[FFmpeg-trac] #8196(ffmpeg:new): ffmpeg X11 grab video too fast

[FFmpeg]

#8196: ffmpeg X11 grab video too fast
-------------------------------------+-------------------------------------
             Reporter:  fld          |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  ffmpeg       |                  Version:  4.0.4
             Keywords:  x11grab      |               Blocked By:
  bitrate speed                      |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug:

 This issue appears only from time to time grabbing X11 displays in an
 automated process running inside docker images.

 This is the command line we use:
 {{{
 % ffmpeg -y -thread_queue_size 512 -f pulse -i pa.monitor -f x11grab -r 30
 -video_size 1280x720 -draw_mouse 0 -i 127.0.0.1:1 -filter:v
 setpts=N/FRAME_RATE/TB -filter:a asetpts=(SR-4)/SR*N/SR/TB -codec:v
 libx264 -preset veryfast -tune film -pix_fmt yuv420p -threads 3 -y
 -framerate 30 -codec:a aac -movflags +faststart -shortest out.mp4
 }}}
 Most of the time (>99%) the video and audio are ok.
 But from time to time, we have a video that appears like being 2x and
 audio is at normal speed

 During that capture, the ffmpeg logs show something like :
 {{{
 frame=  645 fps= 13 q=29.0 size=    2304kB time=00:00:20.84 bitrate=
 905.7kbits/s speed=0.43x
 }}}
 while usually we have:
 {{{
 frame=  638 fps= 30 q=29.0 size=    1536kB time=00:00:20.60 bitrate=
 610.7kbits/s speed=0.972x
 }}}
 (see difference in bitrate and speed)

 Unfortunately I haven't managed to reproduce it manually and cannot
 provide any of the impacted video files.

 Any idea what could be causing this and how to avoid it? Or what kind of
 debug option I can use to try to troubleshoot it?

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Ticket URL: <https://trac.ffmpeg.org/ticket/8196>
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