1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #include "chrome/browser/thumbnails/simple_thumbnail_crop.h"
7 #include "base/metrics/histogram.h"
8 #include "content/public/browser/browser_thread.h"
9 #include "skia/ext/platform_canvas.h"
10 #include "ui/gfx/color_utils.h"
11 #include "ui/gfx/image/image_skia.h"
12 #include "ui/gfx/screen.h"
13 #include "ui/gfx/scrollbar_size.h"
14 #include "ui/gfx/size_conversions.h"
15 #include "ui/gfx/skbitmap_operations.h"
18 static const char kThumbnailHistogramName[] = "Thumbnail.ComputeMS";
21 namespace thumbnails {
23 SimpleThumbnailCrop::SimpleThumbnailCrop(const gfx::Size& target_size)
24 : target_size_(target_size) {
25 DCHECK(!target_size.IsEmpty());
28 ClipResult SimpleThumbnailCrop::GetCanvasCopyInfo(
29 const gfx::Size& source_size,
30 ui::ScaleFactor scale_factor,
31 gfx::Rect* clipping_rect,
32 gfx::Size* target_size) const {
33 DCHECK(!source_size.IsEmpty());
34 ClipResult clip_result = thumbnails::CLIP_RESULT_NOT_CLIPPED;
35 *clipping_rect = GetClippingRect(source_size, target_size_, &clip_result);
36 *target_size = GetCopySizeForThumbnail(scale_factor, target_size_);
40 void SimpleThumbnailCrop::ProcessBitmap(
41 scoped_refptr<ThumbnailingContext> context,
42 const ConsumerCallback& callback,
43 const SkBitmap& bitmap) {
44 DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
45 if (bitmap.isNull() || bitmap.empty())
48 SkBitmap thumbnail = CreateThumbnail(
50 ComputeTargetSizeAtMaximumScale(target_size_),
51 &context->clip_result);
53 context->score.boring_score = CalculateBoringScore(thumbnail);
54 context->score.good_clipping =
55 (context->clip_result == CLIP_RESULT_WIDER_THAN_TALL ||
56 context->clip_result == CLIP_RESULT_TALLER_THAN_WIDE ||
57 context->clip_result == CLIP_RESULT_NOT_CLIPPED);
59 callback.Run(*context.get(), thumbnail);
62 double SimpleThumbnailCrop::CalculateBoringScore(const SkBitmap& bitmap) {
63 if (bitmap.isNull() || bitmap.empty())
65 int histogram[256] = {0};
66 color_utils::BuildLumaHistogram(bitmap, histogram);
68 int color_count = *std::max_element(histogram, histogram + 256);
69 int pixel_count = bitmap.width() * bitmap.height();
70 return static_cast<double>(color_count) / pixel_count;
73 SkBitmap SimpleThumbnailCrop::GetClippedBitmap(const SkBitmap& bitmap,
76 ClipResult* clip_result) {
77 gfx::Rect clipping_rect =
78 GetClippingRect(gfx::Size(bitmap.width(), bitmap.height()),
79 gfx::Size(desired_width, desired_height),
81 SkIRect src_rect = { clipping_rect.x(), clipping_rect.y(),
82 clipping_rect.right(), clipping_rect.bottom() };
83 SkBitmap clipped_bitmap;
84 bitmap.extractSubset(&clipped_bitmap, src_rect);
85 return clipped_bitmap;
88 // Returns the size used by RenderWidgetHost::CopyFromBackingStore.
90 // The size is calculated in such a way that the copied size in pixel becomes
91 // equal to (f * kThumbnailWidth, f * kThumbnailHeight), where f is the scale
92 // of ui::SCALE_FACTOR_200P. Since RenderWidgetHost::CopyFromBackingStore takes
93 // the size in DIP, we need to adjust the size based on |view|'s device scale
94 // factor in order to copy the pixels with the size above.
96 // The copied size was chosen for the following reasons.
98 // 1. When the scale factor of the primary monitor is ui::SCALE_FACTOR_200P, the
99 // generated thumbnail size is (f * kThumbnailWidth, f * kThumbnailHeight).
100 // In order to avoid degrading the image quality by magnification, the size
101 // of the copied pixels should be equal to or larger than this thumbnail size.
103 // 2. RenderWidgetHost::CopyFromBackingStore can be costly especially when
104 // it is necessary to read back the web contents image data from GPU. As the
105 // cost is roughly propotional to the number of the copied pixels, the size of
106 // the copied pixels should be as small as possible.
108 // When the scale factor of the primary monitor is ui::SCALE_FACTOR_100P,
109 // we still copy the pixels with the same size as ui::SCALE_FACTOR_200P because
110 // the resampling method used in RenderWidgetHost::CopyFromBackingStore is not
111 // good enough for the resampled image to be used directly for the thumbnail
112 // (http://crbug.com/141235). We assume this is not an issue in case of
113 // ui::SCALE_FACTOR_200P because the high resolution thumbnail on high density
114 // display alleviates the aliasing.
115 // TODO(mazda): Copy the pixels with the smaller size in the case of
116 // ui::SCALE_FACTOR_100P once the resampling method has been improved.
118 gfx::Size SimpleThumbnailCrop::GetCopySizeForThumbnail(
119 ui::ScaleFactor scale_factor,
120 const gfx::Size& thumbnail_size) {
121 gfx::Size copy_size(thumbnail_size);
122 switch (scale_factor) {
123 case ui::SCALE_FACTOR_100P:
124 copy_size = gfx::ToFlooredSize(gfx::ScaleSize(
125 copy_size, ui::GetImageScale(ui::SCALE_FACTOR_200P)));
127 case ui::SCALE_FACTOR_200P:
128 // Use the size as-is.
131 DLOG(WARNING) << "Unsupported scale factor. Use the same copy size as "
132 << "ui::SCALE_FACTOR_100P";
133 copy_size = gfx::ToFlooredSize(gfx::ScaleSize(
134 copy_size, gfx::ImageSkia::GetMaxSupportedScale()));
140 gfx::Rect SimpleThumbnailCrop::GetClippingRect(const gfx::Size& source_size,
141 const gfx::Size& desired_size,
142 ClipResult* clip_result) {
145 float desired_aspect =
146 static_cast<float>(desired_size.width()) / desired_size.height();
148 // Get the clipping rect so that we can preserve the aspect ratio while
149 // filling the destination.
150 gfx::Rect clipping_rect;
151 if (source_size.width() < desired_size.width() ||
152 source_size.height() < desired_size.height()) {
153 // Source image is smaller: we clip the part of source image within the
154 // dest rect, and then stretch it to fill the dest rect. We don't respect
155 // the aspect ratio in this case.
156 clipping_rect = gfx::Rect(desired_size);
157 *clip_result = thumbnails::CLIP_RESULT_SOURCE_IS_SMALLER;
160 static_cast<float>(source_size.width()) / source_size.height();
161 if (src_aspect > desired_aspect) {
162 // Wider than tall, clip horizontally: we center the smaller
163 // thumbnail in the wider screen.
164 int new_width = static_cast<int>(source_size.height() * desired_aspect);
165 int x_offset = (source_size.width() - new_width) / 2;
166 clipping_rect.SetRect(x_offset, 0, new_width, source_size.height());
167 *clip_result = (src_aspect >= ThumbnailScore::kTooWideAspectRatio) ?
168 thumbnails::CLIP_RESULT_MUCH_WIDER_THAN_TALL :
169 thumbnails::CLIP_RESULT_WIDER_THAN_TALL;
170 } else if (src_aspect < desired_aspect) {
172 gfx::Rect(source_size.width(), source_size.width() / desired_aspect);
173 *clip_result = thumbnails::CLIP_RESULT_TALLER_THAN_WIDE;
175 clipping_rect = gfx::Rect(source_size);
176 *clip_result = thumbnails::CLIP_RESULT_NOT_CLIPPED;
179 return clipping_rect;
183 gfx::Size SimpleThumbnailCrop::ComputeTargetSizeAtMaximumScale(
184 const gfx::Size& given_size) {
185 // TODO(mazda|oshima): Update thumbnail when the max scale factor changes.
187 float max_scale_factor = gfx::ImageSkia::GetMaxSupportedScale();
188 return gfx::ToFlooredSize(gfx::ScaleSize(given_size, max_scale_factor));
191 SimpleThumbnailCrop::~SimpleThumbnailCrop() {
194 // Creates a downsampled thumbnail from the given bitmap.
195 // store. The returned bitmap will be isNull if there was an error creating it.
196 SkBitmap SimpleThumbnailCrop::CreateThumbnail(const SkBitmap& bitmap,
197 const gfx::Size& desired_size,
198 ClipResult* clip_result) {
199 base::TimeTicks begin_compute_thumbnail = base::TimeTicks::Now();
201 SkBitmap clipped_bitmap;
202 if (*clip_result == thumbnails::CLIP_RESULT_UNPROCESSED) {
203 // Clip the pixels that will commonly hold a scrollbar, which looks bad in
205 int scrollbar_size = gfx::scrollbar_size();
206 SkIRect scrollbarless_rect =
208 std::max(1, bitmap.width() - scrollbar_size),
209 std::max(1, bitmap.height() - scrollbar_size) };
211 bitmap.extractSubset(&bmp, scrollbarless_rect);
213 clipped_bitmap = GetClippedBitmap(
214 bmp, desired_size.width(), desired_size.height(), clip_result);
216 clipped_bitmap = bitmap;
219 // Need to resize it to the size we want, so downsample until it's
220 // close, and let the caller make it the exact size if desired.
221 SkBitmap result = SkBitmapOperations::DownsampleByTwoUntilSize(
222 clipped_bitmap, desired_size.width(), desired_size.height());
223 #if !defined(USE_AURA)
224 // This is a bit subtle. SkBitmaps are refcounted, but the magic
225 // ones in PlatformCanvas can't be assigned to SkBitmap with proper
226 // refcounting. If the bitmap doesn't change, then the downsampler
227 // will return the input bitmap, which will be the reference to the
228 // weird PlatformCanvas one insetad of a regular one. To get a
229 // regular refcounted bitmap, we need to copy it.
231 // On Aura, the PlatformCanvas is platform-independent and does not have
232 // any native platform resources that can't be refounted, so this issue does
235 // Note that GetClippedBitmap() does extractSubset() but it won't copy
236 // the pixels, hence we check result size == clipped_bitmap size here.
237 if (clipped_bitmap.width() == result.width() &&
238 clipped_bitmap.height() == result.height())
239 clipped_bitmap.copyTo(&result, kPMColor_SkColorType);
242 HISTOGRAM_TIMES(kThumbnailHistogramName,
243 base::TimeTicks::Now() - begin_compute_thumbnail);
247 } // namespace thumbnails