WIP: Implement APNG

third_party/WebKit/Source/platform/image-decoders/png/PNGImageReader.cpp

+/*
+ * Copyright (C) 2006 Apple Computer, Inc.
+ * Copyright (C) Research In Motion Limited 2009-2010. All rights reserved.
+ *
+ * Portions are Copyright (C) 2001 mozilla.org
+ *
+ * Other contributors:
+ *   Stuart Parmenter <stuart@mozilla.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ * Alternatively, the contents of this file may be used under the terms
+ * of either the Mozilla Public License Version 1.1, found at
+ * http://www.mozilla.org/MPL/ (the "MPL") or the GNU General Public
+ * License Version 2.0, found at http://www.fsf.org/copyleft/gpl.html
+ * (the "GPL"), in which case the provisions of the MPL or the GPL are
+ * applicable instead of those above.  If you wish to allow use of your
+ * version of this file only under the terms of one of those two
+ * licenses (the MPL or the GPL) and not to allow others to use your
+ * version of this file under the LGPL, indicate your decision by
+ * deletingthe provisions above and replace them with the notice and
+ * other provisions required by the MPL or the GPL, as the case may be.
+ * If you do not delete the provisions above, a recipient may use your
+ * version of this file under any of the LGPL, the MPL or the GPL.
+ */
+
+#include "platform/image-decoders/png/PNGImageReader.h"
+
+#include "platform/image-decoders/png/PNGImageDecoder.h"
+#include "platform/image-decoders/FastSharedBufferReader.h"
+#include "png.h"
+#include "wtf/PtrUtil.h"
+#include <memory>
+
+#if !defined(PNG_LIBPNG_VER_MAJOR) || !defined(PNG_LIBPNG_VER_MINOR)
+#error version error: compile against a versioned libpng.
+#endif
+#if USE(QCMSLIB)
+#include "qcms.h"
+#endif
+
+#if PNG_LIBPNG_VER_MAJOR > 1 || (PNG_LIBPNG_VER_MAJOR == 1 && PNG_LIBPNG_VER_MINOR >= 4)
+#define JMPBUF(png_ptr) png_jmpbuf(png_ptr)
+#else
+#define JMPBUF(png_ptr) png_ptr->jmpbuf
+#endif
+
+namespace {
+
+inline blink::PNGImageDecoder* imageDecoder(png_structp png)
+{
+  return static_cast<blink::PNGImageDecoder*>(png_get_progressive_ptr(png));
+}
+
+void PNGAPI pngHeaderAvailable(png_structp png, png_infop)
+{
+  imageDecoder(png)->headerAvailable();
+}
+
+void PNGAPI pngRowAvailable(png_structp png, png_bytep row,
+                            png_uint_32 rowIndex, int state)
+{
+  imageDecoder(png)->rowAvailable(row, rowIndex, state);
+}
+
+void PNGAPI pngComplete(png_structp png, png_infop)
+{
+  imageDecoder(png)->complete();
+}
+
+void PNGAPI pngFailed(png_structp png, png_const_charp err)
+{
+  longjmp(JMPBUF(png), 1);
+}
+
+} // namespace
+
+namespace blink {
+
+// This is the callback function for unknown PNG chunks, which is used to
+// extract the animation chunks.
+static int readAnimationChunk(png_structp png_ptr, png_unknown_chunkp chunk)
+{
+  PNGImageReader* reader = (PNGImageReader*) png_get_user_chunk_ptr(png_ptr);
+  reader->parseAnimationChunk((const char*) chunk->name, chunk->data,
+                              chunk->size);
+  return 1;
+}
+
+PNGImageReader::PNGImageReader(PNGImageDecoder* decoder, size_t initialOffset)
+  : m_decoder(decoder)
+  , m_initialOffset(initialOffset)
+  , m_readOffset(initialOffset)
+  , m_progressiveDecodeOffset(0)
+  , m_idatOffset(0)
+  , m_hasAlpha(false)
+  , m_idatIsPartOfAnimation(false)
+  , m_isAnimated(false)
+  , m_parsedSignature(false)
+  , m_parseCompleted(false)
+#if USE(QCMSLIB)
+  , m_rowBuffer()
+#endif
+{
+  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);
+
+  // Keep the chunks which are of interest for APNG. We don't need to keep
+  // the fdAT chunks, since they are converted to IDAT's by the frame decoder.
+  png_byte apngChunks[] = {"acTL\0fcTL\0"};
+  png_set_keep_unknown_chunks(m_png, PNG_HANDLE_CHUNK_NEVER, apngChunks, 2);
+  png_set_read_user_chunk_fn(m_png, (png_voidp) this, readAnimationChunk);
+}
+
+PNGImageReader::~PNGImageReader()
+{
+  png_destroy_read_struct(m_png ? &m_png : 0, m_info ? &m_info : 0, 0);
+  ASSERT(!m_png && !m_info);
+}
+
+// This method reads from the FastSharedBufferReader, starting at offset,
+// and returns |length| bytes in the form of a pointer to a const png_byte*.
+// This function is used to make it easy to access data from the reader in a
+// png friendly way, and pass it to libpng for decoding.
+//
+// Pre-conditions before using this:
+// - |reader|.size() >= |readOffset| + |length|
+// - |buffer|.size() >= |length|
+// - |length| <= |kBufferSize|
+//
+// The reason for the last two precondition is that currently the png signature
+// plus IHDR chunk (8B + 25B = 33B) is the largest chunk that is read using this
+// method. If the data is not consecutive, it is stored in |buffer|, which must
+// have the size of (at least) |length|, but there's no need for it to be larger
+// than |kBufferSize|.
+static constexpr size_t kBufferSize = 33;
+const png_byte* readAsConstPngBytep(const FastSharedBufferReader& reader,
+                                    size_t readOffset, size_t length,
+                                    char* buffer)
+{
+  ASSERT(length <= kBufferSize);
+  return reinterpret_cast<const png_byte*>(
+      reader.getConsecutiveData(readOffset, length, buffer));
+}
+
+// This is used as a value for the byteLength of a frameInfo struct to
+// indicate that it is the first frame, and we still need to set byteLength
+// to the correct value as soon as the parser knows it. 1 is a safe value
+// since the byteLength field of a frame is at least 12, in the case of an
+// empty fdAT or IDAT chunk.
+static constexpr size_t kFirstFrameIndicator = 1;
+
+void PNGImageReader::decode(SegmentReader& data, size_t index)
+{
+  if (index >= m_frameInfo.size())
+    return;
+
+  // When decoding by libpng fails in either the non-animated branch or the
+  // animated branch, the decoder needs to be set to the failed state.
+  if (setjmp(JMPBUF(m_png))) {
+    m_decoder->setFailed();
+    return;
+  }
+  
+  // For non animated PNGs, resume decoding where we left off in parse(), at
+  // the beginning of the IDAT chunk. Recreating a png struct would either
+  // result in wasted work, by reprocessing all header bytes, or decoding the
+  // wrong data.
+  if (!m_isAnimated) {
+    m_progressiveDecodeOffset += processData(
+      data, m_frameInfo[0].startOffset + m_progressiveDecodeOffset, 0);
+    return;
+  }
+
+  // Progressive decoding is only done if both of the following are true:
+  // - It is the first frame, thus |index| == 0, AND
+  // - The byteLength of the first frame is not yet known, *or* it is known
+  //   but we're only partway in a progressive decode, started earlier.
+  bool firstFrameLengthKnown = m_frameInfo[0].byteLength
+                                 != kFirstFrameIndicator;
+  bool progressiveDecodingAlreadyStarted = m_progressiveDecodeOffset > 0;
+  bool progressiveDecode = (index == 0
+      && (!firstFrameLengthKnown || progressiveDecodingAlreadyStarted));
+  
+  // Initialize a new png struct for this frame. For a progressive decode of
+  // the first frame, we only need to do this once.
+  // @FIXME(joostouwerling) check if the existing png struct can be reused. 
+  if (!progressiveDecode || !progressiveDecodingAlreadyStarted)
+    startFrameDecoding(data, index);
+  
+  bool decodedFrameCompletely;
+  if (progressiveDecode) {
+    decodedFrameCompletely = progressivelyDecodeFirstFrame(data);
+    // If progressive decoding processed all data for this frame, reset
+    // |m_progressiveDecodeOffset|, so |progressiveDecodingAlreadyStarted|
+    // will be false for later calls to decode frame 0.
+    if (decodedFrameCompletely)
+      m_progressiveDecodeOffset = 0;
+  } else {
+    decodeFrame(data, index);
+    // For a non-progressive decode, we already have all the data we are
+    // going to get, so consider the frame complete.
+    decodedFrameCompletely = true;
+  }
+  
+  // Send the IEND chunk if the frame is completely decoded, so the complete
+  // callback in |m_decoder| will be called.
+  if (decodedFrameCompletely)
+    endFrameDecoding();
+}
+
+void PNGImageReader::startFrameDecoding(SegmentReader& data, size_t index)
+{
+  // Each frame is processed as if it were a complete, single frame png image.
+  // To accomplish this, destroy the current |m_png| and |m_info| structs and
+  // create new ones. CRC errors are ignored, so fdAT chunks can be processed
+  // as IDATs without recalculating the CRC value. 
+  png_destroy_read_struct(m_png ? &m_png : 0, m_info ? &m_info : 0, 0);
+  m_png = png_create_read_struct(PNG_LIBPNG_VER_STRING, 0, pngFailed, 0);
+  m_info = png_create_info_struct(m_png);
+  png_set_crc_action(m_png, PNG_CRC_QUIET_USE, PNG_CRC_QUIET_USE);
+  png_set_progressive_read_fn(m_png, m_decoder, pngHeaderAvailable,
+                              pngRowAvailable, pngComplete);
+   
+  // If the frame is the size of the whole image, we don't need to modify any
+  // data in the IHDR chunk. This means it suffices to re-process all header
+  // data up to the first frame, for mimicking a png image.
+  const IntRect& frameRect = m_frameInfo[index].frameRect;
+  if (frameRect.location() == IntPoint()
+      && frameRect.size() == m_decoder->size()) {
+    processData(data, m_initialOffset, m_idatOffset); 
+    return;
+  }
+
+  // Process the IHDR chunk, but change the width and height so it reflects
+  // the frame's width and height. Image Decoder will apply the x,y offset.
+  // This step is omitted if the width and height are equal to the image size,
+  // which is done in the block above.
+  FastSharedBufferReader reader(&data);
+  char readBuffer[kBufferSize];
+
+  // |headerSize| is equal to |kBufferSize|, but adds more semantic insight.
+  constexpr size_t headerSize = 33;
+  png_byte header[headerSize];
+  const png_byte* chunk = readAsConstPngBytep(reader, m_initialOffset,
+                                              headerSize, readBuffer);
+  memcpy(header, chunk, headerSize);
+  
+  // Write the unclipped width and height. Clipping happens in the decoder.
+  png_save_uint_32(header + 16, frameRect.width());
+  png_save_uint_32(header + 20, frameRect.height());
+  png_process_data(m_png, m_info, header, headerSize);
+
+  // Process the rest of the header chunks. Start after the PNG signature and
+  // IHDR chunk, 33B, and process up to the first data chunk. The number of 
+  // bytes up to the first data chunk is stored in |m_idatOffset|.
+  processData(data, m_initialOffset + headerSize, m_idatOffset - headerSize);
+}
+
+// Determine if the bytes 4 to 7 of |chunk| indicate that it is a |tag| chunk.
+// - The length of |chunk| must be >= 8
+// - The length of |tag| must be = 4
+static inline bool isChunk(const png_byte* chunk, const char* tag)

[scroggo_chromium, 2016/11/07 13:02:16]

I like this :)

[joostouwerling, 2016/11/08 21:58:17]

Acknowledged.

+{
+  return memcmp(chunk + 4, tag, 4) == 0;
+}
+
+bool PNGImageReader::progressivelyDecodeFirstFrame(SegmentReader& data)
+{
+  FastSharedBufferReader reader(&data);
+  char readBuffer[8]; // large enough to identify a chunk.
+  size_t offset = m_frameInfo[0].startOffset;
+
+  // Loop while there is enough data to do progressive decoding.
+  while (data.size() >= offset + 8) {
+
+    // At the beginning of each loop, the offset is at the start of a chunk.
+    const png_byte* chunk = readAsConstPngBytep(reader, offset, 8,
+                                                readBuffer);
+    const png_uint_32 length = png_get_uint_32(chunk);
+    
+    // When an fcTL or IEND chunk is encountered, the frame data has ended.
+    // Return true, since all frame data is decoded.
+    if (isChunk(chunk, "fcTL") || isChunk(chunk, "IEND"))

[scroggo_chromium, 2016/11/07 13:02:16]

Previously, you were comparing the first four bytes, and now you're comparing
the second four bytes? (I know the check against IEND was wrong, but I thought
the one for fcTL was correct?)

[joostouwerling, 2016/11/08 21:58:17]

No, that one was wrong as well, since the first four bytes contain the length.
The previous if-condition would never evaluate to true, since the first memcmp
would always compare a number with "fcTL" and the second memcmp would always
return 0 since zero bytes were compared.

This was not caught by the tests since, frankly, nothing broke. This loop would
continue to decode chunks, skipping all unknown fcTL and fdAT chunks
(PNGImageReader::startFrameDecoding() does not specify to keep unknown chunks),
until it ends up at the IEND chunk, and through scenario 3, would process it
normally. This would invoke the frameComplete callback and thus the status of
the frame buffer would be set to FrameComplete.
PNGImageReader::progressivelyDecodeFirstFrame() would return false, but that
only results in PNGImageReader::endFrameDecoding() not being called, but the
effect of that method was already achieved by processing the IEND chunk.

[scroggo_chromium, 2016/11/09 13:42:50]

Please add a test that verifies you got this correct.

[joostouwerling, 2016/11/11 20:22:18]

As discussed in person, this is hard to test without exposing the private parts
of PNGImageReader.

+      return true;
+    
+    // If this chunk was already decoded, move on to the next.
+    if (m_progressiveDecodeOffset >= offset + length + 12) {
+      offset += length + 12;
+      continue;
+    }
+
+    // At this point, three scenarios are possible:
+    // 1) Some bytes of this chunk were already decoded in a previous call,
+    //    so we need to continue from there.
+    // 2) This is an fdAT chunk, so we need to convert it to an IDAT chunk
+    //    before we can decode it.
+    // 3) This is any other chunk, most likely an IDAT chunk.
+    //
+    // In each scenario, we want to decode as much data as possible. In each
+    // one, do the scenario specific work and set |offset| to where decoding
+    // needs to continue. From there, decode until the end of the chunk, if
+    // possible. If the whole chunk is decoded, continue to the next loop.
+    // Otherwise, store how far we've come in |m_progressiveDecodeOffset| and
+    // return false to indicate to the caller that the frame is partially
+    // decoded.
+
+    size_t endOffsetChunk = offset + length + 12;
+
+    // Scenario 1: |m_progressiveDecodeOffset| is ahead of the chunk tag.
+    if (m_progressiveDecodeOffset >= offset + 8) {
+      offset = m_progressiveDecodeOffset;
+    
+    // Scenario 2: we need to convert the fdAT to an IDAT chunk. For an
+    // explanation of the numbers, see the comments in decodeFrame().
+    } else if (isChunk(chunk, "fdAT")) {

[joostouwerling, 2016/11/08 21:58:17]

Same holds here, as for what you mentioned above. The postmortem analysis here
is that my tests didn't catch this because I was only testing an image where the
first frame is an IDAT chunk.

+      png_byte chunkIDAT[] = {0, 0, 0, 0, 'I', 'D', 'A', 'T'};
+      png_save_uint_32(chunkIDAT, length - 4);
+      png_process_data(m_png, m_info, chunkIDAT, 8);
+      // Skip the sequence number
+      offset += 12;
+    
+    // Scenario 3: for any other chunk type, process the first 8 bytes.
+    } else {
+      png_process_data(m_png, m_info, const_cast<png_byte*>(chunk), 8);
+      offset += 8;
+    }
+
+    size_t bytesLeftInChunk = endOffsetChunk - offset;
+    size_t bytesDecoded = processData(data, offset, bytesLeftInChunk);
+    m_progressiveDecodeOffset = offset + bytesDecoded;
+    if (bytesDecoded < bytesLeftInChunk)
+      return false;
+    offset += bytesDecoded;
+  }    
+
+  return false;
+}
+
+void PNGImageReader::decodeFrame(SegmentReader& data, size_t index)
+{
+  // From the frame info that was gathered during parsing, it is known at
+  // what offset the frame data starts and how many bytes are in the stream
+  // before the frame ends. Using this, we process all chunks that fall in
+  // this interval. We catch every fdAT chunk and transform it to an IDAT
+  // chunk, so libpng will decode it like a non-animated PNG image.
+  size_t offset = m_frameInfo[index].startOffset;
+  size_t endOffset = offset + m_frameInfo[index].byteLength;
+  char readBuffer[8];
+  FastSharedBufferReader reader(&data);
+
+  while (offset < endOffset) {
+    const png_byte* chunk = readAsConstPngBytep(reader, offset, 8, readBuffer);
+    const png_uint_32 length = png_get_uint_32(chunk);
+    if (isChunk(chunk, "fdAT")) {
+      // An fdAT chunk is build up as follows: 
+      // - |length| (4B)
+      // - fdAT tag (4B)
+      // - sequence number (4B) 
+      // - frame data (|length| - 4B)
+      // - CRC (4B)
+      // Thus, to reformat this into an IDAT chunk, we need to:
+      // - write |length| - 4 as the new length, since the sequence number
+      //   must be removed.
+      // - change the tag to IDAT.
+      // - omit the sequence number from the data part of the chunk.
+      png_byte chunkIDAT[] = {0, 0, 0, 0, 'I', 'D', 'A', 'T'};
+      png_save_uint_32(chunkIDAT, length - 4);
+      png_process_data(m_png, m_info, chunkIDAT, 8);
+      // The frame data and the CRC span |length| bytes, so skip the
+      // sequence number and process |length| bytes to decode the frame.
+      processData(data, offset + 12, length);
+    } else {
+      png_process_data(m_png, m_info, const_cast<png_byte*>(chunk), 8);
+      processData(data, offset + 8, length + 4);
+    }
+    offset += 12 + length; 
+  }
+}
+
+void PNGImageReader::endFrameDecoding()
+{
+  png_byte IEND[12] = {0, 0, 0, 0, 'I', 'E', 'N', 'D', 174, 66, 96, 130};
+  png_process_data(m_png, m_info, IEND, 12);
+}
+
+bool PNGImageReader::parse(SegmentReader& data,

[scroggo_chromium, 2016/11/07 13:02:16]

I think early on in this method you want to check for m_parseCompleted and
return if it's true.

[joostouwerling, 2016/11/08 21:58:17]

Done.

+                           PNGImageDecoder::PNGParseQuery query)
+{
+  if (setjmp(JMPBUF(m_png)))
+    return m_decoder->setFailed();
+
+  // If the size has not been parsed, do that first, since it's necessary
+  // for both the Size and MetaData query. If parseSize returns false,
+  // it failed because of a lack of data so we can return false at this point.
+  if (!m_decoder->isDecodedSizeAvailable() && !parseSize(data))
+    return false;
+  
+  if (query == PNGImageDecoder::PNGParseQuery::PNGSizeQuery)
+    return m_decoder->isDecodedSizeAvailable();
+  
+  // For non animated images (identified by no acTL chunk before the IDAT), 
+  // we create one frame. This saves some processing time since we don't need
+  // to go over the stream to find chunks.
+  if (!m_isAnimated) {
+    if (m_frameInfo.isEmpty()) {
+      FrameInfo frame;
+      // This needs to be plus 8 since the first 8 bytes of the IDAT chunk
+      // are already processed in parseSize().
+      frame.startOffset = m_readOffset + 8;
+      frame.frameRect = IntRect(IntPoint(), m_decoder->size());
+      frame.duration = 0;
+      frame.alphaBlend = ImageFrame::AlphaBlendSource::BlendAtopBgcolor;
+      frame.disposalMethod = ImageFrame::DisposalMethod::DisposeNotSpecified;
+      m_frameInfo.append(frame);
+      // When the png is not animated, no extra parsing is necessary.
+      m_parseCompleted = true;
+    }
+    return true;
+  }
+
+  FastSharedBufferReader reader(&data);
+  char readBuffer[kBufferSize];
+
+  // At this point, the query is FrameMetaDataQuery. Loop over the data and
+  // register all frames we can find. A frame is registered on the next fcTL
+  // chunk or when the IEND chunk is found. This ensures that only complete
+  // frames are reported, unless there is an error in the stream.
+  while (reader.size() >= m_readOffset + 8) {
+    const png_byte* chunk = readAsConstPngBytep(reader, m_readOffset, 8,
+                                                readBuffer);
+    const size_t length = png_get_uint_32(chunk);
+    
+    // When we find an IDAT chunk (when the IDAT is part of the animation),
+    // or an fdAT chunk, and the readOffset field of the newFrame is 0,
+    // we have found the beginning of a new block of frame data.
+    const bool isFrameData = isChunk(chunk, "fdAT")
+      || (isChunk(chunk, "IDAT") && m_idatIsPartOfAnimation);
+    if (m_newFrame.startOffset == 0 && isFrameData) {
+      m_newFrame.startOffset = m_readOffset;
+      
+      // When the |frameInfo| vector is empty, the first frame needs to be
+      // reported as soon as possible, even before all frame data is in
+      // |data|, so the first frame can be decoded progressively. 
+      if (m_frameInfo.isEmpty()) {
+        m_newFrame.byteLength = kFirstFrameIndicator;
+        m_frameInfo.append(m_newFrame); 
+      }
+
+    // An fcTL or IEND marks the end of the previous frame. Thus, the
+    // FrameInfo data in m_newFrame is submitted to the m_frameInfo vector.
+    //
+    // Furthermore, an fcTL chunk indicates a new frame is coming,
+    // so the m_newFrame variable is prepared accordingly by setting the 
+    // readOffset field to 0, which indicates that the frame control info
+    // is available but that we haven't seen any frame data yet.
+    } else if (isChunk(chunk, "fcTL") || isChunk(chunk, "IEND")) {
+      if (m_newFrame.startOffset != 0) {
+        m_newFrame.byteLength = m_readOffset - m_newFrame.startOffset;
+        if (m_frameInfo[0].byteLength == kFirstFrameIndicator)
+          m_frameInfo[0].byteLength = m_newFrame.byteLength;
+        else
+          m_frameInfo.append(m_newFrame);
+
+        m_newFrame.startOffset = 0;
+      }
+
+      if (reader.size() < m_readOffset + 12 + length)
+        return false;
+      
+      if (isChunk(chunk, "IEND")) {
+        // The PNG image ends at the IEND chunk, so all parsing is completed.
+        m_parseCompleted = true;
+        return true;
+      }
+
+      // At this point, we're dealing with an fcTL chunk, since the above
+      // statement already returns on IEND chunks.
+      
+      // If the fcTL chunk is not 26 bytes long, we can't process it.
+      if (length != 26)
+        return m_decoder->setFailed();
+
+      chunk = readAsConstPngBytep(reader, m_readOffset + 8, length, readBuffer);
+      parseFrameInfo(chunk);
+
+    }
+    m_readOffset += 12 + length;
+  }
+  return false; 
+}
+
+// If |length| == 0, read until the stream ends.
+// @return: number of bytes processed.
+size_t PNGImageReader::processData(SegmentReader& data, size_t offset,
+                                   size_t length)
+{
+  const char* segment;
+  size_t totalProcessedBytes = 0;
+  while (size_t segmentLength = data.getSomeData(segment, offset)) {
+    if (length > 0 && segmentLength + totalProcessedBytes > length)
+      segmentLength = length - totalProcessedBytes;
+    
+    png_process_data(m_png, m_info,
+                     reinterpret_cast<png_byte*>(const_cast<char*>(segment)),
+                     segmentLength);
+    offset += segmentLength;
+    totalProcessedBytes += segmentLength;
+    if (totalProcessedBytes == length)
+      return length;
+  }
+  return totalProcessedBytes;
+}
+
+// This methods reads through the stream until it has parsed the image size.
+// @return true when it succeeds in parsing the size.
+//         false when:
+//         A) not enough data is provided
+//         B) decoding by libpng fails. In the this case, it will also call
+//            setFailed on m_decoder. 
+bool PNGImageReader::parseSize(SegmentReader &data)
+{
+  FastSharedBufferReader reader(&data);
+  char readBuffer[kBufferSize];
+
+  // Process the PNG signature and the IHDR with libpng, such that this code 
+  // does not need to be bothered with parsing the contents. This also enables
+  // the reader to use the existing headerAvailable callback in the decoder.
+  //
+  // When we already have decoded the signature, we don't need to do it again.
+  // By setting a flag for this we allow for byte by byte parsing.
+  if (!m_parsedSignature) {
+    if (reader.size() < m_readOffset + 8)
+      return false;
+    
+    const png_byte* chunk = readAsConstPngBytep(reader, m_readOffset, 8,
+                                                readBuffer);
+    png_process_data(m_png, m_info, const_cast<png_byte*>(chunk), 8);
+    m_readOffset += 8;
+    m_parsedSignature = true;
+    // Initialize the newFrame by setting the readOffset to 0.
+    m_newFrame.startOffset = 0;
+  }
+
+  // This loop peeks at the chunk tag until the IDAT chunk is found. When
+  // a different tag is encountered, pass it on to libpng for general parsing.
+  // We can peek at chunks by looking at the first 8 bytes, which contain the
+  // length and the chunk tag.
+  //
+  // When an fcTL (frame control) is encountered before the IDAT, the frame
+  // data in the IDAT chunk is part of the animation. This case is flagged
+  // and the frame info is stored by parsing the fcTL chunk.
+  while (reader.size() >= m_readOffset + 8) {
+    const png_byte* chunk = readAsConstPngBytep(reader, m_readOffset, 8,
+                                                readBuffer);
+    const png_uint_32 length = png_get_uint_32(chunk);
+
+    // If we encounter the IDAT chunk, we're done with the png header
+    // chunks. Indicate this to libpng by sending the beginning of the IDAT
+    // chunk, which will trigger libpng to call the headerAvailable
+    // callback on m_decoder. This provides the size to the decoder.
+    if (isChunk(chunk, "IDAT")) {
+      m_idatOffset = m_readOffset;
+      png_process_data(m_png, m_info, const_cast<png_byte*>(chunk), 8);
+      return true;
+    }
+
+    // Consider the PNG image animated if an acTL chunk of the correct
+    // length is present. Parsing the acTL content is done by 
+    // parseAnimationControl, called by libpng's png_process_data.
+    if (isChunk(chunk, "acTL") && length == 8)
+      m_isAnimated = true;
+
+    // We don't need to check for |length| here, because the decoder will
+    // fail later on for invalid fcTL chunks.
+    if (isChunk(chunk, "fcTL"))
+      m_idatIsPartOfAnimation = true;
+    
+    // 12 is the length, tag and crc part of the chunk, which are all 4B.
+    if (reader.size() < m_readOffset + length + 12)
+      break;  
+
+    png_process_data(m_png, m_info, const_cast<png_byte*>(chunk), 8);
+    processData(data, m_readOffset + 8, length + 4);
+    m_readOffset += length + 12;
+  }
+
+  // If we end up here, not enough data was available for the IDAT chunk
+  // So libpng would not have called headerAvailable yet.
+  return false;
+} 
+
+
+void PNGImageReader::parseAnimationChunk(const char tag[],
+                                         const void* data_chunk,
+                                         size_t length)
+{
+  const png_byte* data = static_cast<const png_byte*>(data_chunk);
+
+  // The number of frames as indicated in the animation control chunk (acTL) 
+  // is ignored, and the number of frames that are actually present is used.
+  // For now, when the number of indicated frames is different from the
+  // number of supplied frames, the number of supplied frames is what is
+  // provided to the decoder. Therefore, it does not add any benefit of
+  // looking at the value of the indicated framecount. A note here is that
+  // there may be optimisations available, for example, prescaling vectors.
+  if (strcmp(tag, "acTL") == 0 && length == 8) {
+    png_uint_32 repetitionCount = png_get_uint_32(data + 4);
+    m_decoder->setRepetitionCount(repetitionCount);
+  
+  // For fcTL, decoding fails if it does not have the correct length. It is
+  // impossible to make a guess about the frame if not all data is available.
+  // Use longjmp to get back to parse(), which is necessary since this method
+  // is called by a libpng callback.
+  } else if (strcmp(tag, "fcTL") == 0) {
+    if (length != 26)
+      longjmp(JMPBUF(m_png), 1);
+    parseFrameInfo(data);
+  }
+}
+
+void PNGImageReader::clearDecodeState(size_t frameIndex) {
+  if (frameIndex == 0)
+    m_progressiveDecodeOffset = 0;
+}
+
+size_t PNGImageReader::frameCount() const
+{
+  return m_frameInfo.size();
+}
+
+const PNGImageReader::FrameInfo& PNGImageReader::frameInfo(size_t index) const
+{
+  ASSERT(index < m_frameInfo.size());
+  return m_frameInfo[index];
+}
+
+// Extract the frame control info and store it in m_newFrame. The length check
+// on the data chunk has been done in parseAnimationChunk.
+// The fcTL specification used can be found at:
+// https://wiki.mozilla.org/APNG_Specification#.60fcTL.60:_The_Frame_Control_Chunk
+void PNGImageReader::parseFrameInfo(const png_byte* data)
+{
+  png_uint_32 width, height, xOffset, yOffset;
+  png_uint_16 delayNumerator, delayDenominator;
+  width             = png_get_uint_32(data + 4);
+  height            = png_get_uint_32(data + 8);
+  xOffset           = png_get_uint_32(data + 12);
+  yOffset           = png_get_uint_32(data + 16);
+  delayNumerator    = png_get_uint_16(data + 20);
+  delayDenominator  = png_get_uint_16(data + 22);
+
+  m_newFrame.duration       = (delayDenominator == 0) ? delayNumerator * 10
+                                : delayNumerator * 1000 / delayDenominator;
+  m_newFrame.frameRect      = IntRect(xOffset, yOffset, width, height);
+  m_newFrame.disposalMethod = data[24];
+  m_newFrame.alphaBlend     = data[25];
+}
+
+}; // namespace blink