Decode GIF image frames on demand.

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 b98e45a3fa8f93c9a9ccfaba21d6749ce34e02fa..09fe111a8136045c864cc48fd328314c418ac5d7 100644
--- a/Source/core/platform/image-decoders/ImageDecoder.cpp
+++ b/Source/core/platform/image-decoders/ImageDecoder.cpp
@@ -127,7 +127,7 @@ bool ImageDecoder::frameIsCompleteAtIndex(size_t index) const
 
 unsigned ImageDecoder::frameBytesAtIndex(size_t index) const
 {
-    if (m_frameBufferCache.size() <= index)
+    if (m_frameBufferCache.size() <= index || m_frameBufferCache[index].status() == ImageFrame::FrameEmpty)
         return 0;
     // FIXME: Use the dimension of the requested frame.
     return m_size.area() * sizeof(ImageFrame::PixelData);
@@ -140,4 +140,73 @@ void ImageDecoder::reportMemoryUsage(MemoryObjectInfo* memoryObjectInfo) const
     info.addMember(m_frameBufferCache, "frameBufferCache");
 }
 
+size_t ImageDecoder::clearCacheExceptFrame(size_t clearExceptFrame)
+{
+    // Don't clear if there is no or only one frame.

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

Nit: "...are no frames or only one frame."

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

Done.

+    if (m_frameBufferCache.size() <= 1)
+        return 0;
+

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

In theory, it's technically possible for ICOs to reach here, although that ought
not to happen today because while such images have multiple frames, they aren't
animated.

To be safe and clear, however, you should probably modify the ICO image decoder
to explicitly setRequiredPreviousFrameIndex(notFound) on each frame (since all
frames in ICOs are independent).  In a release build, this will have no effect,
but in a debug build it will result in marking each frame's previous frame index
as valid.

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

Done.

+    // We need to preserve frames such that:
+    //  1. We don't clear |clearExceptFrame|;
+    //  2. We don't clear any frame from which a future initFrameBuffer() call
+    //     will copy bitmap data;

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

Nit: ; -> .

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

Done.

+    // All other frames can be cleared.
+    size_t frameToBeRequired = notFound;

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

Nit: You can actually make this even simpler by relying on the fact that
clearing an empty frame is effectively a no-op, and that WTF::notFound will
always evaluate > the framebuffer cache size.  Replace the code from here on
with:

    while ((clearExceptFrame < m_frameBufferCache.size()) &&
(m_frameBufferCache[clearExceptFrame].status() == ImageFrame::FrameEmpty))
        clearExceptFrame =
m_frameBufferCache[clearExceptFrame].requiredPreviousFrameIndex();

    size_t frameBytesCleared = 0;
    for (size_t i = 0; i < m_frameBufferCache.size(); ++i) {
        if (i != clearExceptFrame) {
    ...

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

Done.

+    if (clearExceptFrame < m_frameBufferCache.size() && m_frameBufferCache[clearExceptFrame].status() == ImageFrame::FrameEmpty) {
+        frameToBeRequired = clearExceptFrame;
+        do {
+            frameToBeRequired = m_frameBufferCache[frameToBeRequired].requiredPreviousFrameIndex();
+        } while (frameToBeRequired != notFound && m_frameBufferCache[frameToBeRequired].status() == ImageFrame::FrameEmpty);
+    }
+
+    size_t frameBytesCleared = 0;
+    for (size_t i = 0; i < m_frameBufferCache.size(); ++i) {
+        if (i != clearExceptFrame && i != frameToBeRequired) {
+            frameBytesCleared += frameBytesAtIndex(i);
+            clearFrameBuffer(i);
+        }
+    }
+    return frameBytesCleared;
+}
+
+void ImageDecoder::clearFrameBuffer(size_t frameIndex)
+{
+    m_frameBufferCache[frameIndex].clearPixelData();
+}
+
+size_t ImageDecoder::findRequiredPreviousFrame(size_t frameIndex)
+{
+    ASSERT(frameIndex <= m_frameBufferCache.size());
+    if (!frameIndex) {
+        // The first frame doesn't rely on any previous data.
+        return notFound;
+    }
+
+    // The starting state for this frame depends on the previous frame's
+    // disposal method.
+    size_t prevFrame = frameIndex - 1;
+    const ImageFrame* prevBuffer = &m_frameBufferCache[prevFrame];

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

Here you should also ASSERT(prevBuffer->m_requiredPreviousFrameIndexValid). 
This requires exposing a debug-only accessor for it.

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

Done.

+    switch (prevBuffer->disposalMethod()) {
+    case ImageFrame::DisposeNotSpecified:
+    case ImageFrame::DisposeKeep:
+        // prevFrame will be used as the starting state for this frame.
+        // FIXME: Be even smarter by checking the frame sizes and/or alpha-containing regions.
+        return prevFrame;
+    case ImageFrame::DisposeOverwritePrevious:
+        // Frames that use the DisposeOverwritePrevious method are effectively
+        // no-ops in terms of changing the starting state of a frame compared to
+        // the starting state of the previous frame, so skip over them and
+        // return the required previous frame of it.
+        return prevBuffer->requiredPreviousFrameIndex();
+    case ImageFrame::DisposeOverwriteBgcolor:
+        // If the previous frame fills the whole image, then the current frame
+        // can be decoded alone. Otherwise, prevFrame contributes to this frame.
+        // However, the first frame always clears even it partially covers.

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

We can actually be even smarter here by handling cases where the previous frame
isn't the whole image, but the image's starting state was empty -- basically, a
generalization of the "first frame" case.  Like so:

        // If the previous frame fills the whole image, then the current frame
        // can be decoded alone.  Likewise, if the previous frame could be
        // decoded without reference to any prior frame, the starting state for
        // this frame is a blank frame, so it can again be decoded alone.
        // Otherwise, the previous frame contributes to this frame.
        return (prevBuffer->originalFrameRect().contains(IntRect(IntPoint(),
size()))
            || (prevBuffer->requiredPreviousFrameIndex() == notFound)) ?
notFound : prefFrame;

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

Done. Thanks for the idea.

+        return !prevFrame || prevBuffer->originalFrameRect().contains(IntRect(IntPoint(), size())) ? notFound : prevFrame;
+    default:
+        ASSERT_NOT_REACHED();
+        return notFound;
+    }
+}
+
 } // namespace WebCore