box, 5);
+
+  // Allocate a source image and set to 0.
+  int src_row_stride = kImgWidth * kChannelCount + kStrideSlack;
+  int src_byte_count = src_row_stride * kImgHeight;
+  std::vector<unsigned char> input(src_byte_count);
+
+  // Draw a frame around the image.
+  for (int i = 0; i < src_byte_count; ++i) {
+    int row = i / src_row_stride;
+    int col = i % src_row_stride / kChannelCount;
+    int channel = i % src_row_stride % kChannelCount;
+    if (channel != kChannel || col > kImgWidth) {
+      input[i] = 255;
+    } else if (row == 0 || col == 0 ||
+               col == kImgWidth - 1 || row == kImgHeight - 1) {
+      input[i] = 100;
+    } else if (row == 1 || col == 1 ||
+               col == kImgWidth - 2 || row == kImgHeight - 2) {
+      input[i] = 200;
+    } else {
+      input[i] = 0;
+    }
+  }
+
+  // Destination will be a single channel image with stide matching width.
+  int dest_row_stride = kImgWidth;
+  int dest_byte_count = dest_row_stride * kImgHeight;
+  std::vector<unsigned char> output;
+  output.resize(dest_byte_count);
+
+  // Apply convolution in X.
+  SingleChannelConvolveX1D(&input[0], src_row_stride, 1, kChannelCount,
+                           filter, SkISize::Make(kImgWidth, kImgHeight),
+                           &output[0], dest_row_stride, 0, 1, false);
+
+  // Sadly, comparison is not as simple as retaining all values.
+  int invalid_values = 0;
+  const unsigned char first_value = output[0];
+  EXPECT_TRUE(std::abs(100 - first_value) <= 1);
+  for (int i = 0; i < dest_row_stride; ++i) {
+    if (output[i] != first_value)
+      ++invalid_values;
+  }
+  EXPECT_EQ(0, invalid_values);
+
+  int test_row = 22;
+  EXPECT_NEAR(output[test_row * dest_row_stride], 100, 1);
+  EXPECT_NEAR(output[test_row * dest_row_stride + 1], 80, 1);
+  EXPECT_NEAR(output[test_row * dest_row_stride + 2], 60, 1);
+  EXPECT_NEAR(output[test_row * dest_row_stride + 3], 40, 1);
+  EXPECT_NEAR(output[(test_row + 1) * dest_row_stride - 1], 100, 1);
+  EXPECT_NEAR(output[(test_row + 1) * dest_row_stride - 2], 80, 1);
+  EXPECT_NEAR(output[(test_row + 1) * dest_row_stride - 3], 60, 1);
+  EXPECT_NEAR(output[(test_row + 1) * dest_row_stride - 4], 40, 1);
+
+  SingleChannelConvolveY1D(&input[0], src_row_stride, 1, kChannelCount,
+                           filter, SkISize::Make(kImgWidth, kImgHeight),
+                           &output[0], dest_row_stride, 0, 1, false);
+
+  int test_column = 42;
+  EXPECT_NEAR(output[test_column], 100, 1);
+  EXPECT_NEAR(output[test_column + dest_row_stride], 80, 1);
+  EXPECT_NEAR(output[test_column + dest_row_stride * 2], 60, 1);
+  EXPECT_NEAR(output[test_column + dest_row_stride * 3], 40, 1);
+
+  EXPECT_NEAR(output[test_column + dest_row_stride * (kImgHeight - 1)], 100, 1);
+  EXPECT_NEAR(output[test_column + dest_row_stride * (kImgHeight - 2)], 80, 1);
+  EXPECT_NEAR(output[test_column + dest_row_stride * (kImgHeight - 3)], 60, 1);
+  EXPECT_NEAR(output[test_column + dest_row_stride * (kImgHeight - 4)], 40, 1);
+}
+
 }  // namespace skia