Animated PNG implementation

Source/platform/image-decoders/png/PNGImageDecoder.cpp

Index: Source/platform/image-decoders/png/PNGImageDecoder.cpp
diff --git a/Source/platform/image-decoders/png/PNGImageDecoder.cpp b/Source/platform/image-decoders/png/PNGImageDecoder.cpp
index 99fde2f278671211987446b4b73657c8e4f37586..5e830db615f733128d37c385f542ef30b4fc6339 100644
--- a/Source/platform/image-decoders/png/PNGImageDecoder.cpp
+++ b/Source/platform/image-decoders/png/PNGImageDecoder.cpp
@@ -39,6 +39,7 @@
 #include "config.h"
 #include "platform/image-decoders/png/PNGImageDecoder.h"
 
+#include "platform/RuntimeEnabledFeatures.h"
 #include "wtf/PassOwnPtr.h"
 
 #include "png.h"
@@ -92,8 +93,15 @@ public:
     PNGImageReader(PNGImageDecoder* decoder)
         : m_decoder(decoder)
         , m_readOffset(0)
-        , m_currentBufferSize(0)
-        , m_decodingSizeOnly(false)
+        , m_parseOffset(0)
+        , m_infoSize(0)
+        , m_isAnimated(false)
+        , m_isParsed(false)
+        , m_width(0)
+        , m_height(0)
+        , m_repetitionCount(cAnimationNone)
+        , m_posterFrame(0)
+        , m_visibleFrames(1)
         , m_hasAlpha(false)
 #if USE(QCMSLIB)
         , m_transform(0)
@@ -103,6 +111,7 @@ public:
         m_png = png_create_read_struct(PNG_LIBPNG_VER_STRING, 0, pngFailed, 0);
         m_info = png_create_info_struct(m_png);
         png_set_progressive_read_fn(m_png, m_decoder, pngHeaderAvailable, pngRowAvailable, pngComplete);
+        memset(&m_currentFrame, 0, sizeof(m_currentFrame));
     }
 
     ~PNGImageReader()
@@ -110,6 +119,20 @@ public:
         close();
     }
 
+    struct FrameInfo {
+        unsigned start;
+        unsigned finish;
+        unsigned width;
+        unsigned height;
+        unsigned xOffset;
+        unsigned yOffset;
+        unsigned duration;
+        unsigned dispose;
+        unsigned blend;
+    };
+
+    static const unsigned headerSize = 33;
+
     void close()
     {
         if (m_png && m_info)
@@ -119,34 +142,188 @@ public:
         clearColorTransform();
 #endif
         m_readOffset = 0;
+        m_parseOffset = 0;
     }
 
-    bool decode(const SharedBuffer& data, bool sizeOnly)
+    bool parse(const SharedBuffer& data)
     {
-        m_decodingSizeOnly = sizeOnly;
+        if (data.size() < headerSize)
+            return false;
+
+        const unsigned long maxPNGSize = 1000000UL;
+        png_bytep inputData = reinterpret_cast<png_bytep>(const_cast<char*>(data.data()));
+        unsigned inputSize = data.size();
+
+        if (!m_parseOffset) {
+            if (memcmp(data.data() + 12, "IHDR", 4))
+                return false;
+            m_currentFrame.width = m_width = png_get_uint_32(inputData + 16);
+            m_currentFrame.height = m_height = png_get_uint_32(inputData + 20);
+            if (m_width > maxPNGSize || m_height > maxPNGSize)
+                return false;
+            png_byte bitDepth = inputData[24];
+            if (bitDepth != 1 && bitDepth != 2 && bitDepth != 4 && bitDepth != 8 && bitDepth != 16)
+                return false;
+            png_byte colorType = inputData[25];
+            if (colorType == 1 || colorType == 5 || colorType > 6)
+                return false;
+            if (crc32(crc32(0, Z_NULL, 0), inputData + 12, 17) != png_get_uint_32(inputData + 29))
+                return false;
+
+            m_infoSize = m_parseOffset = headerSize;
+        }
+
+        while (!m_isParsed) {
+            if (inputSize < m_parseOffset + 12)
+                return false;
+
+            png_bytep chunk = inputData + m_parseOffset;
+            png_bytep chunkId = chunk + 4;
+            unsigned chunkSize = png_get_uint_32(chunk);
+            unsigned chunkEnd = m_parseOffset + chunkSize + 12;
+
+            if (!memcmp(chunkId, "IEND", 4) && !chunkSize) {
+                if (!m_currentFrame.start)
+                    return false;
+                m_isParsed = true;
+                m_currentFrame.finish = chunkEnd;
+                m_frames.append(m_currentFrame);
+                return true;
+            }
+            if (!memcmp(chunkId, "IDAT", 4) && !m_currentFrame.start) {
+                m_currentFrame.start = m_parseOffset;
+                if (!m_decoder->setSize(m_width, m_height))
+                    return false;
+            }
+
+            if (inputSize < chunkEnd)
+                return false;
+
+            if (RuntimeEnabledFeatures::animatedPNGEnabled()) {
+                if (!memcmp(chunkId, "fdAT", 4) && m_isAnimated && !m_currentFrame.start) {
+                    m_currentFrame.start = m_parseOffset;
+                } else if (!memcmp(chunkId, "acTL", 4) && chunkSize == 8 && !m_isAnimated && !m_currentFrame.start) {
+                    m_isAnimated = true;
+                    m_posterFrame = 1;
+                    m_visibleFrames = 0;
+                    m_repetitionCount = static_cast<int>(png_get_uint_32(chunk + 12)) - 1;
+                } else if (!memcmp(chunkId, "fcTL", 4) && chunkSize == 26 && (m_isAnimated || !m_currentFrame.start)) {
+                    if (m_currentFrame.start) {
+                        m_currentFrame.finish = chunkEnd;
+                        m_frames.append(m_currentFrame);
+                        m_currentFrame.start = 0;
+                    } else if (m_frames.isEmpty()) {
+                        m_posterFrame = 0;
+                    }
+
+                    m_currentFrame.width = png_get_uint_32(chunk + 12);
+                    m_currentFrame.height = png_get_uint_32(chunk + 16);
+                    m_currentFrame.xOffset = png_get_uint_32(chunk + 20);
+                    m_currentFrame.yOffset = png_get_uint_32(chunk + 24);
+                    unsigned numerator = png_get_uint_16(chunk + 28);
+                    unsigned denominator = png_get_uint_16(chunk + 30);
+                    m_currentFrame.duration = (!denominator) ? numerator * 10 : (int)(numerator * 1000.0 / denominator);
+                    m_currentFrame.dispose = chunk[32];
+                    m_currentFrame.blend = chunk[33];
+
+                    if (m_currentFrame.width > maxPNGSize || m_currentFrame.height > maxPNGSize
+                        || m_currentFrame.xOffset > maxPNGSize || m_currentFrame.yOffset > maxPNGSize
+                        || m_currentFrame.xOffset + m_currentFrame.width > m_width
+                        || m_currentFrame.yOffset + m_currentFrame.height > m_height
+                        || m_currentFrame.dispose > 2 || m_currentFrame.blend > 1)
+                        return false;
+
+                    if (!m_visibleFrames) {
+                        m_currentFrame.blend = 0;
+                        if (m_currentFrame.dispose == 2)
+                            m_currentFrame.dispose = 1;
+                    }
+                    m_visibleFrames++;
+                } else if (!m_currentFrame.start && m_frames.isEmpty()) {
+                    m_infoSize = chunkEnd;
+                }
+            }
+
+            m_parseOffset = chunkEnd;
+        }
+        return true;
+    }
+
+    bool decode(const SharedBuffer& data, size_t index)
+    {
+        static png_byte dataIEND[12] = {0, 0, 0, 0, 73, 69, 78, 68, 174, 66, 96, 130};
+
+        index += m_posterFrame;
+        if (index && index >= m_frames.size())
+            return true;
+
+        if (index || !m_readOffset) {
+            png_destroy_read_struct(&m_png, &m_info, 0);
+            m_png = png_create_read_struct(PNG_LIBPNG_VER_STRING, 0, pngFailed, 0);
+            m_info = png_create_info_struct(m_png);
+            png_set_progressive_read_fn(m_png, m_decoder, pngHeaderAvailable, pngRowAvailable, pngComplete);
+        }
 
-        // We need to do the setjmp here. Otherwise bad things will happen.
         if (setjmp(JMPBUF(m_png)))
-            return m_decoder->setFailed();
-
-        const char* segment;
-        while (unsigned segmentLength = data.getSomeData(segment, m_readOffset)) {
-            m_readOffset += segmentLength;
-            m_currentBufferSize = m_readOffset;
-            png_process_data(m_png, m_info, reinterpret_cast<png_bytep>(const_cast<char*>(segment)), segmentLength);
-            if (sizeOnly ? m_decoder->isDecodedSizeAvailable() : m_decoder->frameIsCompleteAtIndex(0))
+            return false;
+
+        if (!index) {
+            const char* segment;
+            unsigned endOffset = (!m_frames.isEmpty()) ? m_frames[0].finish : 0;
+            while (unsigned segmentLength = data.getSomeData(segment, m_readOffset)) {
+                if (endOffset && m_readOffset + segmentLength > endOffset)
+                    segmentLength = endOffset - m_readOffset;
+
+                m_readOffset += segmentLength;
+                png_process_data(m_png, m_info, reinterpret_cast<png_bytep>(const_cast<char*>(segment)), segmentLength);
+
+                if (m_readOffset == endOffset) {
+                    png_process_data(m_png, m_info, dataIEND, 12);
+                    break;
+                }
+            }
+        } else {
+            m_readOffset = 0;
+            png_bytep inputData = reinterpret_cast<png_bytep>(const_cast<char*>(data.data()));
+            png_bytep chunk = inputData + m_frames[index].start;
+            png_bytep finish = inputData + m_frames[index].finish;
+            if (inputData + data.size() < finish)
                 return true;
+
+            png_set_crc_action(m_png, PNG_CRC_QUIET_USE, PNG_CRC_QUIET_USE);
+            if (m_frames[index].width != m_width || m_frames[index].height != m_height) {
+                unsigned char header[headerSize];
+                memcpy(&header[0], inputData, headerSize);
+                png_save_uint_32(&header[16], m_frames[index].width);
+                png_save_uint_32(&header[20], m_frames[index].height);
+                png_process_data(m_png, m_info, header, headerSize);
+                png_process_data(m_png, m_info, inputData + headerSize, m_infoSize - headerSize);
+            } else {
+                png_process_data(m_png, m_info, inputData, m_infoSize);
+            }
+
+            png_byte dataIDAT[8] = {0, 0, 0, 0, 73, 68, 65, 84};
+            while (chunk < finish) {
+                unsigned size = png_get_uint_32(chunk);
+                if (!memcmp(chunk + 4, "fdAT", 4)) {
+                    png_save_uint_32(&dataIDAT[0], size - 4);
+                    png_process_data(m_png, m_info, dataIDAT, 8);
+                    png_process_data(m_png, m_info, chunk + 12, size);
+                }
+                chunk += size + 12;
+            }
+            png_process_data(m_png, m_info, dataIEND, 12);
         }
 
-        return false;
+        return true;
     }
 
     png_structp pngPtr() const { return m_png; }
     png_infop infoPtr() const { return m_info; }
 
-    void setReadOffset(unsigned offset) { m_readOffset = offset; }
-    unsigned currentBufferSize() const { return m_currentBufferSize; }
-    bool decodingSizeOnly() const { return m_decodingSizeOnly; }
+    int repetitionCount() const { return m_repetitionCount; }
+    size_t imagesCount() const { return m_frames.isEmpty() ? 1 : m_frames.size() - m_posterFrame; }
+    const FrameInfo * frameAtIndex(size_t index) const { return m_frames.isEmpty() ? &m_currentFrame : &m_frames[index + m_posterFrame]; }
     void setHasAlpha(bool hasAlpha) { m_hasAlpha = hasAlpha; }
     bool hasAlpha() const { return m_hasAlpha; }
 
@@ -194,8 +371,17 @@ private:
     png_infop m_info;
     PNGImageDecoder* m_decoder;
     unsigned m_readOffset;
-    unsigned m_currentBufferSize;
-    bool m_decodingSizeOnly;
+    unsigned m_parseOffset;
+    unsigned m_infoSize;
+    bool m_isAnimated;
+    bool m_isParsed;
+    unsigned m_width;
+    unsigned m_height;
+    int m_repetitionCount;
+    size_t m_posterFrame;
+    size_t m_visibleFrames;
+    FrameInfo m_currentFrame;
+    Vector<FrameInfo> m_frames;
     bool m_hasAlpha;
     OwnPtr<png_byte[]> m_interlaceBuffer;
 #if USE(QCMSLIB)
@@ -207,6 +393,7 @@ private:
 PNGImageDecoder::PNGImageDecoder(AlphaOption alphaOption, GammaAndColorProfileOption colorOptions, size_t maxDecodedBytes)
     : ImageDecoder(alphaOption, colorOptions, maxDecodedBytes)
     , m_hasColorProfile(false)
+    , m_currentFrame(0)
 {
 }
 
@@ -214,6 +401,48 @@ PNGImageDecoder::~PNGImageDecoder()
 {
 }
 
+bool PNGImageDecoder::frameIsCompleteAtIndex(size_t index) const
+{
+    if (!m_reader || failed())
+        return false;
+    if (m_frameBufferCache.size() <= 1)
+        return ImageDecoder::frameIsCompleteAtIndex(index);
+    bool frameIsLoadedAtIndex = index < m_frameBufferCache.size();
+    return frameIsLoadedAtIndex;
+}
+
+float PNGImageDecoder::frameDurationAtIndex(size_t index) const
+{
+    return (index < m_frameBufferCache.size()) ? m_frameBufferCache[index].duration() : 0;
+}
+
+int PNGImageDecoder::repetitionCount() const
+{
+    return (!m_reader || failed()) ? cAnimationNone : m_reader->repetitionCount();
+}
+
+size_t PNGImageDecoder::clearCacheExceptFrame(size_t clearExceptFrame)
+{
+    if (clearExceptFrame >= m_frameBufferCache.size() || m_frameBufferCache.size() <= 1)
+        return 0;
+
+    size_t prevFrame = clearExceptFrame;
+    if (m_frameBufferCache[clearExceptFrame].disposalMethod() == ImageFrame::DisposeOverwritePrevious)
+        prevFrame = m_frameBufferCache[clearExceptFrame].requiredPreviousFrameIndex();
+
+    while (clearExceptFrame != kNotFound && m_frameBufferCache[clearExceptFrame].status() != ImageFrame::FrameComplete)
+        clearExceptFrame = m_frameBufferCache[clearExceptFrame].requiredPreviousFrameIndex();
+
+    size_t frameBytesCleared = 0;
+    for (size_t i = 0; i < m_frameBufferCache.size(); ++i) {
+        if (i != prevFrame && i != clearExceptFrame) {
+            frameBytesCleared += frameBytesAtIndex(i);
+            clearFrameBuffer(i);
+        }
+    }
+    return frameBytesCleared;
+}
+
 #if USE(QCMSLIB)
 static void getColorProfile(png_structp png, png_infop info, ColorProfile& colorProfile, bool& sRGB)
 {
@@ -255,22 +484,7 @@ void PNGImageDecoder::headerAvailable()
 {
     png_structp png = m_reader->pngPtr();
     png_infop info = m_reader->infoPtr();
-    png_uint_32 width = png_get_image_width(png, info);
-    png_uint_32 height = png_get_image_height(png, info);
-
-    // Protect against large PNGs. See http://bugzil.la/251381 for more details.
-    const unsigned long maxPNGSize = 1000000UL;
-    if (width > maxPNGSize || height > maxPNGSize) {
-        longjmp(JMPBUF(png), 1);
-        return;
-    }
-
-    // Set the image size now that the image header is available.
-    if (!setSize(width, height)) {
-        longjmp(JMPBUF(png), 1);
-        return;
-    }
-
+    png_uint_32 width, height;
     int bitDepth, colorType, interlaceType, compressionType, filterType, channels;
     png_get_IHDR(png, info, &width, &height, &bitDepth, &colorType, &interlaceType, &compressionType, &filterType);
 
@@ -337,17 +551,6 @@ void PNGImageDecoder::headerAvailable()
     ASSERT(channels == 3 || channels == 4);
 
     m_reader->setHasAlpha(channels == 4);
-
-    if (m_reader->decodingSizeOnly()) {
-        // If we only needed the size, halt the reader.
-#if PNG_LIBPNG_VER_MAJOR > 1 || (PNG_LIBPNG_VER_MAJOR == 1 && PNG_LIBPNG_VER_MINOR >= 5)
-        // '0' argument to png_process_data_pause means: Do not cache unprocessed data.
-        m_reader->setReadOffset(m_reader->currentBufferSize() - png_process_data_pause(png, 0));
-#else
-        m_reader->setReadOffset(m_reader->currentBufferSize() - png->buffer_size);
-        png->buffer_size = 0;
-#endif
-    }
 }
 
 void PNGImageDecoder::rowAvailable(unsigned char* rowBuffer, unsigned rowIndex, int)
@@ -356,7 +559,7 @@ void PNGImageDecoder::rowAvailable(unsigned char* rowBuffer, unsigned rowIndex,
         return;
 
     // Initialize the framebuffer if needed.
-    ImageFrame& buffer = m_frameBufferCache[0];
+    ImageFrame& buffer = m_frameBufferCache[m_currentFrame];
     if (buffer.status() == ImageFrame::FrameEmpty) {
         png_structp png = m_reader->pngPtr();
         if (!buffer.setSize(size().width(), size().height())) {
@@ -365,8 +568,10 @@ void PNGImageDecoder::rowAvailable(unsigned char* rowBuffer, unsigned rowIndex,
         }
 
         unsigned colorChannels = m_reader->hasAlpha() ? 4 : 3;
-        if (PNG_INTERLACE_ADAM7 == png_get_interlace_type(png, m_reader->infoPtr())) {
-            m_reader->createInterlaceBuffer(colorChannels * size().width() * size().height());
+        if (PNG_INTERLACE_ADAM7 == png_get_interlace_type(png, m_reader->infoPtr())
+            || m_currentFrame) {
+            if (!m_reader->interlaceBuffer())
+                m_reader->createInterlaceBuffer(colorChannels * size().width() * size().height());
             if (!m_reader->interlaceBuffer()) {
                 longjmp(JMPBUF(png), 1);
                 return;
@@ -375,7 +580,8 @@ void PNGImageDecoder::rowAvailable(unsigned char* rowBuffer, unsigned rowIndex,
 
 #if USE(QCMSLIB)
         if (m_reader->colorTransform()) {
-            m_reader->createRowBuffer(colorChannels * size().width());
+            if (!m_reader->rowBuffer())
+                m_reader->createRowBuffer(colorChannels * size().width());
             if (!m_reader->rowBuffer()) {
                 longjmp(JMPBUF(png), 1);
                 return;
@@ -385,8 +591,10 @@ void PNGImageDecoder::rowAvailable(unsigned char* rowBuffer, unsigned rowIndex,
         buffer.setStatus(ImageFrame::FramePartial);
         buffer.setHasAlpha(false);
 
-        // For PNGs, the frame always fills the entire image.
-        buffer.setOriginalFrameRect(IntRect(IntPoint(), size()));
+        if (!initFrameBuffer()) {
+            longjmp(JMPBUF(png), 1);
+            return;
+        }
     }
 
     /* libpng comments (here to explain what follows).
@@ -438,6 +646,10 @@ void PNGImageDecoder::rowAvailable(unsigned char* rowBuffer, unsigned rowIndex,
         png_progressive_combine_row(m_reader->pngPtr(), row, rowBuffer);
     }
 
+    // Only do incremental image display for the first frame.
+    if (m_currentFrame)
+        return;
+
 #if USE(QCMSLIB)
     if (qcms_transform* transform = m_reader->colorTransform()) {
         qcms_transform_data(transform, row, m_reader->rowBuffer(), size().width());
@@ -481,30 +693,171 @@ void PNGImageDecoder::complete()
     if (m_frameBufferCache.isEmpty())
         return;
 
-    m_frameBufferCache[0].setStatus(ImageFrame::FrameComplete);
+    ImageFrame& buffer = m_frameBufferCache[m_currentFrame];
+    buffer.setStatus(ImageFrame::FrameComplete);
+
+    if (!m_currentFrame)
+        return;
+
+    const IntRect& rect = buffer.originalFrameRect();
+    bool hasAlpha = m_reader->hasAlpha();
+    unsigned colorChannels = hasAlpha ? 4 : 3;
+    unsigned alphaMask = 255;
+    ImageFrame::AlphaBlendSource blendMethod = buffer.alphaBlendSource();
+
+    if (blendMethod == ImageFrame::BlendAtopPreviousFrame && !hasAlpha)
+        blendMethod = ImageFrame::BlendAtopBgcolor;
+
+    png_bytep row = m_reader->interlaceBuffer();
+    for (int y = rect.y(); y < rect.maxY(); ++y, row += colorChannels * size().width()) {
+        png_bytep pixel = row;
+#if USE(QCMSLIB)
+        if (qcms_transform* transform = m_reader->colorTransform()) {
+            qcms_transform_data(transform, row, m_reader->rowBuffer(), size().width());
+            pixel = m_reader->rowBuffer();
+        }
+#endif
+        ImageFrame::PixelData* address = buffer.getAddr(rect.x(), y);
+        if (hasAlpha) {
+            if (blendMethod == ImageFrame::BlendAtopBgcolor) {
+                if (buffer.premultiplyAlpha()) {
+                    for (int x = rect.x(); x < rect.maxX(); ++x, pixel += 4) {
+                        buffer.setRGBAPremultiply(address++, pixel[0], pixel[1], pixel[2], pixel[3]);
+                        alphaMask &= pixel[3];
+                    }
+                } else {
+                    for (int x = rect.x(); x < rect.maxX(); ++x, pixel += 4) {
+                        buffer.setRGBARaw(address++, pixel[0], pixel[1], pixel[2], pixel[3]);
+                        alphaMask &= pixel[3];
+                    }
+                }
+            } else {
+                if (buffer.premultiplyAlpha()) {
+                    for (int x = rect.x(); x < rect.maxX(); ++x, pixel += 4) {
+                        buffer.overRGBAPremultiply(address++, pixel[0], pixel[1], pixel[2], pixel[3]);
+                        alphaMask &= pixel[3];
+                    }
+                } else {
+                    for (int x = rect.x(); x < rect.maxX(); ++x, pixel += 4) {
+                        buffer.overRGBARaw(address++, pixel[0], pixel[1], pixel[2], pixel[3]);
+                        alphaMask &= pixel[3];
+                    }
+                }
+            }
+        } else {
+            for (int x = rect.x(); x < rect.maxX(); ++x, pixel += 3) {
+                buffer.setRGBARaw(address++, pixel[0], pixel[1], pixel[2], 255);
+            }
+        }
+    }
+
+    if (alphaMask == 255) {
+        if (buffer.originalFrameRect().contains(IntRect(IntPoint(), size()))) {
+            buffer.setHasAlpha(false);
+        } else {
+            size_t frameIndex = m_currentFrame;
+            const ImageFrame* prevBuffer = &m_frameBufferCache[--frameIndex];
+            while (frameIndex && (prevBuffer->disposalMethod() == ImageFrame::DisposeOverwritePrevious))
+                prevBuffer = &m_frameBufferCache[--frameIndex];
+            if ((prevBuffer->disposalMethod() == ImageFrame::DisposeOverwriteBgcolor)
+                && !prevBuffer->hasAlpha() && buffer.originalFrameRect().contains(prevBuffer->originalFrameRect()))
+                buffer.setHasAlpha(false);
+        }
+    } else if (blendMethod == ImageFrame::BlendAtopBgcolor && !buffer.hasAlpha()) {
+        buffer.setHasAlpha(true);
+    }
+}
+
+size_t PNGImageDecoder::decodeFrameCount()
+{
+    return parse() ? m_reader->imagesCount() : m_frameBufferCache.size();
+}
+
+void PNGImageDecoder::decode(size_t index)
+{
+    if (!m_reader || failed())
+        return;
+
+    Vector<size_t> framesToDecode;
+    size_t frameToDecode = index;
+    do {
+        framesToDecode.append(frameToDecode);
+        frameToDecode = m_frameBufferCache[frameToDecode].requiredPreviousFrameIndex();
+    } while (frameToDecode != kNotFound && m_frameBufferCache[frameToDecode].status() != ImageFrame::FrameComplete);
+
+    for (auto i = framesToDecode.rbegin(); i != framesToDecode.rend(); ++i) {
+        m_currentFrame = *i;
+        if (!m_reader->decode(*m_data, m_currentFrame)) {
+            setFailed();
+            break;
+        }
+
+        // We need more data to continue decoding.
+        if (m_frameBufferCache[m_currentFrame].status() != ImageFrame::FrameComplete)
+            break;
+    }
+}
+
+void PNGImageDecoder::initializeNewFrame(size_t index)
+{
+    ImageFrame* buffer = &m_frameBufferCache[index];
+    bool frameRectIsOpaque = (buffer->status() == ImageFrame::FrameComplete) ? !m_reader->hasAlpha() : false;
+    buffer->setRequiredPreviousFrameIndex(findRequiredPreviousFrame(index, frameRectIsOpaque));
+    const PNGImageReader::FrameInfo* frame = m_reader->frameAtIndex(index);
+    IntRect frameRect(frame->xOffset, frame->yOffset, frame->width, frame->height);
+    buffer->setOriginalFrameRect(intersection(frameRect, IntRect(IntPoint(), size())));
+    buffer->setDuration(frame->duration);
+    buffer->setDisposalMethod((frame->dispose == 2) ? ImageFrame::DisposeOverwritePrevious : (frame->dispose == 1) ? ImageFrame::DisposeOverwriteBgcolor : ImageFrame::DisposeKeep);
+    buffer->setAlphaBlendSource((frame->blend == 1) ? ImageFrame::BlendAtopPreviousFrame : ImageFrame::BlendAtopBgcolor);
 }
 
-inline bool isComplete(const PNGImageDecoder* decoder)
+bool PNGImageDecoder::initFrameBuffer()
 {
-    return decoder->frameIsCompleteAtIndex(0);
+    ImageFrame& buffer = m_frameBufferCache[m_currentFrame];
+
+    const size_t requiredPreviousFrameIndex = buffer.requiredPreviousFrameIndex();
+    if (requiredPreviousFrameIndex == kNotFound) {
+        // This frame doesn't rely on any previous data.
+        buffer.zeroFillPixelData();
+        if (!m_currentFrame)
+            buffer.setHasAlpha(false);
+    } else {
+        const ImageFrame& prevBuffer = m_frameBufferCache[requiredPreviousFrameIndex];
+        ASSERT(prevBuffer.status() == ImageFrame::FrameComplete);
+
+        // 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();
+            ASSERT(!prevRect.contains(IntRect(IntPoint(), size())));
+            buffer.zeroFillFrameRect(prevRect);
+        }
+    }
+    return true;
 }
 
-void PNGImageDecoder::decode(bool onlySize)
+bool PNGImageDecoder::parse()
 {
     if (failed())
-        return;
+        return false;
 
     if (!m_reader)
         m_reader = adoptPtr(new PNGImageReader(this));
 
     // If we couldn't decode the image but have received all the data, decoding
     // has failed.
-    if (!m_reader->decode(*m_data, onlySize) && isAllDataReceived())
+    if (!m_reader->parse(*m_data) && isAllDataReceived())
         setFailed();
 
-    // If decoding is done or failed, we don't need the PNGImageReader anymore.
-    if (isComplete(this) || failed())
+    if (failed()) {
         m_reader.clear();
+        return false;
+    }
+    return true;
 }
 
 } // namespace blink