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 724e27c918dc3e3c7a5dcd00f2ef98bfa3cd2aeb..d57464e311533caafaf0d8cfd05eb1bf2f688287 100644
--- a/Source/core/platform/image-decoders/ImageDecoder.h
+++ b/Source/core/platform/image-decoders/ImageDecoder.h
@@ -115,6 +115,11 @@ namespace WebCore {
         FrameDisposalMethod disposalMethod() const { return m_disposalMethod; }
         bool premultiplyAlpha() const { return m_premultiplyAlpha; }
         void reportMemoryUsage(MemoryObjectInfo*) const;
+        size_t requiredPreviousFrameIndex() const
+        {
+            ASSERT(m_requiredPreviousFrameIndexValid);
+            return m_requiredPreviousFrameIndex;
+        }
 
         void setHasAlpha(bool alpha);
         void setOriginalFrameRect(const IntRect& r) { m_originalFrameRect = r; }
@@ -122,6 +127,13 @@ 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;
+#ifndef NDEBUG
+            m_requiredPreviousFrameIndexValid = true;
+#endif
+        }
 
         inline void setRGBA(int x, int y, unsigned r, unsigned g, unsigned b, unsigned a)
         {
@@ -200,6 +212,13 @@ 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.

[Peter Kasting, 2013/05/29 02:02:18]

Nit: Add this at end:

This is used by ImageDecoder::clearCacheExceptFrame(), and will never be read
for image formats that do not have multiple frames.

[Xianzhu, 2013/05/29 18:37:01]

Done.

+        size_t m_requiredPreviousFrameIndex;
+#ifndef NDEBUG
+        bool m_requiredPreviousFrameIndexValid;
+#endif
     };
 
     // ImageDecoder is a base for all format-specific decoders
@@ -358,10 +377,12 @@ 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) { }
+        // compositing). Pass a value greater than the frame count to clear
+        // decoded pixel data of all frames.
+        // Returns number of bytes of the cleared frames.

[Peter Kasting, 2013/05/29 02:02:18]

Nit: How about this comment:

Clears decoded pixel data from all frames except the provided frame, unless that
frame has status FrameEmpty, in which case we instead preserve the most recent
frame whose data is required in order to decode this frame.  Callers may pass
WTF::notFound to clear all frames.

Returns the number of bytes of frame data actually cleared.

[Xianzhu, 2013/05/29 18:37:01]

Done.

+        size_t clearCacheExceptFrame(size_t);
 
         // If the image has a cursor hot-spot, stores it in the argument
         // and returns true. Otherwise returns false.
@@ -378,6 +399,18 @@ namespace WebCore {
         }
 
     protected:
+        // Finds the previous required frame for decoding the given frame based
+        // on the parsed meta data. 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,
+        // and must ensure all previous frame's required previous frame has been
+        // cached before calling this method.

[Peter Kasting, 2013/05/29 02:02:18]

Nit: How about this comment:

Calculates the most recent frame whose image data may be needed in order to
decode frame |frameIndex|, based on frame disposal methods.  If no previous
frame's data is required, returns WTF::notFound.

This function requires that the previous frame's |m_requiredPreviousFrameIndex|
member has been set correctly.  The easiest way to ensure this is for subclasses
to call this method and store the result on the frame via
setRequiredPreviousFrameIndex() as soon as the frame has been created and parsed
sufficiently to determine the disposal method; assuming this happens for all
frames in order, the required invariant will hold.

Image formats which do not use more than one frame do not need to worry about
this; see comments on ImageFrame::m_requiredPreviousFrameIndex.

[Xianzhu, 2013/05/29 18:37:01]

Done.

+        size_t findRequiredPreviousFrame(size_t frameIndex);
+
+        virtual void clearFrameBuffer(size_t frameIndex);
+
         RefPtr<SharedBuffer> m_data; // The encoded data.
         Vector<ImageFrame, 1> m_frameBufferCache;
         bool m_premultiplyAlpha;