Save a bitmap copy when advancing to dependent GIF and WebP animation frames

third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp

Index: third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
diff --git a/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp b/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
index 72f2c834ae95c11df7e2e03cee1975a0e5591c08..7b4f71f0a7fa3aa837fba4315afc042910cd12a0 100644
--- a/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
+++ b/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
@@ -356,12 +356,21 @@ bool GIFImageDecoder::initFrameBuffer(size_t frameIndex)
         if (!buffer->setSize(size().width(), size().height()))
             return setFailed();
     } else {
-        const ImageFrame* prevBuffer = &m_frameBufferCache[requiredPreviousFrameIndex];
+        ImageFrame* prevBuffer = &m_frameBufferCache[requiredPreviousFrameIndex];
         ASSERT(prevBuffer->getStatus() == ImageFrame::FrameComplete);
 
-        // Preserve the last frame as the starting state for this frame.
-        if (!buffer->copyBitmapData(*prevBuffer))
-            return setFailed();
+        if (buffer->getDisposalMethod() == ImageFrame::DisposeOverwritePrevious) {
+            // The next frame will also use |prevBuffer| as its starting state,
+            // so we can't take over its image data as we do below.  Copy the
+            // data instead.
+            if (!buffer->copyBitmapData(*prevBuffer))
+                return setFailed();
+        } else {
+            // This is the only frame to use |prevBuffer| as its starting state,
+            // and we'll always clear the old frame data after initializing this
+            // frame anyway, so we can save time by just moving its data over.

[Peter Kasting, 2016/07/20 01:00:28]

Nit: I still think you should explicitly say what ensures we always clear the
old frame data, so if that changes in the future it will be more clear.

It doesn't seem like this is true because of m_purgeAggressively, since that
itself is not always set.  So it seems like this must be due to
ImageFrameGenerator::decode() calling clearCacheExceptFrame(), or else your
statement that this always happens is in fact incorrect.

+            buffer->takeBitmapData(prevBuffer);
+        }
 
         if (prevBuffer->getDisposalMethod() == ImageFrame::DisposeOverwriteBgcolor) {
             // We want to clear the previous frame to transparent, without