VideoToolbox encoder for cast senders.

media/cast/sender/h264_vt_encoder.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "media/cast/sender/h264_vt_encoder.h"
+
+#include <algorithm>
+#include <vector>

[miu, 2014/08/25 19:21:17]

Looks like std::vector isn't being used anywhere.

[jfroy, 2014/08/25 20:59:13]

Done.

+
+#include "base/big_endian.h"
+#include "base/bind.h"
+#include "base/bind_helpers.h"
+#include "base/location.h"
+#include "base/logging.h"
+
+namespace media {
+namespace cast {
+
+namespace {
+
+bool SetSessionProperty(VTSessionRef session, CFStringRef key, uint32_t value) {
+  base::ScopedCFTypeRef<CFNumberRef> cfvalue(
+      CFNumberCreate(NULL, kCFNumberSInt32Type, &value));
+  return VTSessionSetProperty(session, key, cfvalue) == noErr;
+}
+
+bool SetSessionProperty(VTSessionRef session, CFStringRef key, bool value) {
+  CFBooleanRef cfvalue = (value) ? kCFBooleanTrue : kCFBooleanFalse;
+  return VTSessionSetProperty(session, key, cfvalue) == noErr;
+}
+
+bool SetSessionProperty(VTSessionRef session,
+                        CFStringRef key,
+                        CFStringRef value) {
+  return VTSessionSetProperty(session, key, value) == noErr;
+}
+
+base::ScopedCFTypeRef<CFDictionaryRef> DictionaryWithKeyValue(CFTypeRef key,
+                                                              CFTypeRef value) {
+  CFTypeRef keys[1] = {key};
+  CFTypeRef values[1] = {value};
+  return base::ScopedCFTypeRef<CFDictionaryRef>(
+      CFDictionaryCreate(kCFAllocatorDefault,
+                         keys,
+                         values,
+                         1,
+                         &kCFTypeDictionaryKeyCallBacks,
+                         &kCFTypeDictionaryValueCallBacks));
+}
+
+struct FrameContext {
+  base::TimeTicks capture_time;
+  media::cast::VideoEncoder::FrameEncodedCallback frame_encoded_callback;
+};
+
+}  // namespace
+
+H264VideoToolboxEncoder::H264VideoToolboxEncoder(
+    scoped_refptr<CastEnvironment> cast_environment,
+    const VideoSenderConfig& video_config)
+    : cast_environment_(cast_environment),
+      cast_config_(video_config),
+      frame_id_(kStartFrameId),
+      last_keyframe_id_(kStartFrameId),
+      encode_next_frame_as_keyframe_(false) {
+  Initialize();
+}
+
+H264VideoToolboxEncoder::~H264VideoToolboxEncoder() {
+  Teardown();
+}
+
+CVPixelBufferPoolRef H264VideoToolboxEncoder::cv_pixel_buffer_pool() const {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(compression_session_);
+  return VTCompressionSessionGetPixelBufferPool(compression_session_);
+}
+
+void H264VideoToolboxEncoder::Initialize() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(!compression_session_);
+
+  // Note that the encoder object is given to the compression session as the
+  // callback context using a raw pointer. The C API does not allow us to use
+  // a smart pointer, nor is this encoder ref counted. However, this is still
+  // safe, because we 1) we own the compression session and 2) we tear it down
+  // safely. When destructing the encoder, the compression session is flushed
+  // and invalidated. Internally, VideoToolbox will join all of its threads
+  // before returning to the client. Therefore, when control returns to us, we
+  // are guaranteed that the output callback will not execute again.
+
+  // On OS X, allow the hardware encoder. Don't require it, it does not support
+  // all configurations (some of which are used for testing).
+  base::ScopedCFTypeRef<CFDictionaryRef> encoder_spec;
+#if !defined(OS_IOS)
+  encoder_spec = DictionaryWithKeyValue(
+      kVTVideoEncoderSpecification_EnableHardwareAcceleratedVideoEncoder,
+      kCFBooleanTrue);
+#endif
+
+  VTCompressionSessionRef session;
+  OSStatus status =
+      VTCompressionSessionCreate(kCFAllocatorDefault,
+                                 cast_config_.width,
+                                 cast_config_.height,
+                                 kCMVideoCodecType_H264,
+                                 encoder_spec,
+                                 NULL /* sourceImageBufferAttributes */,
+                                 NULL /* compressedDataAllocator */,
+                                 CompressionCallback,
+                                 reinterpret_cast<void*>(this),
+                                 &session);
+  if (status != noErr) {
+    DLOG(ERROR) << " VTCompressionSessionCreate failed: " << status;
+    return;
+  }
+  compression_session_.reset(session);
+
+  ConfigureSession();
+}
+
+void H264VideoToolboxEncoder::ConfigureSession() {
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_ProfileLevel,
+                     kVTProfileLevel_H264_Main_AutoLevel);
+  SetSessionProperty(
+      compression_session_, kVTCompressionPropertyKey_RealTime, true);
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_AllowFrameReordering,
+                     false);
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_MaxKeyFrameInterval,
+                     240u);
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_MaxKeyFrameIntervalDuration,
+                     240u);
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_AverageBitRate,
+                     static_cast<uint32_t>(cast_config_.start_bitrate));
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_ExpectedFrameRate,
+                     static_cast<uint32_t>(cast_config_.max_frame_rate));
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_ColorPrimaries,
+                     kCVImageBufferColorPrimaries_ITU_R_709_2);
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_TransferFunction,
+                     kCVImageBufferTransferFunction_ITU_R_709_2);
+  SetSessionProperty(compression_session_,
+                     kVTCompressionPropertyKey_YCbCrMatrix,
+                     kCVImageBufferYCbCrMatrix_ITU_R_709_2);
+}
+
+void H264VideoToolboxEncoder::Teardown() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  // If the compression session exists, invalidate it. This blocks until all
+  // pending output callbacks have returned and any internal threads have
+  // joined, ensuring no output callback ever sees a dangling encoder pointer.
+  if (compression_session_) {
+    VTCompressionSessionInvalidate(compression_session_);
+    compression_session_.reset();
+  }
+}
+
+bool H264VideoToolboxEncoder::EncodeVideoFrame(
+    const scoped_refptr<media::VideoFrame>& video_frame,
+    const base::TimeTicks& capture_time,
+    const FrameEncodedCallback& frame_encoded_callback) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(!capture_time.is_null());
+
+  if (!compression_session_) {
+    DLOG(ERROR) << " compression session is null";
+    return false;
+  }
+
+  base::ScopedCFTypeRef<CVPixelBufferRef> pixel_buffer(
+      video_frame->cv_pixel_buffer(), base::scoped_policy::RETAIN);
+  if (!pixel_buffer) {
+    pixel_buffer = WrapVideoFrame(video_frame);

[miu, 2014/08/25 22:22:09]

Just to retain the findings of our discussion (what was primarily my confusion),
can you add a comment here like: "VideoFrame was not generated from the
accelerated capture code path that provides a CVPixelBuffer, so create an
adapter."

[jfroy, 2014/08/25 22:30:38]

Done.

+    if (!pixel_buffer) {
+      return false;
+    }
+  }
+
+  CMTime timestamp_cm =
+      CMTimeMake(capture_time.ToInternalValue(), USEC_PER_SEC);
+
+  scoped_ptr<FrameContext> frame_context(new FrameContext());
+  frame_context->capture_time = capture_time;
+  frame_context->frame_encoded_callback = frame_encoded_callback;
+
+  base::ScopedCFTypeRef<CFDictionaryRef> frame_props;
+  if (encode_next_frame_as_keyframe_) {
+    frame_props = DictionaryWithKeyValue(kVTEncodeFrameOptionKey_ForceKeyFrame,
+                                         kCFBooleanTrue);
+    encode_next_frame_as_keyframe_ = false;
+  }
+
+  VTEncodeInfoFlags info;
+  OSStatus status = VTCompressionSessionEncodeFrame(
+      compression_session_,
+      pixel_buffer,
+      timestamp_cm,
+      kCMTimeInvalid,
+      frame_props,
+      reinterpret_cast<void*>(frame_context.release()),
+      &info);
+  if (status != noErr) {
+    DLOG(ERROR) << " VTCompressionSessionEncodeFrame failed: " << status;
+    return false;
+  }
+  if ((info & kVTEncodeInfo_FrameDropped)) {
+    DLOG(ERROR) << " frame dropped";
+    return false;
+  }
+
+  return true;
+}
+
+void H264VideoToolboxEncoder::SetBitRate(int new_bit_rate) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  // VideoToolbox does not seem to support bitrate reconfiguration.
+}
+
+void H264VideoToolboxEncoder::GenerateKeyFrame() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(compression_session_);
+
+  encode_next_frame_as_keyframe_ = true;
+}
+
+void H264VideoToolboxEncoder::LatestFrameIdToReference(uint32 /*frame_id*/) {
+  // Not supported by VideoToolbox in any meaningful manner.
+}
+
+static void VideoFramePixelBufferReleaseCallback(void* frame_ref,
+                                                 const void* data,
+                                                 size_t size,
+                                                 size_t num_planes,
+                                                 const void* planes[]) {
+  free(const_cast<void*>(data));
+  reinterpret_cast<media::VideoFrame*>(frame_ref)->Release();
+}
+
+base::ScopedCFTypeRef<CVPixelBufferRef> H264VideoToolboxEncoder::WrapVideoFrame(

[miu, 2014/08/25 19:21:17]

IMHO, it would be cleaner for the CVPixelBuffer to be owned by
media::VideoFrame.  VideoFrame could create the CVPixelBuffer lazily, and keep
it around until the VideoFrame dies.  This would allow the same CVPixelBuffer to
be re-used rather than having to create a new one for each use.

In other words, this should be a method of VideoFrame.

[jfroy, 2014/08/25 20:59:13]

I really don't want to burden VideoFrame with more platform-specific code (this
sentiment has been expressed by others as well reviewing the other patch).

In addition, the CVPixelBuffer must own the VideoFrame in this situation because
VideoToolbox manages the lifetime of CVPixelBuffers, not VideoFrames. The
encoder may hold on to input frames for some time, and the encoding process is
asynchronous. It is very likely that the VideoFrame would be destroyed before
the CVPixelBuffer, if the pixel buffer did not take a reference on the
VideoFrame.

[miu, 2014/08/25 21:47:40]

Okay.  I'm fine with this scheme.  But, I don't think you should check in the
other change (https://codereview.chromium.org/446163003/) which does the
wrapping in the other order (VideoFrame wrapping CVPixelBuffer).

[jfroy, 2014/08/25 21:57:56]

This is essentially a fallback function. It is expected that in the final
production version, this code path will not execute because the VideoFrames will
wrap a CVPixelBuffer backed by the encoder's CVPixelBufferPool. It is there to
ensure the encoder will not completely fail if the optimization enabled by
450693006 is not used by some client (perhaps Chrome for tab casting).

[miu, 2014/08/25 22:22:09]

I understand now.  Thanks for being patient.

[jfroy, 2014/08/25 22:30:38]

No problem :)

+    const scoped_refptr<media::VideoFrame>& frame) {
+  static const size_t MAX_PLANES = 3;
+
+  media::VideoFrame::Format format = frame->format();
+  size_t num_planes = media::VideoFrame::NumPlanes(format);
+  DCHECK_LE(num_planes, MAX_PLANES);
+  gfx::Size coded_size = frame->coded_size();
+
+  // media::VideoFrame only supports YUV formats, so there is no way to
+  // leverage VideoToolbox's ability to convert RGBA formats automatically. In
+  // addition, most of the media::VideoFrame formats are YVU, which VT does not
+  // support. Finally, media::VideoFrame formats do not carry any information
+  // about the color space, transform or any other colorimetric information
+  // that is generally needed to fully specify the input data. So essentially
+  // require that the input be YCbCr 4:2:0 (either planar or biplanar) and
+  // assume the standard video dynamic range for samples (although most modern
+  // HDTVs support full-range video these days).
+  OSType pixel_format;
+  if (format == media::VideoFrame::Format::I420) {
+    pixel_format = kCVPixelFormatType_420YpCbCr8Planar;
+  } else if (format == media::VideoFrame::Format::NV12) {
+    pixel_format = kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange;
+  } else {
+    DLOG(ERROR) << " unsupported frame format: " << format;
+    return base::ScopedCFTypeRef<CVPixelBufferRef>(NULL);
+  }
+
+  // TODO(jfroy): Support extended pixels (i.e. padding).
+  if (frame->coded_size() != frame->visible_rect().size()) {
+    DLOG(ERROR) << " frame with extended pixels not supported: "
+                << " coded_size: " << coded_size.ToString()
+                << ", visible_rect: " << frame->visible_rect().ToString();
+    return base::ScopedCFTypeRef<CVPixelBufferRef>(NULL);
+  }
+
+  void* plane_ptrs[MAX_PLANES];
+  size_t plane_widths[MAX_PLANES];
+  size_t plane_heights[MAX_PLANES];
+  size_t plane_bytes_per_row[MAX_PLANES];
+  for (size_t plane_i = 0; plane_i < num_planes; ++plane_i) {
+    plane_ptrs[plane_i] = frame->data(plane_i);
+    gfx::Size plane_size =
+        media::VideoFrame::PlaneSize(format, plane_i, coded_size);
+    plane_widths[plane_i] = plane_size.width();
+    plane_heights[plane_i] = plane_size.height();
+    plane_bytes_per_row[plane_i] = frame->stride(plane_i);
+  }
+
+  // CVPixelBufferCreateWithPlanarBytes needs a dummy plane descriptor or the
+  // release callback will not execute. The descriptor is freed in the callback.
+  void* descriptor =
+      calloc(1,
+             std::max(sizeof(CVPlanarPixelBufferInfo_YCbCrPlanar),
+                      sizeof(CVPlanarPixelBufferInfo_YCbCrBiPlanar)));
+
+  // Wrap the frame's data in a CVPixelBuffer. Because this is a C API, we can't
+  // give it a smart pointer to the frame, so instead pass a raw pointer and
+  // increment the frame's reference count manually.
+  CVPixelBufferRef pixel_buffer;
+  CVReturn result =
+      CVPixelBufferCreateWithPlanarBytes(kCFAllocatorDefault,
+                                         coded_size.width(),
+                                         coded_size.height(),
+                                         format,
+                                         descriptor,
+                                         0,
+                                         num_planes,
+                                         plane_ptrs,
+                                         plane_widths,
+                                         plane_heights,
+                                         plane_bytes_per_row,
+                                         VideoFramePixelBufferReleaseCallback,
+                                         frame.get(),
+                                         NULL,
+                                         &pixel_buffer);
+  if (result != kCVReturnSuccess) {
+    DLOG(ERROR) << " CVPixelBufferCreateWithPlanarBytes failed: " << result;
+    return base::ScopedCFTypeRef<CVPixelBufferRef>(NULL);
+  }
+
+  // The CVPixelBuffer now references the data of the frame, so increment its
+  // reference count manually. The release callback set on the pixel buffer will
+  // release the frame.
+  frame->AddRef();
+
+  return base::ScopedCFTypeRef<CVPixelBufferRef>(pixel_buffer);
+}
+
+void H264VideoToolboxEncoder::CompressionCallback(void* encoder_opaque,
+                                                  void* frame_opaque,
+                                                  OSStatus status,
+                                                  VTEncodeInfoFlags info,
+                                                  CMSampleBufferRef sbuf) {
+  H264VideoToolboxEncoder* encoder =
+      reinterpret_cast<H264VideoToolboxEncoder*>(encoder_opaque);
+  scoped_ptr<FrameContext> frame_context(
+      reinterpret_cast<FrameContext*>(frame_opaque));
+
+  if (status != noErr) {
+    DLOG(ERROR) << " encoding failed: " << status;
+    return;
+  }
+  if ((info & kVTEncodeInfo_FrameDropped)) {
+    DVLOG(2) << " frame dropped";
+    return;
+  }
+  DCHECK_EQ(CMSampleBufferGetNumSamples(sbuf), 1);
+
+  CFDictionaryRef sample_attachments =
+      static_cast<CFDictionaryRef>(CFArrayGetValueAtIndex(
+          CMSampleBufferGetSampleAttachmentsArray(sbuf, true), 0));
+
+  // If the NotSync key is not present, it implies Sync, which indicates a
+  // keyframe (at least I think, VT documentation is, erm, sparse). Could
+  // alternatively use kCMSampleAttachmentKey_DependsOnOthers == false.
+  bool keyframe =
+      CFDictionaryContainsKey(sample_attachments,
+                              kCMSampleAttachmentKey_NotSync) == false;
+
+  // Generate a frame id and update the last keyframe id if needed. VideoToolbox
+  // calls the output callback serially, so this is safe.
+  uint32 frame_id = ++encoder->frame_id_;
+  if (keyframe) {
+    encoder->last_keyframe_id_ = frame_id;
+  }
+
+  CMSampleTimingInfo timing_info;
+  CMSampleBufferGetSampleTimingInfo(sbuf, 0, &timing_info);
+
+  scoped_ptr<EncodedFrame> encoded_frame(new EncodedFrame());
+  encoded_frame->frame_id = frame_id;
+  encoded_frame->reference_time = frame_context->capture_time;
+  encoded_frame->rtp_timestamp =
+      GetVideoRtpTimestamp(frame_context->capture_time);
+  if (keyframe) {
+    encoded_frame->dependency = EncodedFrame::KEY;
+    encoded_frame->referenced_frame_id = frame_id;
+  } else {
+    encoded_frame->dependency = EncodedFrame::DEPENDENT;
+    // H.264 supports complex frame reference schmes (multiple reference frames,
+    // slice references, backward and forward references, etc). This encoder
+    // compromises by using the last keyframe as the reference frame. This will
+    // force retransmission of keyframes, which are necessary for decoding of
+    // any following frames since parameter sets are attached to them, while
+    // allowing other frames to be dropped (which may force the receiver to drop
+    // frames at decode time). Keyframes are emitted at a regular interval, so
+    // this should only cause temporary frame drops.
+    encoded_frame->referenced_frame_id = encoder->last_keyframe_id_;

[miu, 2014/08/25 19:21:17]

If this is true, the encoder might not be very space-efficient since it would
have to: 1) encode key frames more often; and/or 2) encode larger deltas.  You
may want to take a look at what the Android folks are doing for their H264
encoder.

[jfroy, 2014/08/25 20:59:13]

In my experimentation (and there is no documentation for this) is that
VideoToolbox does not report to the client the exact frame dependencies it used.
Furthermore, if a frame depends on more than one frame, then a single
"referenced frame" field is not going to be sufficient.

As far as I understand it, this field is used for frame retransmission, so using
the last encoded keyframe is a reasonable compromise. It will allow the stream
to recover reasonably quickly from a burst of lost packets, which I understand
is the typical scenario in a busy WiFi environment.

The rate at which keyframes are encoded is under our control (see
ConfigureSession), currently set at every 240 frames or 240 seconds. I admit I
have done no performance analysis for these rates yet. I'll try to get in touch
with the Android team to see if they have any (I don't have access to their
source). I also don't know to what extent the hardware encoder respects these
parameters.

[miu, 2014/08/25 21:47:40]

Can it be determined from the encoded data?  (read below)
 
No, that's not at all the case.  These are true frame dependencies, where the
system must absolutely have frame X in order to make frame Y decodable.  IIRC,
the Android folks at one time always made frame X depend on frame X-1 (except
for key frames).  Not sure whether they ever resolved that.

If there's no way to determine the true frame dependencies, you'll have no
choice but to have every non-key frame depend on its immediately prior frame; or
else bad things will happen.

[jfroy, 2014/08/25 21:57:56]

I'd rather not have to decode the video layer data and put a ton of H.264
decoding code here or elsewhere, so conservatively no.
OK, I'll follow what the Android implementation did. I'm getting in touch with
them regarding this specific issue (and other details).

+  }
+
+  CopySampleBufferToAnnexBBuffer(sbuf, &encoded_frame->data, keyframe);
+
+  encoder->cast_environment_->PostTask(
+      CastEnvironment::MAIN,
+      FROM_HERE,
+      base::Bind(frame_context->frame_encoded_callback,
+                 base::Passed(&encoded_frame)));
+}
+
+template <typename NalSizeType>
+static void CopyNalsToAnnexB(char* avcc_buffer,
+                             const size_t avcc_size,
+                             std::string* annexb_buffer) {
+  COMPILE_ASSERT(sizeof(NalSizeType) == 1 || sizeof(NalSizeType) == 2 ||
+                     sizeof(NalSizeType) == 4,
+                 "NAL size type has unsupported size");
+  static const char startcode_3[3] = {0, 0, 1};
+  DCHECK(avcc_buffer);
+  DCHECK(annexb_buffer);
+  size_t bytes_left = avcc_size;
+  while (bytes_left > 0) {
+    DCHECK_GT(bytes_left, sizeof(NalSizeType));
+    NalSizeType nal_size;
+    base::ReadBigEndian(avcc_buffer, &nal_size);
+    bytes_left -= sizeof(NalSizeType);
+    avcc_buffer += sizeof(NalSizeType);
+
+    DCHECK_GE(bytes_left, nal_size);
+    annexb_buffer->append(startcode_3, sizeof(startcode_3));
+    annexb_buffer->append(avcc_buffer, nal_size);
+    bytes_left -= nal_size;
+    avcc_buffer += nal_size;
+  }
+}
+
+void H264VideoToolboxEncoder::CopySampleBufferToAnnexBBuffer(
+    CMSampleBufferRef sbuf,
+    std::string* annexb_buffer,
+    bool keyframe) {
+  // Perform two pass, one to figure out the total output size, and another to
+  // copy the data after having performed a single output allocation. Note that
+  // we'll allocate a bit more because we'll count 4 bytes instead of 3 for
+  // video NALs.
+
+  // TODO(jfroy): There is a bug in
+  // CMVideoFormatDescriptionGetH264ParameterSetAtIndex, iterate until fail.
+
+  OSStatus status;
+
+  // Get the sample buffer's block buffer and format description.
+  CMBlockBufferRef bb = CMSampleBufferGetDataBuffer(sbuf);
+  DCHECK(bb);
+  CMFormatDescriptionRef fdesc = CMSampleBufferGetFormatDescription(sbuf);
+  DCHECK(fdesc);
+
+  size_t bb_size = CMBlockBufferGetDataLength(bb);
+  size_t total_bytes = bb_size;
+
+  size_t pset_count;
+  int nal_size_field_bytes;
+  status = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(
+      fdesc, 0, NULL, NULL, &pset_count, &nal_size_field_bytes);
+  if (status == kCMFormatDescriptionBridgeError_InvalidParameter) {
+    DLOG(WARNING) << " assuming 2 parameter sets and 4 bytes NAL length header";
+    pset_count = 2;
+    nal_size_field_bytes = 4;
+  } else if (status != noErr) {
+    DLOG(ERROR)
+        << " CMVideoFormatDescriptionGetH264ParameterSetAtIndex failed: "
+        << status;
+    return;
+  }
+
+  if (keyframe) {
+    const uint8_t* pset;
+    size_t pset_size;
+    for (size_t pset_i = 0; pset_i < pset_count; ++pset_i) {
+      status = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(
+          fdesc, pset_i, &pset, &pset_size, NULL, NULL);
+      if (status != noErr) {
+        DLOG(ERROR)
+            << " CMVideoFormatDescriptionGetH264ParameterSetAtIndex failed: "
+            << status;
+        return;
+      }
+      total_bytes += pset_size + nal_size_field_bytes;
+    }
+  }
+
+  annexb_buffer->reserve(total_bytes);
+
+  // Copy all parameter sets before keyframes.
+  if (keyframe) {
+    const uint8_t* pset;
+    size_t pset_size;
+    for (size_t pset_i = 0; pset_i < pset_count; ++pset_i) {
+      status = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(
+          fdesc, pset_i, &pset, &pset_size, NULL, NULL);
+      if (status != noErr) {
+        DLOG(ERROR)
+            << " CMVideoFormatDescriptionGetH264ParameterSetAtIndex failed: "
+            << status;
+        return;
+      }
+      static const char startcode_4[4] = {0, 0, 0, 1};
+      annexb_buffer->append(startcode_4, sizeof(startcode_4));
+      annexb_buffer->append(reinterpret_cast<const char*>(pset), pset_size);
+    }
+  }
+
+  // Block buffers can be composed of non-contiguous chunks. For the sake of
+  // keeping this code simple, flatten non-contiguous block buffers.
+  base::ScopedCFTypeRef<CMBlockBufferRef> contiguous_bb(
+      bb, base::scoped_policy::RETAIN);
+  if (!CMBlockBufferIsRangeContiguous(bb, 0, 0)) {
+    contiguous_bb.reset();
+    status = CMBlockBufferCreateContiguous(kCFAllocatorDefault,
+                                           bb,
+                                           kCFAllocatorDefault,
+                                           NULL,
+                                           0,
+                                           0,
+                                           0,
+                                           contiguous_bb.InitializeInto());
+    if (status != noErr) {
+      DLOG(ERROR) << " CMBlockBufferCreateContiguous failed: " << status;
+      return;
+    }
+  }
+
+  // Copy all the NAL units. In the process convert them from AVCC format
+  // (length header) to AnnexB format (start code).
+  char* bb_data;
+  status = CMBlockBufferGetDataPointer(contiguous_bb, 0, NULL, NULL, &bb_data);
+  if (status != noErr) {
+    DLOG(ERROR) << " CMBlockBufferGetDataPointer failed: " << status;
+    return;
+  }
+
+  if (nal_size_field_bytes == 1) {
+    CopyNalsToAnnexB<uint8_t>(bb_data, bb_size, annexb_buffer);
+  } else if (nal_size_field_bytes == 2) {
+    CopyNalsToAnnexB<uint16_t>(bb_data, bb_size, annexb_buffer);
+  } else if (nal_size_field_bytes == 4) {
+    CopyNalsToAnnexB<uint32_t>(bb_data, bb_size, annexb_buffer);
+  }
+}
+
+}  // namespace cast
+}  // namespace media