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• What determine number of cpu core usage of filter select=gt(scene,0.1) ?

  15 décembre 2022, par kocoten1992

  I've notice that when using filter gt(scene,0.1), for example :

  


  ffmpeg -i big_buck_bunny.mkv -filter:v "select='gt(scene,0.1)',showinfo" -f null -

  


  Depends on the video, number of cpu cores usage varies extremely (sometimes it 3 cores usage - other time 12 cores usage in different video).

  


  Would like to ask what determine that logic ?

  


  I try to read ffmpeg source code but not familiar with it, a general explanation would be enough, but much appreciate if you point out the line/directory determine that logic in https://github.com/FFmpeg/FFmpeg.

  


  (Also not asking how to reduce cpu usage, interested in the logic determine that).

  


• How does MPG determine its default audio track ?

  1er octobre 2022, par codedread

  I have some mpg files that I transcoded from DVDs I bought a long time ago (maybe 20 years ago). ffprobe :

  


  Input #0, mpeg, from 'da-orig.mpg':
  Duration: 00:06:59.44, start: 0.044100, bitrate: 6354 kb/s
  Stream #0:0[0x1e0]: Video: mpeg2video (Main), yuv420p(tv, progressive), 720x480 [SAR 8:9 DAR 4:3], Closed Captions, 31 fps, 59.94 tbr, 90k tbn
    Side data:
      cpb: bitrate max/min/avg: 7500000/0/0 buffer size: 1835008 vbv_delay: N/A
  Stream #0:1[0x85]: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
  Stream #0:2[0x83]: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
  Stream #0:3[0x81]: Audio: ac3, 48000 Hz, mono, fltp, 192 kb/s
  Stream #0:4[0x80]: Audio: ac3, 48000 Hz, mono, fltp, 192 kb/s


  


  This shows there are 4 audio streams. When I play this file in VLC / QuickTime it seems that Audio Track 4 is the default. I'd like to understand how this is chosen. Is it something within the mpg container format or are players choosing the stream that has the lowest id (0x80) ?

  


  More background, when I try to turn this into a mp4 file with the following command :

  


  ffmpeg -i da-orig.mpg -c copy -map 0 da-copy.mp4

  


  I get roughly the same size file, but the default audio track is stream #0:1[0x85].

  


  What I want is an equivalent mp4 file (so the same audio track chosen).

  


• avcodec/vorbisdec : don't use a flag to determine if frames have been output

  8 septembre 2022, par Jyrki Vesterinen

  avcodec/vorbisdec : don't use a flag to determine if frames have been output
  If a developer using FFmpeg libraries seeks into an earlier position and calls
  
  avcodec_flush_buffers() afterwards as recommended, the Vorbis decoder will drop
  
  the next frame, since buffer flushing clears the first_frame flag. As a result,
  
  the audio samples the calling code receives may be ahead of the requested seek
  
  position, which is unacceptable in some use cases such as playing a looping
  
  sound effect.
  This commit records the presentation timestamp of the first frame and
  
  determines after that if the new frame is the first frame (possible after
  
  seeking to the start) by comparing its pts to the stored pts.
  

  • [DH] libavcodec/vorbisdec.c