Refactored-out the code of thumbnaling algorithm from thumbnail_tab_helper.

chrome/browser/thumbnails/simple_thumbnail_crop.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/simple_thumbnail_crop.h"
+
+#include "base/metrics/histogram.h"
+#include "content/public/browser/browser_thread.h"
+#include "skia/ext/platform_canvas.h"
+#include "ui/gfx/color_utils.h"
+#include "ui/gfx/screen.h"
+#include "ui/gfx/scrollbar_size.h"
+#include "ui/gfx/size_conversions.h"
+#include "ui/gfx/skbitmap_operations.h"
+
+namespace {
+static const char kThumbnailHistogramName[] = "Thumbnail.ComputeMS";
+}
+
+namespace thumbnails {
+
+SimpleThumbnailCrop::SimpleThumbnailCrop(const gfx::Size& target_size)
+    : target_size_(target_size) {
+}

[sky, 2013/01/17 20:18:35]

DCHECK size not empty

[motek., 2013/01/18 18:38:41]

Done.

+
+SimpleThumbnailCrop::~SimpleThumbnailCrop() {

[sky, 2013/01/17 20:18:35]

Order of methods in .cc should match header.

[motek., 2013/01/18 18:38:41]

Done.

+}
+
+void SimpleThumbnailCrop::ProcessBitmap(ThumbnailingContext* context,
+                                        const SkBitmap& bitmap,
+                                        const ConsumerCallback& callback) {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+  if (bitmap.isNull() || bitmap.empty())
+    return;
+
+  SkBitmap thumbnail = CreateThumbnail(bitmap,
+                                       GetThumbnailSizeInPixel(),
+                                       &context->clip_result);
+
+  context->score.boring_score = CalculateBoringScore(thumbnail);
+  context->score.good_clipping =
+      (context->clip_result == kWiderThanTall ||
+       context->clip_result == kTallerThanWide ||
+       context->clip_result == kNotClipped);
+
+  callback.Run(context, thumbnail);
+}
+
+ClipResult SimpleThumbnailCrop::GetCanvasCopyInfo(
+    const gfx::Size& source_size,
+    ui::ScaleFactor scale_factor,
+    gfx::Rect* clipping_rect,
+    gfx::Size* target_size) const {
+  ClipResult clip_result = thumbnails::kNotClipped;

[sky, 2013/01/17 20:18:36]

Are you guaranteed source_size is not empty? If so add a DCHECK, otherwise
return.

[motek., 2013/01/18 18:38:41]

This is a good question. This is refactored code, so I am trying to reproduce
pre-existing model. It appears that the condition where source_size.IsEmpty (or
rather, when any dimension of source_size is less than (non-empty!) target_size
is treated explicitly by GetClippingRect. 
I checked follow-ups in CopyFromBackingStore and found that this is not handled
consistently. 
So, I made it here required AND added check for that at the call point.

+  *clipping_rect = GetClippingRect(source_size, target_size_, &clip_result);
+  *target_size = GetCopySizeForThumbnail(scale_factor, target_size_);
+  return clip_result;
+}
+
+gfx::Rect SimpleThumbnailCrop::GetClippingRect(const gfx::Size& source_size,
+                                               const gfx::Size& desired_size,
+                                               ClipResult* clip_result) {
+  DCHECK(clip_result);
+
+  float desired_aspect =
+      static_cast<float>(desired_size.width()) / desired_size.height();
+
+  // Get the clipping rect so that we can preserve the aspect ratio while
+  // filling the destination.
+  gfx::Rect clipping_rect;
+  if (source_size.width() < desired_size.width() ||
+      source_size.height() < desired_size.height()) {
+    // Source image is smaller: we clip the part of source image within the
+    // dest rect, and then stretch it to fill the dest rect. We don't respect
+    // the aspect ratio in this case.
+    clipping_rect = gfx::Rect(desired_size);
+    *clip_result = thumbnails::kSourceIsSmaller;
+  } else {
+    float src_aspect =
+        static_cast<float>(source_size.width()) / source_size.height();
+    if (src_aspect > desired_aspect) {
+      // Wider than tall, clip horizontally: we center the smaller
+      // thumbnail in the wider screen.
+      int new_width = static_cast<int>(source_size.height() * desired_aspect);
+      int x_offset = (source_size.width() - new_width) / 2;
+      clipping_rect.SetRect(x_offset, 0, new_width, source_size.height());
+      *clip_result = (src_aspect >= ThumbnailScore::kTooWideAspectRatio) ?
+          thumbnails::kTooWiderThanTall :
+          thumbnails::kWiderThanTall;
+    } else if (src_aspect < desired_aspect) {
+      clipping_rect =
+          gfx::Rect(source_size.width(), source_size.width() / desired_aspect);
+      *clip_result = thumbnails::kTallerThanWide;
+    } else {
+      clipping_rect = gfx::Rect(source_size);
+      *clip_result = thumbnails::kNotClipped;
+    }
+  }
+  return clipping_rect;
+}
+
+// Returns the size used by RenderWidgetHost::CopyFromBackingStore.
+//
+// The size is calculated in such a way that the copied size in pixel becomes
+// equal to (f * kThumbnailWidth, f * kThumbnailHeight), where f is the scale
+// of ui::SCALE_FACTOR_200P. Since RenderWidgetHost::CopyFromBackingStore takes
+// the size in DIP, we need to adjust the size based on |view|'s device scale
+// factor in order to copy the pixels with the size above.
+//
+// The copied size was chosen for the following reasons.
+//
+// 1. When the scale factor of the primary monitor is ui::SCALE_FACTOR_200P, the
+// generated thumbnail size is (f * kThumbnailWidth, f * kThumbnailHeight).
+// In order to avoid degrading the image quality by magnification, the size
+// of the copied pixels should be equal to or larger than this thumbnail size.
+//
+// 2. RenderWidgetHost::CopyFromBackingStore can be costly especially when
+// it is necessary to read back the web contents image data from GPU. As the
+// cost is roughly propotional to the number of the copied pixels, the size of
+// the copied pixels should be as small as possible.
+//
+// When the scale factor of the primary monitor is ui::SCALE_FACTOR_100P,
+// we still copy the pixels with the same size as ui::SCALE_FACTOR_200P because
+// the resampling method used in RenderWidgetHost::CopyFromBackingStore is not
+// good enough for the resampled image to be used directly for the thumbnail
+// (http://crbug.com/141235). We assume this is not an issue in case of
+// ui::SCALE_FACTOR_200P because the high resolution thumbnail on high density
+// display alleviates the aliasing.
+// TODO(mazda): Copy the pixels with the smaller size in the case of
+// ui::SCALE_FACTOR_100P once the resampling method has been improved.
+// static
+gfx::Size SimpleThumbnailCrop::GetCopySizeForThumbnail(
+    ui::ScaleFactor scale_factor, const gfx::Size& thumbnail_size) {

[sky, 2013/01/17 20:18:36]

wrap each param to its own line.

[motek., 2013/01/18 18:38:41]

Done.

+  gfx::Size copy_size(thumbnail_size);
+  switch (scale_factor) {
+    case ui::SCALE_FACTOR_100P:
+      copy_size = gfx::ToFlooredSize(gfx::ScaleSize(
+          copy_size, ui::GetScaleFactorScale(ui::SCALE_FACTOR_200P)));
+      break;
+    case ui::SCALE_FACTOR_200P:
+      // Use the size as-is.
+      break;
+    default:
+      DLOG(WARNING) << "Unsupported scale factor. Use the same copy size as "
+                    << "ui::SCALE_FACTOR_100P";
+      copy_size = gfx::ToFlooredSize(gfx::ScaleSize(
+          copy_size, ui::GetScaleFactorScale(ui::SCALE_FACTOR_200P)));
+      break;
+  }
+  return copy_size;
+}
+
+// Returns the size of the thumbnail stored in the database in pixel.
+gfx::Size SimpleThumbnailCrop::GetThumbnailSizeInPixel() const {
+  // Determine the resolution of the thumbnail based on the maximum scale
+  // factor.
+  // TODO(mazda|oshima): Update thumbnail when the max scale factor changes.
+  // crbug.com/159157.
+  float max_scale_factor =
+      ui::GetScaleFactorScale(ui::GetMaxScaleFactor());
+  return gfx::ToFlooredSize(gfx::ScaleSize(target_size_, max_scale_factor));
+}
+
+SkBitmap SimpleThumbnailCrop::GetClippedBitmap(const SkBitmap& bitmap,
+                                               int desired_width,
+                                               int desired_height,
+                                               ClipResult* clip_result) {
+  gfx::Rect clipping_rect =
+      GetClippingRect(gfx::Size(bitmap.width(), bitmap.height()),
+                      gfx::Size(desired_width, desired_height),
+                      clip_result);
+  SkIRect src_rect = { clipping_rect.x(), clipping_rect.y(),
+                       clipping_rect.right(), clipping_rect.bottom() };
+  SkBitmap clipped_bitmap;
+  bitmap.extractSubset(&clipped_bitmap, src_rect);
+  return clipped_bitmap;
+}
+
+// Creates a downsampled thumbnail from the given bitmap.
+// store. The returned bitmap will be isNull if there was an error creating it.
+SkBitmap SimpleThumbnailCrop::CreateThumbnail(const SkBitmap& bitmap,
+                                              const gfx::Size& desired_size,
+                                              ClipResult* clip_result) {
+  base::TimeTicks begin_compute_thumbnail = base::TimeTicks::Now();
+
+  SkBitmap clipped_bitmap;
+  if (*clip_result == thumbnails::kUnprocessed) {
+    // Clip the pixels that will commonly hold a scrollbar, which looks bad in
+    // thumbnails.
+    int scrollbar_size = gfx::scrollbar_size();
+    SkIRect scrollbarless_rect =
+        { 0, 0,
+          std::max(1, bitmap.width() - scrollbar_size),
+          std::max(1, bitmap.height() - scrollbar_size) };
+    SkBitmap bmp;
+    bitmap.extractSubset(&bmp, scrollbarless_rect);
+
+    clipped_bitmap = GetClippedBitmap(
+        bmp, desired_size.width(), desired_size.height(), clip_result);
+  } else {
+    clipped_bitmap = bitmap;
+  }
+
+  // Need to resize it to the size we want, so downsample until it's
+  // close, and let the caller make it the exact size if desired.
+  SkBitmap result = SkBitmapOperations::DownsampleByTwoUntilSize(
+      clipped_bitmap, desired_size.width(), desired_size.height());
+#if !defined(USE_AURA)
+  // This is a bit subtle. SkBitmaps are refcounted, but the magic
+  // ones in PlatformCanvas can't be assigned to SkBitmap with proper
+  // refcounting.  If the bitmap doesn't change, then the downsampler
+  // will return the input bitmap, which will be the reference to the
+  // weird PlatformCanvas one insetad of a regular one. To get a
+  // regular refcounted bitmap, we need to copy it.
+  //
+  // On Aura, the PlatformCanvas is platform-independent and does not have
+  // any native platform resources that can't be refounted, so this issue does
+  // not occur.
+  //
+  // Note that GetClippedBitmap() does extractSubset() but it won't copy
+  // the pixels, hence we check result size == clipped_bitmap size here.
+  if (clipped_bitmap.width() == result.width() &&
+      clipped_bitmap.height() == result.height())
+    clipped_bitmap.copyTo(&result, SkBitmap::kARGB_8888_Config);
+#endif
+
+  HISTOGRAM_TIMES(kThumbnailHistogramName,
+                  base::TimeTicks::Now() - begin_compute_thumbnail);
+  return result;
+}
+
+double SimpleThumbnailCrop::CalculateBoringScore(const SkBitmap& bitmap) {
+  if (bitmap.isNull() || bitmap.empty())
+    return 1.0;
+  int histogram[256] = {0};
+  color_utils::BuildLumaHistogram(bitmap, histogram);
+
+  int color_count = *std::max_element(histogram, histogram + 256);
+  int pixel_count = bitmap.width() * bitmap.height();
+  return static_cast<double>(color_count) / pixel_count;
+}
+
+}