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• How to extract time-accurate video segments with ffmpeg ?

  30 octobre 2023, par Jim Miller

  This is not a particularly new question area around here, but I've tried what's been suggested there without much luck. So, my story :

  


  I've got a hunk of 15 seconds of straight-from-the-camera.mov video out of which I want to extract a specific chunk, which I can identify by start time and stop time, in seconds. I started by trying to do what I'll call a "copy extraction" : to get seconds 9 to 12,

  


  ffmpeg -i test.mov -vcodec copy -acodec copy -ss 9 -to 12 test-copy.mov


  


  This was a not-bad start, but there are some black frames at the beginning and end of the clip, which I can't have — it has to be a clean edit from the original. So, I tried recoding the original into a new, trimmed clip :

  


  ffmpeg -i test.mov -ss 00:00:09 -t 00:00:03 test-out.mov


  


  This is better, but not quite : There are no longer any black frames at the beginning of the clip, but they're still there at the end.

  


  After some more browsing and reading, I then suspected that the problem is that ffmpeg is having trouble finding the proper points because of a lack of keyframes in the original video. So I recoded the original video to (presumably) add keyframes, in a couple of different ways. Since I want to be able to pick video at boundaries of a second ("from 9 seconds to 12 seconds"), I tried, copying various suggestions around the web,

  


  ffmpeg -i test.mov -force_key_frames "expr:gte(t, n_forced)" test-forced.mp4


  


  and

  


  ffmpeg -i test.mov -g 1 test-g-inserted.mp4


  


  (I built these as mp4's based on some comments about an mp4 container being needed to support the keyframe search, but I'm honestly just hacking here.) I then tried the extraction as before, but on these new videos that presumably now have keyframes in them. No luck — both seem to be about the same ; the start is OK but there are still black frames at the end. (FWIW, both test-forced.mp4 and test-g-inserted.mp4 also have trailing black frames.)

  


  So : I'm still stuck, and would like to not be. Any insights out there as to what I'm doing wrong ? I feel like I'm close, but I really need to get rid of those trailing black frames....

  


• Detecting on which frame there is audio presence on a video

  26 août 2016, par drov

  I have a few video files that corresponds to a tv zapping (one channel with sound, then a black screen without sound, then sound again with the new channel)

  I already detect pretty much everything but I would like to know how long it takes for the audio to appear after the end of the black screen.

  Basically I extract the audio from the video and giving the starting frame I would like to know at which frame there is some audio again.

  Then using that I can easily calculate the time it took for the audio to appear.

• ffmpeg - Drawing rotated text on video with complex filters takes a very long time

  14 mai 2019, par Bedrule Paul

  I am trying to overlap different text pieces on some placeholders in a video, and I am using multiple complex filters of the following type :

  ffmpeg -i ~/Desktop/input.mp4 -filter_complex  \
  
      "color=black@0:100x100,format=yuva444p[c]; \
  
      [c][0]scale2ref[ct][mv31]; \
  
      [ct]setsar=1,split=1[t31];\
  
      [t31]\
  
      drawtext=text='text':x='main_w/2-text_w/2+70':y=210:fontsize="100":fontcolor=black,\
  
      drawtext=text='text2':x='main_w/2-text_w/2+75':y=340:fontsize="100":fontcolor=black,\
  
      rotate=-0.07:ow=rotw(-0.07):oh=roth(-0.07):c=black@0[txta31]; \
  
      [mv31][txta31]overlay=enable='between(t, 0, 1.15)':x='min(0,-H*sin(-0.07))':y='min(0,W*sin(-0.07))':shortest=1" \
  
         ~/Desktop/result.mp4 -y1

  My goal is to write differently rotated texts on different time intervals in the video. The problem is that at about 10-12 [t31]-like pieces(here is an example of only one command), the rendering time of the video is twice the time of the video, whereas drawing straight horizontal text takes about 10-20% of the total video length (examples, for a 1 minute video, it takes about 8-10 seconds to write straight horizontal text, and about 2 minutes to write the same amount of text, but inclined with an angle). Is there any better way to do these multiple rotated text bits with more performance ?