Decode GIF image frames on demand.

Source/core/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 39 matching lines @@
         return;
 
     ImageDecoder::setData(data, allDataReceived);
     if (m_reader)
         m_reader->setData(data);
 }
 
 bool GIFImageDecoder::isSizeAvailable()
 {
     if (!ImageDecoder::isSizeAvailable())
-         decode(0, GIFSizeQuery);
+        parse(GIFSizeQuery);
 
     return ImageDecoder::isSizeAvailable();
 }
 
 bool GIFImageDecoder::setSize(unsigned width, unsigned height)
 {
     if (ImageDecoder::isSizeAvailable() && size() == IntSize(width, height))
         return true;
 
     if (!ImageDecoder::setSize(width, height))
         return false;
 
     prepareScaleDataIfNecessary();
     return true;
 }
 
 size_t GIFImageDecoder::frameCount()
 {
-    decode(std::numeric_limits<unsigned>::max(), GIFFrameCountQuery);
+    parse(GIFFrameCountQuery);
     return m_frameBufferCache.size();
 }
 
 int GIFImageDecoder::repetitionCount() const
 {
     // This value can arrive at any point in the image data stream.  Most GIFs
     // in the wild declare it near the beginning of the file, so it usually is
     // set by the time we've decoded the size, but (depending on the GIF and the
     // packets sent back by the webserver) not always.  If the reader hasn't
     // seen a loop count yet, it will return cLoopCountNotSeen, in which case we
@@ skipping 24 matching lines @@
 }
 
 ImageFrame* GIFImageDecoder::frameBufferAtIndex(size_t index)
 {
     if (index >= frameCount())
         return 0;
 
     ImageFrame& frame = m_frameBufferCache[index];
     if (frame.status() != ImageFrame::FrameComplete) {
         PlatformInstrumentation::willDecodeImage("GIF");
-        decode(index + 1, GIFFullQuery);
+        decode(index);
         PlatformInstrumentation::didDecodeImage();
     }
     return &frame;
 }
 
 bool GIFImageDecoder::frameIsCompleteAtIndex(size_t index) const
 {
     return m_reader && (index < m_reader->imagesCount()) && m_reader->frameContext(index)->isComplete();
 }
 
@@ skipping 47 matching lines @@
     //     has a disposal method other than DisposeOverwritePrevious, stop
     //     scanning, as we'll only need this frame when decoding the next one.
     Vector<ImageFrame>::iterator i(end);
     for (; (i != m_frameBufferCache.begin()) && ((i->status() == ImageFrame::FrameEmpty) || (i->disposalMethod() == ImageFrame::DisposeOverwritePrevious)); --i) {
         if ((i->status() == ImageFrame::FrameComplete) && (i != end))
             i->clearPixelData();
     }
 
     // Now |i| holds the last frame we need to preserve; clear prior frames.
     for (Vector<ImageFrame>::iterator j(m_frameBufferCache.begin()); j != i; ++j) {
         ASSERT(j->status() != ImageFrame::FramePartial);

[Alpha Left Google, 2013/05/18 00:39:35]

This condition can fail now with this change.

Because now you can decode:
0 1 2 3'  where 3' is partially decoded

And then skip ahead to decode:
5 6 7 8

Suppose clearFrameBufferCache(7) is called then this assert will fail.

[Xianzhu, 2013/05/20 04:30:59]

Done.

         if (j->status() != ImageFrame::FrameEmpty)
             j->clearPixelData();
     }
 }
 
-bool GIFImageDecoder::haveDecodedRow(unsigned frameIndex, const Vector<unsigned char>& rowBuffer, size_t width, size_t rowNumber, unsigned repeatCount, bool writeTransparentPixels)
+bool GIFImageDecoder::haveDecodedRow(size_t frameIndex, const Vector<unsigned char>& rowBuffer, size_t width, size_t rowNumber, unsigned repeatCount, bool writeTransparentPixels)
 {
     const GIFFrameContext* frameContext = m_reader->frameContext(frameIndex);
     // The pixel data and coordinates supplied to us are relative to the frame's
     // origin within the entire image size, i.e.
     // (frameContext->xOffset, frameContext->yOffset). There is no guarantee
     // that width == (size().width() - frameContext->xOffset), so
     // we must ensure we don't run off the end of either the source data or the
     // row's X-coordinates.
     int xBegin = upperBoundScaledX(frameContext->xOffset);
     int yBegin = upperBoundScaledY(frameContext->yOffset + rowNumber);
@@ skipping 42 matching lines @@
         ++currentAddress;
     }
 
     // Tell the frame to copy the row data if need be.
     if (repeatCount > 1)
         buffer.copyRowNTimes(xBegin, xEnd, yBegin, yEnd);
 
     return true;
 }
 
-bool GIFImageDecoder::frameComplete(unsigned frameIndex, unsigned frameDuration, ImageFrame::FrameDisposalMethod disposalMethod)
+bool GIFImageDecoder::frameComplete(size_t frameIndex, unsigned frameDuration, ImageFrame::FrameDisposalMethod disposalMethod)
 {
     // Initialize the frame if necessary.  Some GIFs insert do-nothing frames,
     // in which case we never reach haveDecodedRow() before getting here.
     ImageFrame& buffer = m_frameBufferCache[frameIndex];
     if ((buffer.status() == ImageFrame::FrameEmpty) && !initFrameBuffer(frameIndex))
         return false; // initFrameBuffer() has already called setFailed().
 
     buffer.setStatus(ImageFrame::FrameComplete);
     buffer.setDuration(frameDuration);
     buffer.setDisposalMethod(disposalMethod);
@@ skipping 25 matching lines @@
             // it had no alpha, and its rect is contained in the current frame's
             // rect, we know the current frame has no alpha.
             if ((prevBuffer->disposalMethod() == ImageFrame::DisposeOverwriteBgcolor) && !prevBuffer->hasAlpha() && buffer.originalFrameRect().contains(prevBuffer->originalFrameRect()))
                 buffer.setHasAlpha(false);
         }
     }
 
     return true;
 }
 
 void GIFImageDecoder::gifComplete()

[Alpha Left Google, 2013/05/18 00:39:35]

I don't think this method is needed any more. Now a frame count query will scan
the file until the end and hence repetition count will be correct regardless of
whether the entire file is decoded or not.

[Xianzhu, 2013/05/20 04:30:59]

Done.

 {
     // Cache the repetition count, which is now as authoritative as it's ever
     // going to be.
     repetitionCount();
 }
 
-void GIFImageDecoder::decode(unsigned haltAtFrame, GIFQuery query)
+void GIFImageDecoder::parse(GIFParseQuery query)
 {
     if (failed())
         return;
 
     if (!m_reader) {
         m_reader = adoptPtr(new GIFImageReader(this));
         m_reader->setData(m_data);
     }
 
-    if (query == GIFSizeQuery) {
-        if (!m_reader->decode(GIFSizeQuery, haltAtFrame))
-            setFailed();
-        return;
-    }
-
-    if (!m_reader->decode(GIFFrameCountQuery, haltAtFrame)) {
+    if (!m_reader->parse(query)) {
         setFailed();
         return;
     }
 
     const size_t oldSize = m_frameBufferCache.size();
     m_frameBufferCache.resize(m_reader->imagesCount());
     for (size_t i = oldSize; i < m_reader->imagesCount(); ++i)
         m_frameBufferCache[i].setPremultiplyAlpha(m_premultiplyAlpha);
+}
 
-    if (query == GIFFrameCountQuery)
+void GIFImageDecoder::decode(size_t frameIndex)
+{
+    parse(GIFFrameCountQuery);
+
+    if (failed())
         return;
 
-    if (!m_reader->decode(GIFFullQuery, haltAtFrame)) {
+    BitVector frames(frameIndex);

[Alpha Left Google, 2013/05/18 00:39:35]

Instead of a BitVector what about a vector of frame indices? This way you can
call m_reader->decode(...) with those indices in order.

[Xianzhu, 2013/05/20 04:30:59]

Done.

+    getFramesToDecode(frameIndex, &frames);
+
+    if (!m_reader->decode(frames)) {

[Alpha Left Google, 2013/05/18 00:39:35]

I'd prefer to call m_reader->decode(...) multiple times to decode all the
necessary frames.

[Xianzhu, 2013/05/20 04:30:59]

Done.

         setFailed();
         return;
     }
 
     // It is also a fatal error if all data is received and we have decoded all
     // frames available but the file is truncated.
-    if (haltAtFrame >= m_frameBufferCache.size() && isAllDataReceived() && m_reader && !m_reader->parseCompleted())
+    if (frameIndex >= m_frameBufferCache.size() - 1 && isAllDataReceived() && m_reader && !m_reader->parseCompleted())
         setFailed();

[Alpha Left Google, 2013/05/18 00:39:35]

The reason for calling setFailed() here is to stop animation such that for
truncated GIF files we only animate once.

I think this will look much simpler if we intead do a condition in
repetitionCount() that:
if (isAllDataReceived() && m_reader && !m_reader->parseCompleted())
  return cAnimationLoopOnce;
It's not necessary to do this change now though.

 }
 
-bool GIFImageDecoder::initFrameBuffer(unsigned frameIndex)
+void GIFImageDecoder::getFramesToDecode(size_t frameIndex, BitVector* frames)
+{
+    while (m_frameBufferCache[frameIndex].status() != ImageFrame::FrameComplete) {
+        frames->set(frameIndex);
+        frameIndex = getPreviousRequiredFrame(frameIndex);
+        if (frameIndex == notFound)
+            break;
+    }
+}
+
+size_t GIFImageDecoder::getPreviousRequiredFrame(size_t frameIndex)

[Alpha Left Google, 2013/05/18 00:39:35]

I see this method will be called multiple times for the same |frameIndex|. Can
you make an optimization here by caching the |previousRequiredFrame|? In fact
after each frame count query you can go over the new frames and compute each
|previousRequiredFrame| quickly.

The current algorithm is to scan backward for each frame and in the worst case
this is O(n^2).

[Xianzhu, 2013/05/20 04:30:59]

Done.

+{
+    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.
+    //
+    // 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.  (If the
+    // first frame specifies this method, it will get treated like
+    // DisposeOverwriteBgcolor below and reset to a completely empty image.)
+    size_t prevFrame = frameIndex - 1;
+    const ImageFrame* prevBuffer = &m_frameBufferCache[prevFrame];
+    ImageFrame::FrameDisposalMethod prevMethod = prevBuffer->disposalMethod();
+    while (prevFrame && (prevMethod == ImageFrame::DisposeOverwritePrevious)) {
+        prevBuffer = &m_frameBufferCache[--prevFrame];
+        prevMethod = prevBuffer->disposalMethod();
+    }
+
+    if (prevMethod == ImageFrame::DisposeNotSpecified || prevMethod == ImageFrame::DisposeKeep) {
+        // prevFrame will be used as the starting state for this frame.
+        return prevFrame;
+    }
+
+    ASSERT(prevMethod == ImageFrame::DisposeOverwriteBgcolor);
+    const IntRect& prevRect = prevBuffer->originalFrameRect();
+    const IntSize& bufferSize = scaledSize();
+    if (!frameIndex || prevRect.contains(IntRect(IntPoint(), scaledSize()))) {
+        // The first frame or a frame covering the whole image will be cleared
+        // to background, so this frame doesn't rely on any previous data.
+        return notFound;
+    }
+
+    // prevFrame only clears part of the image to background, so it contributes
+    // to the starting state of this frame.
+    return prevFrame;
+}
+
+bool GIFImageDecoder::initFrameBuffer(size_t frameIndex)
 {
     // Initialize the frame rect in our buffer.
     const GIFFrameContext* frameContext = m_reader->frameContext(frameIndex);
     IntRect frameRect(frameContext->xOffset, frameContext->yOffset, frameContext->width, frameContext->height);
 
     // Make sure the frameRect doesn't extend outside the buffer.
     if (frameRect.maxX() > size().width())
         frameRect.setWidth(size().width() - frameContext->xOffset);
     if (frameRect.maxY() > size().height())
         frameRect.setHeight(size().height() - frameContext->yOffset);
 
     ImageFrame* const buffer = &m_frameBufferCache[frameIndex];
     int left = upperBoundScaledX(frameRect.x());
     int right = lowerBoundScaledX(frameRect.maxX(), left);
     int top = upperBoundScaledY(frameRect.y());
     int bottom = lowerBoundScaledY(frameRect.maxY(), top);
     buffer->setOriginalFrameRect(IntRect(left, top, right - left, bottom - top));
-    
-    if (!frameIndex) {
-        // This is the first frame, so we're not relying on any previous data.
+
+    size_t previousRequiredFrame = getPreviousRequiredFrame(frameIndex);
+    if (previousRequiredFrame == notFound) {
+        // This frame doesn't rely on any previous data.
         if (!buffer->setSize(scaledSize().width(), scaledSize().height()))
             return setFailed();
     } else {
-        // The starting state for this frame depends on the previous frame's
-        // disposal method.
-        //
-        // 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.  (If the
-        // first frame specifies this method, it will get treated like
-        // DisposeOverwriteBgcolor below and reset to a completely empty image.)
-        const ImageFrame* prevBuffer = &m_frameBufferCache[--frameIndex];
-        ImageFrame::FrameDisposalMethod prevMethod = prevBuffer->disposalMethod();
-        while (frameIndex && (prevMethod == ImageFrame::DisposeOverwritePrevious)) {
-            prevBuffer = &m_frameBufferCache[--frameIndex];
-            prevMethod = prevBuffer->disposalMethod();
-        }
+        const ImageFrame* prevBuffer = &m_frameBufferCache[previousRequiredFrame];
         ASSERT(prevBuffer->status() == ImageFrame::FrameComplete);
 
-        if ((prevMethod == ImageFrame::DisposeNotSpecified) || (prevMethod == ImageFrame::DisposeKeep)) {
-            // Preserve the last frame as the starting state for this frame.
-            if (!buffer->copyBitmapData(*prevBuffer))
-                return setFailed();
-        } else {
+        // Preserve the last frame as the starting state for this frame.
+        if (!buffer->copyBitmapData(*prevBuffer))
+            return setFailed();
+
+        if (prevBuffer->disposalMethod() == ImageFrame::DisposeOverwriteBgcolor) {
             // We want to clear the previous frame to transparent, without
             // affecting pixels in the image outside of the frame.
             const IntRect& prevRect = prevBuffer->originalFrameRect();
-            const IntSize& bufferSize = scaledSize();
-            if (!frameIndex || prevRect.contains(IntRect(IntPoint(), scaledSize()))) {
-                // Clearing the first frame, or a frame the size of the whole
-                // image, results in a completely empty image.
-                if (!buffer->setSize(bufferSize.width(), bufferSize.height()))
-                    return setFailed();
-            } else {
-              // Copy the whole previous buffer, then clear just its frame.
-              if (!buffer->copyBitmapData(*prevBuffer))
-                  return setFailed();
-              for (int y = prevRect.y(); y < prevRect.maxY(); ++y) {
-                  for (int x = prevRect.x(); x < prevRect.maxX(); ++x)
-                      buffer->setRGBA(x, y, 0, 0, 0, 0);
-              }
-              if ((prevRect.width() > 0) && (prevRect.height() > 0))
-                  buffer->setHasAlpha(true);
+            for (int y = prevRect.y(); y < prevRect.maxY(); ++y) {
+                for (int x = prevRect.x(); x < prevRect.maxX(); ++x)
+                    buffer->setRGBA(x, y, 0, 0, 0, 0);
             }
+            if ((prevRect.width() > 0) && (prevRect.height() > 0))
+                buffer->setHasAlpha(true);
         }
     }
 
     // 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 WebCore