Early reporting of opaqueness by decoders.

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

 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 70 matching lines @@
     else if (m_reader && m_reader->loopCount() != cLoopCountNotSeen)
         m_repetitionCount = m_reader->loopCount();
     return m_repetitionCount;
 }
 
 bool GIFImageDecoder::frameIsCompleteAtIndex(size_t index) const
 {
     return m_reader && (index < m_reader->imagesCount()) && m_reader->frameContext(index)->isComplete();
 }
 
+bool GIFImageDecoder::frameHasAlphaAtIndex(size_t index) const
+{
+    if (!frameIsCompleteAtIndex(index))
+        return true;
+    if (m_reader) {
+        const GIFFrameContext& frameContext = *m_reader->frameContext(index);
+        const GIFColorMap::Table& colorTable = frameContext.localColorMap().isDefined() ? frameContext.localColorMap().getTable() : m_reader->globalColorMap().getTable();
+        if (frameContext.transparentPixel() >= colorTable.size())

[scroggo_chromium, 2016/05/16 20:38:20]

Isn't it also possible that this frame does not cover the frame behind it, which
may have transparency? (Or, for that matter, could it have no frames behind it?
Wouldn't that also result in transparency?)

+            return false;
+    }
+    return m_frameBufferCache[index].hasAlpha();
+}
+
 float GIFImageDecoder::frameDurationAtIndex(size_t index) const
 {
     return (m_reader && (index < m_reader->imagesCount()) &&
         m_reader->frameContext(index)->isHeaderDefined()) ?
         m_reader->frameContext(index)->delayTime() : 0;
 }
 
 bool GIFImageDecoder::setFailed()
 {
     m_reader.clear();
@@ skipping 267 matching lines @@
 
     // Update our status to be partially complete.
     buffer->setStatus(ImageFrame::FramePartial);
 
     // Reset the alpha pixel tracker for this frame.
     m_currentBufferSawAlpha = false;
     return true;
 }
 
 } // namespace blink