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

ui/gfx/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 "ui/gfx/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 InputIterator::value_type, int> >
+      deque_type;
+  deque_type slider;
+  int front_tail_length = window_size / 2;
+  int i = 0;
+  DCHECK(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 color_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());
+
+  int tail_length = static_cast<int>(4.0f * kernel_sigma + 0.5f);
+  int kernel_size = tail_length * 2 + 1;
+  float smoother_weights[kernel_size];
+  float sigmasq = kernel_sigma * kernel_sigma;
+  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;
+
+  float gradient_weights[kernel_size];
+  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, kernel_size);
+  gradient_filter.AddFilter(0, gradient_weights, kernel_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::SingleChannelConvolve1D_X(
+      static_cast<const uint8*>(input_bitmap->getPixels()),

[Stephen White, 2013/04/10 15:20:31]

Suggestion:  use input_bitmap->getAddr8(0, 0) to hide the cast.  (Here, and many
places below.)

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

Done.

+      static_cast<int>(input_bitmap->rowBytes()),
+      0, input_bitmap->bytesPerPixel(),
+      smoothing_filter,
+      image_size,
+      static_cast<uint8*>(intermediate.getPixels()),
+      static_cast<int>(intermediate.rowBytes()),
+      0, intermediate.bytesPerPixel(), false);
+  skia::SingleChannelConvolve1D_Y(
+      static_cast<const uint8*>(intermediate.getPixels()),
+      static_cast<int>(intermediate.rowBytes()),
+      0, intermediate.bytesPerPixel(),
+      smoothing_filter,
+      image_size,
+      static_cast<uint8*>(input_bitmap->getPixels()),
+      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::SingleChannelConvolve1D_X(
+      static_cast<const uint8*>(input_bitmap->getPixels()),
+      static_cast<int>(input_bitmap->rowBytes()),
+      0, input_bitmap->bytesPerPixel(),
+      gradient_filter,
+      image_size,
+      static_cast<uint8*>(intermediate.getPixels()),
+      static_cast<int>(intermediate.rowBytes()),
+      0, intermediate.bytesPerPixel(), true);
+  skia::SingleChannelConvolve1D_Y(
+      static_cast<const uint8*>(input_bitmap->getPixels()),
+      static_cast<int>(input_bitmap->rowBytes()),
+      0, input_bitmap->bytesPerPixel(),
+      gradient_filter,
+      image_size,
+      static_cast<uint8*>(intermediate2.getPixels()),
+      static_cast<int>(intermediate2.rowBytes()),
+      0, intermediate2.bytesPerPixel(), true);
+
+  uint grad_max = 0;
+  for (int r = 0; r < image_size.height(); ++r) {
+    const uint8* grad_x_row = static_cast<const uint8*>(
+        intermediate.getPixels()) + r * intermediate.rowBytes();
+    const uint8* grad_y_row = static_cast<const uint8*>(
+        intermediate2.getPixels()) + r * intermediate2.rowBytes();
+    for (int c = 0; c < image_size.width(); ++c) {
+      uint grad_x = grad_x_row[c];
+      uint 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;

[Stephen White, 2013/04/10 15:20:31]

Not that it really matters, but these could probably just be log, instead of
log10.

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

They certainly could. I need log2 which is not available, so I use the property
of logarithm in general. I dunno, log10 seemed just more 'decimal' than at
natural base. 

+  for (int r = 0; r < image_size.height(); ++r) {
+    const uint8* grad_x_row = static_cast<const uint8*>(
+        intermediate.getPixels()) + r * intermediate.rowBytes();

[Stephen White, 2013/04/10 15:20:31]

intermediate.getAddr8(0, r) should work here, and below.

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

Done.

+    const uint8* grad_y_row = static_cast<const uint8*>(
+        intermediate2.getPixels()) + r * intermediate2.rowBytes();
+    uint8* target_row = static_cast<uint8*>(
+        input_bitmap->getPixels()) + r * input_bitmap->rowBytes();
+    for (int c = 0; c < image_size.width(); ++c) {
+      uint grad_x = grad_x_row[c];
+      uint 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->resize(0);
+  columns->resize(0);
+  rows->resize(area.height(), 0);
+  columns->resize(area.width(), 0);
+
+  for (int r = 0; r < area.height(); ++r) {
+    const uint8* image_row = static_cast<const uint8*>(
+        input_bitmap.getPixels()) +
+        (r + area.y()) * input_bitmap.rowBytes() +
+        area.x();  // Points to the first byte of the row in the rectangle.
+    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() + 0.5);

[Stephen White, 2013/04/10 15:20:31]

0.5 should probably be 0.5f, unless you really need the last + to be done in
doubles.

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

Done.

+    int row_window_size = 1 +
+        2 * static_cast<int>(0.5f * area.height() / target_size.height() + 0.5);

[Stephen White, 2013/04/10 15:20:31]

Same here.

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

Done.

+
+    // 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) {

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

Be consistent whether you use iterators or indices, either use indices
everywhere or iterators everywhere.

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

I think I am consistent. I use iterators everywhere except that iteration over
histogram below, where I actually need the index as a result. 

+    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 (unsigned i = 1; i < histogram.size(); ++i) {

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

Use size_t, here and throughout for the indices.

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

Done.

+    w2 += histogram[i];
+    t2 += (0.5 + i) * histogram[i];
+  }
+
+  int 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 (unsigned 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 tgt_row_count = std::count(rows.begin(), rows.end(), true);
+  unsigned tgt_column_count = std::count(columns.begin(), columns.end(), true);
+
+  if (tgt_row_count == 0 || tgt_column_count == 0)
+    return NULL;  // Not quite an error, so no DCHECK. Just return null.
+
+  if (tgt_row_count == rows.size() && tgt_column_count == columns.size())
+    return NULL;  // Equivalent of the situation where the target is empty.
+
+  // Declare and allocate the target image.
+  SkBitmap target;
+  target.setConfig(bitmap.config(), tgt_column_count, tgt_row_count);
+  target.allocPixels();
+
+  int tgt_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()) + tgt_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);
+    }
+    tgt_row++;
+  }
+
+  return new SkBitmap(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 (!ComputePrincipalComponentImage(source_bitmap, &reduced_color)) {
+    // CCIR601 luminance conversion vector.
+    gfx::Vector3dF transform(0.299f, 0.587f, 0.114f);
+    if (!ApplyColorReduction(source_bitmap, transform, true, &reduced_color)) {
+      DLOG(WARNING) << "Failed to compute luminance image from a screenshot. "
+                    << "Cannot compute retargetted thumbnail.";
+      return NULL;
+    }
+    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);
+}
+
+}  // color_utils