media/capture: Adding WebmMuxer class and unittests

media/capture/webm_muxer.cc

+// Copyright 2015 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/capture/webm_muxer.h"
+
+#include <limits>
+
+namespace media {
+
+WebmMuxer::WebmMuxer(const WriteDataCB& write_data_callback)
+    : write_data_callback_(write_data_callback),
+      position_(0) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  segment_.Init(this);
+  segment_.set_mode(mkvmuxer::Segment::kLive);
+  segment_.OutputCues(false);
+
+  mkvmuxer::SegmentInfo* const info = segment_.GetSegmentInfo();
+  info->set_writing_app("Chrome");
+  info->set_muxing_app("Chrome");
+}
+
+WebmMuxer::~WebmMuxer() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  segment_.Finalize();
+}
+
+mkvmuxer::uint64 WebmMuxer::AddVideoTrack(const gfx::Size& frame_size,

[DaleCurtis, 2015/07/20 20:23:07]

Since this is the Chrome side interface, can we just return uint64_t? Ditto for
the OnEncodedVideo.  We should try to avoid the spread of these outside of this
class.

[mcasas, 2015/07/21 12:56:20]

Done.

+                                          double frame_rate) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  const mkvmuxer::uint64 track_index =
+      segment_.AddVideoTrack(frame_size.width(), frame_size.height(), 0);
+
+  mkvmuxer::VideoTrack* const video_track =
+      reinterpret_cast<mkvmuxer::VideoTrack*>(
+          segment_.GetTrackByNumber(track_index));
+  DCHECK(video_track);
+  video_track->set_codec_id(mkvmuxer::Tracks::kVp8CodecId);
+  DCHECK_EQ(video_track->crop_right(), 0ull);
+  DCHECK_EQ(video_track->crop_left(), 0ull);
+  DCHECK_EQ(video_track->crop_top(), 0ull);
+  DCHECK_EQ(video_track->crop_bottom(), 0ull);
+
+  video_track->set_frame_rate(frame_rate);
+  video_track->set_default_duration(base::Time::kMicrosecondsPerSecond /
+                                    frame_rate);
+  // Segment's timestamps should be in milliseconds, dcheck it. See
+  // http://www.webmproject.org/docs/container/#muxer-guidelines
+  DCHECK_EQ(segment_.GetSegmentInfo()->timecode_scale(), 1000000ull);
+
+  return track_index;
+}
+
+void WebmMuxer::OnEncodedVideo(mkvmuxer::uint64 track_number,
+                               const std::string& encoded_data,

[DaleCurtis, 2015/07/20 20:23:07]

Do you want std::string& here? Should you instead be passing a ptr+size or
something like a DecoderBuffer / RefCountedMemory?  We're going to want to
minimize copies along this path likely.

[mcasas, 2015/07/21 12:56:20]

Hmm at this point I'm happy with std::string&, reasons:
- http://crrev.com/1233033002 VideoTrackRecorder,
 libvpx decode likes to "add" encoded data into a buffer,
and a growing buffer suits an std::string. To make it
more evident, I changed to buffer ptr and size.
- I played around with RefCountedString::TakeString()
and it didn't clarify much the mechanism.
- Encode and Muxing happen in a single thread and
seems reasonable that this will be the normal case,
so encode-mux is a one-to-one synchronous call,
hence no need for ref counting etc. 
All of this notwithstanding that when we bring the 
whole shebang together, we might feel like some
extra optimisation.

WDYT?

[DaleCurtis, 2015/07/21 16:52:30]

I didn't realize that about the vp8 encode api, so I defer to Tom as to the best
way of moving these bits around post-encoding. Mostly I'd worry about
unintentional copies sneaking in (use w/ Bind(), lost const&); you could use
StringPiece for a stronger guarantee.

[mcasas, 2015/07/21 19:46:59]

Acknowledged.

[Tom Finegan, 2015/07/21 21:19:09]

By default, this is true, but you can make the library encode into your buffers
(and decode, but that's a typo, right?). The encoding app uses
vpx_codec_set_cx_data_buf() to give the encoder buffers, calls
vpx_codec_encode(), then calls vpx_codec_get_data() (i.e. almost the same as
what you have now, just with the extra call to pass in your buffer).

I don't think it's necessary to do this now, but maybe in a future optimization
pass it would be worthwhile. Since you're dealing with compressor output the
buffers aren't going to be very big anyway. As Knuth said, premature
optimization is the root of all evil. It's probably too early a stage to be
worrying about this copy.

Otoh, you guys might feel differently. The thing to keep in mind is requested
datarate vs frame rate. For example, 1 megabit at 15 fps you end up with an
average frame size of just 8.3k. Is jumping through hoops to save that one copy
worth it yet?

Also, I'm hesitant to recommend the use of vpx_codec_set_cx_data_buf(). It
doesn't have test coverage, and I have no knowledge of its use in the wild.

+                               const base::TimeDelta& timestamp,
+                               bool keyframe) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  // |track_number|, a caller-side identifier, cannot be zero (!), see
+  // http://www.matroska.org/technical/specs/index.html#Tracks
+  DCHECK_GT(track_number, 0u);
+  DCHECK(segment_.GetTrackByNumber(track_number));
+
+  segment_.AddFrame(reinterpret_cast<const uint8_t*>(encoded_data.data()),
+                    encoded_data.size(),
+                    track_number,
+                    timestamp.InMilliseconds(),
+                    keyframe);
+}
+
+mkvmuxer::int32 WebmMuxer::Write(const void* buf, mkvmuxer::uint32 len) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  write_data_callback_.Run(static_cast<const char*>(buf), len);
+  position_ += len;
+  return 0;
+}
+
+mkvmuxer::int64 WebmMuxer::Position() const {
+  return position_.ValueOrDie();
+}
+
+mkvmuxer::int32 WebmMuxer::Position(mkvmuxer::int64 position) {
+  // The stream is not Seekable() so indicate we cannot set the position.
+  return -1;
+}
+
+bool WebmMuxer::Seekable() const {
+  return false;
+}
+
+void WebmMuxer::ElementStartNotify(mkvmuxer::uint64 element_id,
+                                   mkvmuxer::int64 position) {
+  // This method gets pinged before items are sent to |write_data_callback_|.
+  DCHECK_GE(position, position_.ValueOrDefault(0))
+      << "Can't go back in a Live Mkv stream.";
+}
+
+}  // namespace media