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FFMPEG-Plugin: Simplify start time calculation of video frame.

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- We let FFMPEG keep track of the correct relationship between presentation
  start time of the encoded video frame and the resulting decoded video frame.
  This simplyfies our code, meaning less lines of code to maintain :)

- Update documentation and pointing out some corner cases when calculating the
  correct presentation start time of a decoded video frame under certain
  circumstances.

- Fix doxygen: Use doxygen style instead of javadoc style.

- No functional change intended.

Signed-off-by: Colin Günther <coling@gmx.de>
This commit is contained in:
Colin Günther 2014-07-26 14:53:33 +02:00
parent 254a53409e
commit 97f5a12f36
1 changed files with 47 additions and 10 deletions

57
src/add-ons/media/plugins/ffmpeg/AVCodecDecoder.cpp

@ -705,14 +705,32 @@ AVCodecDecoder::_DecodeVideo(void* outBuffer, int64* outFrameCount,
To every decoded video frame there is a media_header populated in
fHeader, containing the corresponding video frame properties.
@return B_OK, when we successfully decoded one video frame
Normally every decoded video frame has a start_time field populated in the
associated fHeader, that determines the presentation time of the frame.
This relationship will only hold true, when each data chunk that is
provided via GetNextChunk() contains data for exactly one encoded video
frame (one complete frame) - not more and not less.
We can decode data chunks that contain partial video frame data, too. In
that case, you cannot trust the value of the start_time field in fHeader.
We simply have no logic in place to establish a meaningful relationship
between an incomplete frame and the start time it should be presented.
Though this might change in the future.
We can decode data chunks that contain more than one video frame, too. In
that case, you cannot trust the value of the start_time field in fHeader.
We simply have no logic in place to track the start_time across multiple
video frames. So a meaningful relationship between the 2nd, 3rd, ... frame
and the start time it should be presented isn't established at the moment.
Though this might change in the future.
\return B_OK, when we successfully decoded one video frame
*/
status_t
AVCodecDecoder::_DecodeNextVideoFrame()
{
assert(fTempPacket.size >= 0);
bool firstRun = true;
while (true) {
media_header chunkMediaHeader;
@ -730,6 +748,31 @@ AVCodecDecoder::_DecodeNextVideoFrame()
fChunkBuffer));
fTempPacket.size = fChunkBufferSize;
fContext->reordered_opaque = chunkMediaHeader.start_time;
// Let ffmpeg handle the relationship between start_time and
// decoded video frame.
//
// Explanation:
// The received chunk buffer may not contain the next video
// frame to be decoded, due to frame reordering (e.g. MPEG1/2
// provides encoded video frames in a different order than the
// decoded video frame).
//
// FIXME: Research how to establish a meaningful relationship
// between start_time and decoded video frame when the received
// chunk buffer contains partial video frames. Maybe some data
// formats contain time stamps (ake pts / dts fields) that can
// be evaluated by FFMPEG. But as long as I don't have such
// video data to test it, it makes no sense to implement it.
//
// FIXME: Implement tracking start_time of video frames
// originating in data chunks that encode more than one video
// frame at a time. In that case on would increment the
// start_time for each consecutive frame of such a data chunk
// (like it is done for audio frame decoding). But as long as
// I don't have such video data to test it, it makes no sense
// to implement it.
#ifdef LOG_STREAM_TO_FILE
if (sDumpedPackets < 100) {
sStreamLogFile.Write(fChunkBuffer, fChunkBufferSize);
@ -740,13 +783,6 @@ AVCodecDecoder::_DecodeNextVideoFrame()
#endif
}
if (firstRun) {
firstRun = false;
fHeader.start_time = chunkMediaHeader.start_time;
fStartTime = chunkMediaHeader.start_time;
}
#if DO_PROFILING
bigtime_t startTime = system_time();
#endif
@ -857,12 +893,13 @@ AVCodecDecoder::_UpdateMediaHeaderForVideoFrame()
fHeader.type = B_MEDIA_RAW_VIDEO;
fHeader.file_pos = 0;
fHeader.orig_size = 0;
fHeader.start_time = fRawDecodedPicture->reordered_opaque;
fHeader.u.raw_video.field_gamma = 1.0;
fHeader.u.raw_video.field_sequence = fFrame;
fHeader.u.raw_video.field_number = 0;
fHeader.u.raw_video.pulldown_number = 0;
fHeader.u.raw_video.first_active_line = 1;
fHeader.u.raw_video.line_count = fContext->height;
fHeader.u.raw_video.line_count = fRawDecodedPicture->height;
TRACE("[v] start_time=%02d:%02d.%02d field_sequence=%lu\n",
int((fHeader.start_time / 60000000) % 60),