Decode GIF image frames on demand.

Source/core/platform/image-decoders/gif/GIFImageReader.h

 /* -*- Mode: C; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* ***** BEGIN LICENSE BLOCK *****
  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
  *
  * The contents of this file are subject to the Mozilla Public License Version
  * 1.1 (the "License"); you may not use this file except in compliance with
  * the License. You may obtain a copy of the License at
  * http://www.mozilla.org/MPL/
  *
  * Software distributed under the License is distributed on an "AS IS" basis,
@@ skipping 195 matching lines @@
     bool isComplete() const { return m_isComplete; }
     void setComplete() { m_isComplete = true; }
     bool isHeaderDefined() const { return m_isHeaderDefined; }
     void setHeaderDefined() { m_isHeaderDefined = true; }
     bool isDataSizeDefined() const { return m_isDataSizeDefined; }
     void setDataSize(int size)
     {
         datasize = size;
         m_isDataSizeDefined = true;
     }
-
+    void clearDecodeState() { m_lzwContext.clear(); }
 private:
     OwnPtr<GIFLZWContext> m_lzwContext;
     Vector<GIFLZWBlock> m_lzwBlocks; // LZW blocks for this frame.
     size_t m_currentLzwBlock;
     bool m_isComplete;
     bool m_isHeaderDefined;
     bool m_isDataSizeDefined;
 };
 
 class GIFImageReader {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     GIFImageReader(WebCore::GIFImageDecoder* client = 0)
         : m_client(client)
         , m_state(GIFType)
         , m_bytesToConsume(6) // Number of bytes for GIF type, either "GIF87a" or "GIF89a".
         , m_bytesRead(0)
         , m_screenBgcolor(0)
         , m_version(0)
         , m_screenWidth(0)
         , m_screenHeight(0)
         , m_isGlobalColormapDefined(false)
         , m_globalColormapPosition(0)
         , m_globalColormapSize(0)
         , m_loopCount(cLoopCountNotSeen)
-        , m_currentDecodingFrame(0)
         , m_parseCompleted(false)
     {
     }
 
     ~GIFImageReader()
     {
     }
 
     void setData(PassRefPtr<WebCore::SharedBuffer> data) { m_data = data; }
-    // FIXME: haltAtFrame should be size_t.
-    bool decode(WebCore::GIFImageDecoder::GIFQuery, unsigned haltAtFrame);
+    bool parse(WebCore::GIFImageDecoder::GIFParseQuery);
+    bool decode(size_t frameIndex);

[Peter Kasting, 2013/05/24 03:15:22]

Thumbs up on improving the API here to be noticeably clearer!

 
     size_t imagesCount() const
     {
         if (m_frames.isEmpty())
             return 0;
 
         // This avoids counting an empty frame when the file is truncated right after
         // GIFControlExtension but before GIFImageHeader.
         // FIXME: This extra complexity is not necessary and we should just report m_frames.size().
         return m_frames.last()->isHeaderDefined() ? m_frames.size() : m_frames.size() - 1;
@@ skipping 18 matching lines @@
         return frame->isLocalColormapDefined ? frame->localColormapSize : 0;
     }
 
     const GIFFrameContext* frameContext(size_t index) const
     {
         return index < m_frames.size() ? m_frames[index].get() : 0;
     }
 
     bool parseCompleted() const { return m_parseCompleted; }
 
+    void clearDecodeState(size_t index) { m_frames[index]->clearDecodeState(); }
+
 private:
-    bool parse(size_t dataPosition, size_t len, bool parseSizeOnly);
+    bool parseData(size_t dataPosition, size_t len, WebCore::GIFImageDecoder::GIFParseQuery);
     void setRemainingBytes(size_t);
 
     const unsigned char* data(size_t dataPosition) const
     {
         return reinterpret_cast<const unsigned char*>(m_data->data()) + dataPosition;
     }
 
     void addFrameIfNecessary();
     bool currentFrameIsFirstFrame() const
     {
         return m_frames.isEmpty() || (m_frames.size() == 1u && !m_frames[0]->isComplete());
     }
 
     WebCore::GIFImageDecoder* m_client;
 
     // Parsing state machine.
     GIFState m_state; // Current decoder master state.
     size_t m_bytesToConsume; // Number of bytes to consume for next stage of parsing.
     size_t m_bytesRead; // Number of bytes processed.
     
     // Global (multi-image) state.
     int m_screenBgcolor; // Logical screen background color.

[Peter Kasting, 2013/05/24 03:15:22]

Tangential to your change, but I noticed while looking at how we handle
DisposeOverwriteBgcolor that we don't actually care about this member.  Neither
does Mozilla.  Can you go ahead and nuke this entirely while you're here?

[Xianzhu, 2013/05/28 22:54:21]

Done.

     int m_version; // Either 89 for GIF89 or 87 for GIF87.
     unsigned m_screenWidth; // Logical screen width & height.
     unsigned m_screenHeight;
     bool m_isGlobalColormapDefined;
     size_t m_globalColormapPosition; // (3* MAX_COLORS in size) Default colormap if local not supplied, 3 bytes for each color.
     int m_globalColormapSize; // Size of global colormap array.
     int m_loopCount; // Netscape specific extension block to control the number of animation loops a GIF renders.
     
     Vector<OwnPtr<GIFFrameContext> > m_frames;
-    size_t m_currentDecodingFrame;
 
     RefPtr<WebCore::SharedBuffer> m_data;
     bool m_parseCompleted;
 };
 
 #endif