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• starting ffmpeg on an explicit frame

  15 février 2018, par resatee

  I am trying to slica a file. I want the first part to end after 1550 frames and the second one to go from frame 1551 to 3000. The first part wasn’t a problem, I used that line :

  ffmpeg.exe -i foo.hevc -c copy -frames:v 1549 -s 1920x1080 foo_1sthalf.hevc

  For the second part I tried this :

  ffmpeg.exe -ss 30 -i foo.hevc -c copy -s 1920x1080 foo_2ndhalf.hevc

  and this :

  ffmpeg.exe -ss 00:00:30 -i foo.hevc -c copy -s 1920x1080 foo_2ndhalf.hevc

  and this :

  ffmpeg.exe -start_number 1550 -i foo.hevc -c copy -s 1920x1080 foo_2ndhalf.hevc

  but nothing worked.
  Error message is always :

  could not seek to position 30.000

  Do you know a way to start at a certain frame ?
  I have ffmpeg version 3.2

• Trim with ffmpeg without re-encoding gives issues on the trimming point

  3 mai 2022, par Oleg Merkulov

  I have a video file with following parameters :

  


  Container: mp4
Video: h264
Audio: AAC
Duration: 00:29:38.03


  


  I need to extract 2 parts from it and merge it into one video. I want to keep the quality and make the operation as fast as possible. Thus -> I don't want to re-encode.

  


  I'm using the following :

  


  ffmpeg -y -ss 00:01:00 -to 00:03:00 -i 30.mp4 -c copy part-1.mp4
ffmpeg -y -ss 00:21:00 -to 00:29:00 -i 30.mp4 -c copy part-2.mp4
ffmpeg -y -f concat -safe 0 -i parts.txt -c copy merged.mp4
parts.txt:
file 'part-1.mp4'
file 'part-2.mp4'


  


  the output has 103 warnings like that :

  


  [mp4 @ 0x7fa91d707e40] Non-monotonous DTS in output stream 0:1; previous: 5759491, current: 5719904; changing to 5759492. This may result in incorrect timestamps in the output file.
[mp4 @ 0x7fa91d707e40] Non-monotonous DTS in output stream 0:0; previous: 3598631, current: 3574391; changing to 3598632. This may result in incorrect timestamps in the output file.


  


  merged.mp4 has an issue on 00:02:00, right after the cut.
The audio is not in sync with video for around 5 seconds and then after that it goes perfectly.

  


  What can cause such an issue ? Is there is a way to trim without losing quality ?

  


• ffmpeg video-concatenation - audio-/videotrack Synchronisation

  29 avril 2013, par user1739223

  I'm want to concatenate different video-files of random format. First I convert them to flv, adapt the framerate of each video to lowest of the involved videos. Next step is to decode them and remove the header (but not the header of the first video in the sequence, then I concatenate them with cat and finally I encode the resultsequence.

  But the video- and audio-track are going more and more out of sync from sequence-part to sequence-part.

  Would be glad for any idea.