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• libav MPEGTS demuxing - handle loop/discontinuity

  28 septembre 2021, par Michal Artazov

  I'm writing a video/audio player that uses libav/ffmpeg for demuxing and decoding MPEGTS streams over UDP. One problem that I'm dealing with is that sometimes the stream is looping and when it loops, my player breaks down.

  


  The issue is that once the stream loops, the new packets have widely different dts/pts. My player is relying on pts for video - audio synchronisation so it's important that I can handle pts properly.

  


  Whenever the server loops the stream, it sends a discontinuity flag, which I can confirm is being correctly received by libav mpegts demuxer (I did some digging in the code and inspected the debug logs). However, it seems to me that the demuxer doesn't act on the discontinuity flag much. In other words, from the point of view of the user I can't tell that there's a discontinuity, apart from the dramatic jump in dts/pts.

  


  Is there a way I can reliably tell that there was a discontinuty so I can recalculate my timestamps and continue playback smoothly ?

  


• How do I configure Janus to accept H.264 video with periodic intra refresh enabled ?

  31 août 2021, par CompuCat

  I have a Janus server set up to accept H.264 video over RTP and stream it to clients over WebRTC. This is for a low-latency application, so I would like to enable libx264's periodic intra refresh option, which minimizes "keyframe pumping" and avoids bitrate spikes on keyframes. (When streaming at low latency, there is not much wiggle room in the playback buffer, and bitrate spikes tend to cause playback stutters or dropouts.)

  


  If I enable periodic intra refresh, however, a video client cannot play back the stream unless it is listening form the very beginning of the stream. In other words, a client cannot start viewing video mid-stream. I suspect this is because WebRTC is waiting for a full keyframe to start video...which never comes, as periodic intra refresh replaces keyframes.

  


  The stream is being sent from ffmpeg using these parameters :

  


  ffmpeg -i  -an -s 426x240 -c:v libx264 -profile:v baseline -preset superfast -tune zerolatency -pix_fmt yuv420p -minrate 1000 -maxrate 1000 -bufsize 100 -crf 20 -intra-refresh 1 -f rtp rtp://:<port>?pkt_size=1300</port>

  


  yuv420p is being set to force a color conversion, since my input video is 4:2:2 encoded, and 4:2:2 H.264 over WebRTC causes a segfault on Chrome. ?pkg_size=1300 is being applied to avoid packets exceeding the MTU per this StackOverflow thread.

  


  What can I do to allow clients to start viewing video mid-stream without disabling periodic intra refresh ?

  


• Next drawtext clears the previous one ffmpeg

  27 août 2021, par Макс Шульдинер

  ffmpeg.exe -i ffmpeg_inputs/zheltiy_chelik.mp4  -vf "[in]drawtext=fonts/Temporarium.otf:text='r':fontcolor=orange:fontsize=100:x=(w-text_w)/2+0:y=0:enable='between(t,0, 11)', drawtext =fonts/Temporarium.otf:text = 'r':fontcolor=orange:fontsize=100:x=(w-text_w)/2+20:y=0:enable='between(t,11, 22)', drawtext =fonts/Temporarium.otf:text = 'r':fontcolor=orange:fontsize=100:x=(w-text_w)/2+40:y=0:enable='between(t,22, 33)'[out]" ffmpeg_outputs/test2.mp4 -y 


  


  There is such a ffmpeg line. Its meaning is to draw a letter 20 pixels to the right of the current position of the word every 11 seconds, and in this way "write the word". But here's the problem - when the second letter starts to render, it overwrites the first, and the third, respectively, overwrites the second. The string seems to have prescribed [in] and [out] but does not help, maybe someone has ideas