Animated WebP: Optimize decoding in case of seeking

Source/core/platform/image-decoders/ImageDecoder.cpp

Index: Source/core/platform/image-decoders/ImageDecoder.cpp
diff --git a/Source/core/platform/image-decoders/ImageDecoder.cpp b/Source/core/platform/image-decoders/ImageDecoder.cpp
index 917161f10527eb91c1636fd73868dfe04515499f..65ee260b91ed56fa263f9b9b669a70ab3ee327a4 100644
--- a/Source/core/platform/image-decoders/ImageDecoder.cpp
+++ b/Source/core/platform/image-decoders/ImageDecoder.cpp
@@ -152,7 +152,7 @@ void ImageDecoder::clearFrameBuffer(size_t frameIndex)
     m_frameBufferCache[frameIndex].clearPixelData();
 }
 
-size_t ImageDecoder::findRequiredPreviousFrame(size_t frameIndex)
+size_t ImageDecoder::findRequiredPreviousFrame(size_t frameIndex, bool frameRectIsOpaque)
 {
     ASSERT(frameIndex <= m_frameBufferCache.size());
     if (!frameIndex) {
@@ -160,6 +160,11 @@ size_t ImageDecoder::findRequiredPreviousFrame(size_t frameIndex)
         return notFound;
     }
 
+    const ImageFrame* const currBuffer = &m_frameBufferCache[frameIndex];

[Peter Kasting, 2013/08/23 01:14:02]

Nit: I might avoid this temp just because you only use it twice, in the same
statement...

If you do want to keep it, either remove the second const or make prevBuffer
const.

[urvang (Google), 2013/08/23 02:34:31]

I'd keep it as it makes the next line more readable. Made it "const ImageFrame*
currBuffer" to be consistent with 'prevBuffer'.

+    if ((frameRectIsOpaque || currBuffer->alphaBlendSource() == ImageFrame::BlendAtopBgcolor)
+        && currBuffer->originalFrameRect().contains(IntRect(IntPoint(), size())))
+        return notFound;
+
     // The starting state for this frame depends on the previous frame's
     // disposal method.
     size_t prevFrame = frameIndex - 1;