[libav-api] DTS-PTS Delay on GOP-Level

Jan libav at jd67.de
Tue Mar 19 10:24:21 CET 2013


Hi,

I'm working on a lossless GOP-based cutter to handle TV-recordings. I 
try to handle mpeg2video and h264 as video-streams.

I collect all video-frames/packets within one GOP, starting with first 
withAVPacket->flag & AV_PKT_FLAG_KEY until the AVPacket before the next 
AVPacket->flag & AV_PKT_FLAG_KEY.

Now I like to identify all audio-frames/packets associated with the GOP.

First I calculate the minimum- and maximum-PTS of the GOP and take all 
audio-packets between. This works fine with h264 - every frame has a 
PTS. But using mpeg2video, frames will have PTSwithout value 
(AV_NOPTS_VALUE) and the min/max calculation will be not match to the 
full GOP-duration.

By working on that, I found out, that the delay of PTS to DTS on 
GOP-Level is constant within a video-stream. So it looks like that I can 
fetch audio-frames easily by working with a fixed delay:

----
Assign audio frame to GOP:
assume: audio->pts == audio->dts and audio-stream->timebase == 
video-stream->timebase
note: duration will NOT ignored in real code

if (audio->pts >= video-keyframe[n]->dts + video-dts-pts-delay && 
audio->pts < video-keyframe[n+1]->dts + video-dts-pts-delay) {
     // MATCH
}
----

1. QUESTION:
Is this a good or a bad idea in general?

2. QUESTION:
Is there any way to get a valid delay between DTS and PTS on GOP-Level 
for mpeg2video?

Thanks for your help in advanced

Jan





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