[FFmpeg-user] Help needed to work around incorrect DTS when capturing H264 from a Logitech C920

Peter Rabbitson rabbit+list at rabbit.us
Mon Dec 15 17:46:51 CET 2014

I investigated more and was able to fully recreate the issue in a "lab 
setting". It seems that the C920 is producing invalid PTS values in the 
output H264 stream. What is, however, more interesting is that ffplay is 
somehow able to compensate for it if "treated right".

My initial setup is running the following command and capturing video of 
a high-speed timer running on my screen:

ffmpeg -r 30 -f v4l2 -s 1920x1080 -vcodec h264 -i 
/dev/v4l/by-id/*HD_Pro_Webcam_C920* \
   -c:v copy -f matroska - | \
tee stdin.mkv | \
strace -ttt -ff -o ffplay.trace -s 65536 ffplay -i -

The result is the (incomplete, no index at the end) stdin.mkv [1], a 
perfect smooth video on my screen produced by ffplay, and a bunch of 
tracefiles [2.1]. As described in the previous email [3], if I attempt 
to play stdin.mkv with anything (including ffplay itself via `ffplay -i 
stdin.mkv`) I get micro-freezes every ~2.5 sec as can be seen in the 
playback of [1] on vimeo.

My questions:
1) The PTS of the stream is clearly mangled. If I look at it via ffprobe 
and calculate the PTS differences I end up with [2.2]. Is there a way to 
rewrite the PTS alone without recoding, using a stable clocksource? I 
tried in various combinations the following options - nothing seem to 
change the outcome (is it due to -c:v copy, affecting even input-side 
(input side): -re, -ts abs, -ts mono2abs
(output side): -fflags genpts

2) (the more interesting question) I want to understand what exactly 
causes ffplay to process the apparently broken stream. I took the strace 
of the reader thread, and after some manipulation ended up with a "read 
schedule" as seen in [2.3]. Then I wrote a simple "delayed cat" program 
that reads out [1] as per the schedule - it can be found in [2.4]. The 
result is:

Hang every ~2.5 secs: `cat stdin.mkv | ffplay -i -`
Smooth no-hang video: `./scheduler | ffplay -i -`

If anyone has any ideas - please do share ;)

[1] 7Mb SHA1 649434a5aa0082f81cfc447af173203a970fda84, downloadable from 
https://vimeo.com/114550042 (you need to login to vimeo to see the 
"download original" link)


[2.2] https://gist.github.com/ribasushi/fab13b785c3c6e1b70a3#file-ptsdeltas


[2.4] https://gist.github.com/ribasushi/fab13b785c3c6e1b70a3#file-scheduler


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