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• How to replace a snippet in a video without reencoding the whole video

  3 novembre 2022, par Simon Streicher

  I am trying to edit and replace a section of a video without reencoding the whole video.
Here are the steps I think I need to take :

  


  1. Find keyframes

  


  input :

  


  ffprobe -v error -select_streams v:0 -skip_frame nokey -show_entries frame=pkt_pts_time -of csv=p=0 'example.mkv'


  


  output :

  


  0.000000
1.001000
11.011000
13.430000
20.812000
30.822000
40.832000
50.842000
⋮


  


  2. Export relevant section

  


  For example, export the snippet 40.832000 → 50.842000.

  


  3. Edit and reencode

  


  After editing that section, I need to reencode it to the original codecs for compatibility with the surrounding video. For example, this is the original codecs :

  


  Stream #0:0: Video: hevc (Main 10), yuv420p10le(tv, bt2020nc/bt2020/smpte2084), 3840x1600 [SAR 1:1 DAR 12:5], 23.98 fps, 23.98 tbr, 1k tbn, 23.98 tbc (default)


  


  4. Inject edit back into the video stream

  


  Finally, I need to use FFmpeg somehow to retain everything from the original file (audio, subtitles, chapters, etc.) and to construct a new video stream (for example, Stream #0:0) that is exactly 0 → 40.832000 of the original, the whole edited section, and 50.842000 → end of the original section.

  


  My main questions are :

  


  

  • A. How do I trim the video stream in 2. without reencoding ?

  


  • B. Assuming that the edited video's resolution will remain the same, what is the command for FFmpeg to encode my edit to the codecs in 3. (and would the video be concatenable with the original video) ?

  


  • C. How should I go about glueing the sections together ? Should I simply trim Stream #0:0 into sections v1 = 0 → 40.832 and v2 = 50.842 → end and then concatenate a new stream as new = v1 + edited + v2 ?

  


  • D. How do I replace Stream #0:0 with new ?

  


  


  And probably the most important question : are my assumptions correct and can this be achieved ?

  


• Ffmpeg behaving intermittently

  10 mars 2024, par confused

  I'm noticing something weird, for the first time ever today, and I'm not sure what the problem could even possibly be.

  


  I first noticed it while stripping videos with a python script I wrote. Now I realize it is even happen with command line entry.

  


  Some, but not all videos, and some but not all segments within the same video are not stripping correctly. I'm taking numerous 1-2 hour long videos and cutting segments out of it. When I go to look at the video clip afterwards it automatically jumps to 2, 5, 8, 10 seconds into the video and starts playing there. It depends on the video segment as to how far ahead it skips. If I try to get it to go back and play the first part of the video it won't, it just jumps back to the 2-10 second jump ahead spot and plays from there. In further investigating it is stripping the audio off correctly and keep all the first 2-10 seconds of the audio with the video clip, but it is not picking up the first 2-10 seconds of the video. When I go into VLC Media Player and try to go back to the beginning I have a frozen image on the screen until I get to the 'predestined' 2-10 second mark, and then the video will play fine. When I watch the original video, it is okay but when I strip it the new video doesn't pick up the beginning of the video. Finally had to go to VLC media player to figure that part out as it was the only one who even let me play the underlying audio, every other media player wanted to jump ahead and skip the entire first 2-10 seconds altogether.

  


  This is happening with numerous different videos and video segments within the same video. Some segments it will strip fine others it won't.

  


  This occurs either way, python or command line. I just checked and the underlying video plays fine in the regular media player, until I strip the video, then it wants to skip the first 10 seconds.

  


  How might I fix this problem ?

  


• How to "unconcatenate" MP4 file ?

  17 avril 2023, par arne

  I used Losseless Cut (which uses ffmpeg) to concatenate a bunch of MP4 files using ffmpeg concat demuxer. Unfortunately I didnt' check the result before deleting the source clips. The first clip had different format than the rest. The resulting clip has all the audio and video from the first clip. Rest of the video gives decoding errors.

  


  I'm looking for a way how to "unconcatenate" the file and get back the original video. I believe it's in the container, just incorrectly labeled as 1920x1080x50 hevc video.

  


  I believe I should export the audio and video streams, cut away the first frames from both streams up to the point where the first clip ends, then change the format of the video stream and finally put them back into container.

  


  I'm not sure what tools and commands to use the cut the video stream and what commands and tools to use force the correct video format on the stream.

  


  I've read ffmpeg documentation but it's vast and my use case isn't directly covered. I'm planning to play around with different tools, but I'm new to the subject and thought to ask first.