Decode GIF image frames on demand.

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

Index: Source/core/platform/image-decoders/ImageDecoder.h
diff --git a/Source/core/platform/image-decoders/ImageDecoder.h b/Source/core/platform/image-decoders/ImageDecoder.h
index 83dbdc070a653ec942c629b7e635ae8c4f37560e..1ad13f7820441d17a52618b7775f0b0225f48a09 100644
--- a/Source/core/platform/image-decoders/ImageDecoder.h
+++ b/Source/core/platform/image-decoders/ImageDecoder.h
@@ -117,6 +117,7 @@ namespace WebCore {
         FrameDisposalMethod disposalMethod() const { return m_disposalMethod; }
         bool premultiplyAlpha() const { return m_premultiplyAlpha; }
         void reportMemoryUsage(MemoryObjectInfo*) const;
+        size_t requiredPreviousFrameIndex() const { return m_requiredPreviousFrameIndex; }
 
         void setHasAlpha(bool alpha);
         void setOriginalFrameRect(const IntRect& r) { m_originalFrameRect = r; }
@@ -124,6 +125,7 @@ namespace WebCore {
         void setDuration(unsigned duration) { m_duration = duration; }
         void setDisposalMethod(FrameDisposalMethod method) { m_disposalMethod = method; }
         void setPremultiplyAlpha(bool premultiplyAlpha) { m_premultiplyAlpha = premultiplyAlpha; }
+        void setRequiredPreviousFrameIndex(size_t previousFrameIndex) { m_requiredPreviousFrameIndex = previousFrameIndex; }
 
         inline void setRGBA(int x, int y, unsigned r, unsigned g, unsigned b, unsigned a)
         {
@@ -202,6 +204,10 @@ namespace WebCore {
         unsigned m_duration;
         FrameDisposalMethod m_disposalMethod;
         bool m_premultiplyAlpha;
+
+        // The frame that must be decoded before this frame can be decoded.
+        // |notFound| if this frame doesn't require any previous frame.
+        size_t m_requiredPreviousFrameIndex;
     };
 
     // ImageDecoder is a base for all format-specific decoders
@@ -371,10 +377,10 @@ namespace WebCore {
 
         bool failed() const { return m_failed; }
 
-        // Clears decoded pixel data from before the provided frame unless that
+        // Clears decoded pixel data except the provided frame unless that
         // data may be needed to decode future frames (e.g. due to GIF frame
         // compositing).
-        virtual void clearFrameBufferCache(size_t) { }
+        void clearFrameBufferCache(size_t clearExceptFrame);
 
 #if ENABLE(IMAGE_DECODER_DOWN_SAMPLING)
         void setMaxNumPixels(int m) { m_maxNumPixels = m; }
@@ -395,6 +401,12 @@ namespace WebCore {
         }
 
     protected:
+        // Finds the previous required frame for decoding the given frame.
+        // Returns notFound if this frame doesn't require any previous frame.
+        // Subclasses should call this function and cache the result by calling
+        // ImageFrame::setRequiredPreviousFrame() when a new frame is created.
+        size_t findRequiredPreviousFrame(size_t frameIndex);
+
         void prepareScaleDataIfNecessary();
         int upperBoundScaledX(int origX, int searchStart = 0);
         int lowerBoundScaledX(int origX, int searchStart = 0);