[Libav-user] How to correctly handle Matroska timestamps

[MattKC]

==Summary==

I've been implementing libav into an application, and have run into 
timing-related issues when playing back Matroska/MKV videos created by 
my application in VLC. While it's true that there may be issues with 
VLC's implementation too, I've noticed specifically that videos 
generated by FFmpeg do not cause these issues, while my application and 
the examples in "doc/examples" (which my code is modeled from) do, which 
makes me think this code is missing something crucial.

==Symptoms==

During playback, VLC's playback time will appear to stay frozen at 0:00 
while the video and audio plays normally. It will then jump only in 
increments of a few seconds (0:03, then 0:07, then 0:11, etc.) This 
behavior occurs with videos produced both by my application and by the 
FFmpeg examples programs "transcoding.c" and "remuxing.c".

The fact that this occurs with "remuxing.c" is particularly interesting 
since that would seem to imply it's an issue with the format/container 
writing specifically. If "remuxing.c" is given a correct MKV file (i.e. 
one that does not exhibit this issue) produced by FFmpeg as its input, 
and outputs to a second MKV file, that second MKV will demonstrate this 
issue even though the first one didn't, indicating that something is 
breaking during the remuxing process despite no decoding/encoding occurring.

The worst demonstration of this issue that I've run into is if my 
application writes any audio packets before writing the first video 
packet (with "av_interleaved_write_frame"), the first few seconds of 
video (until that first 0:03) will be considered completely blank by 
VLC. The fact that it will pick up after 0:03, the same time the timer 
will make its first update on other MKV files, leads me to believe it's 
the exact same root cause, just a different presentation of it.

This issue only occurs on MKV files. MP4/MOV appear to have no issues 
whatsoever.

==Conclusion==

Considering FFmpeg produces correct MKV files, I don't believe this to 
be a bug in libav (though it could be considered a bug in the examples). 
I'm assuming the examples and my program are missing something or doing 
something incorrectly.

My code can be seen here, however the examples will almost certainly be 
easier to read and test: 
https://github.com/olive-editor/olive/blob/master/app/codec/ffmpeg/ffmpegencoder.cpp

Thanks in advance for any help, I'm a little confused about this whole 
issue.