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

skia/ext/convolver.cc

Index: skia/ext/convolver.cc
diff --git a/skia/ext/convolver.cc b/skia/ext/convolver.cc
index cbfa9315930edcc9664165a48dc667c2d7fba3b5..8ec2efe62daea4f23b2a92f14a3bbc2f92cc0ccb 100644
--- a/skia/ext/convolver.cc
+++ b/skia/ext/convolver.cc
@@ -6,6 +6,7 @@
 
 #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 +23,17 @@ 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 +283,7 @@ 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 +311,27 @@ 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 +506,165 @@ 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)
+    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;

[Stephen White, 2013/04/12 12:15:21]

Does this handle the case where the image width is less than the filter width?

+      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)
+    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