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• How to averagely extract frames from a video using ffmpeg by specifying "fps"

  14 juin 2021, par alanzzz

  I have a job of using ffmpeg to extract the frames averagely from a video, with different fps. I use this command for it.

  


  ffmpeg -i input.mp4 -r specified_fps -q:v 2 image %4d.png

  


  And I have 3 questions about this task.

  


  

  1. what I expect is that if I double the fps, the number of extracted frames will also get doubled. However, that's not the case. Take one of the input videos as an example.

  


  


  video info

  


  

  • Duration : 1min18s

  


  • Total number of frames : 2340

  


  • Frame rate mode : constant (CFR)

  


  • Frame rate : 30.0 FPS

  


  


  Config setting & results

  


  

  • FPS=1 => number of frames=80

  


  • FPS=2 => number of frames=158 (2x80-2)

  


  • FPS=3 => number of frames=236 (3x80-4)

  


  • FPS=5 => number of frames=392 (5x80-8)

  


  


  Is it possible for me to get the exact doubled number of frames when fps get doubled ? In such case, number of frames is 160 for FPS=2, 240 for FPS=3, 400 for FPS=5.

  


  


  2. I check for the output images, the extracted frames in different fps are totally different from each other. In other words, for example, the 1st image for fps=1 is not the same as the 1st image for fps=2. Is that legitimate ? And is it possible for me the get some identical images for different fps ?

     


  


  3. The last problem is that for some videos I use, the difference between the 1st and 2nd image is different from the difference between the 2nd and 3rd. While for the remaining images, the differences become average. To be specific, there is only a slight change from 1st to 2nd frame, while for 2nd to 3rd, 3rd to 4th, and so on, the changes are the same, which is normally distributed according to the specified FPS. I am wondering why such a case happens ? Does it related to the I-frame, B-frame, P-frame, GOP or IDR ?

     


  


  


  I am new to this field and cannot find some useful info from other places. I've tried my best to describe my questions clearly. Feel free to leave some comments. Any help would do me a great favor. Thanks in advance !

  


• apply ffmpeg to many files

  30 juillet 2015, par puchu

  I have written simple script :

  #!/bin/bash
  
  find . -name "*.m4a" | while read filename;
  
  do
  
      new_filename=$(echo "$filename" | sed "s/^\(.*\)m4a$/\1flac/g");
  
      if [ ! -f "$new_filename" ]
  
      then
  
              #ffmpeg -i "$filename" -acodec flac "$new_filename" > /dev/null 2>&1;
  
              #wait $!;
  
              echo "$filename";
  
              echo "$new_filename";
  
      fi
  
  done

  it outputs correct result :

  ./Equilibrium, ALAC [GER] viking.folk/2003 - Demo 2003, ALAC/04 - Met.m4a
  
  ./Equilibrium, ALAC [GER] viking.folk/2003 - Demo 2003, ALAC/04 - Met.flac
  
  ./Equilibrium, ALAC [GER] viking.folk/2003 - Demo 2003, ALAC/02 - Nach Dem Winter.m4a
  
  ./Equilibrium, ALAC [GER] viking.folk/2003 - Demo 2003, ALAC/02 - Nach Dem Winter.flac

  if uncomment ffmpeg and wait :

  ./Equilibrium, ALAC [GER] viking.folk/2003 - Demo 2003, ALAC/04 - Met.m4a
  
  ./Equilibrium, ALAC [GER] viking.folk/2003 - Demo 2003, ALAC/04 - Met.flac
  
  uilibrium, ALAC [GER] viking.folk/2003 - Demo 2003, ALAC/02 - Nach Dem Winter.m4a
  
  uilibrium, ALAC [GER] viking.folk/2003 - Demo 2003, ALAC/02 - Nach Dem Winter.flac

  And no flacs has been done !

  PS

  #!/bin/bash
  
  find . -name "*.m4a" | while read filename;
  
  do
  
      new_filename=$(echo "$filename" | sed "s/^\(.*\)m4a$/\1flac/g");
  
      if [ ! -f "$new_filename" ]
  
      then
  
              ffmpeg -i "$filename" -acodec flac "$new_filename";
  
              echo "$filename";
  
              echo "$new_filename";
  
      fi
  
      sleep 5;
  
  done

  1) encode start but suddenly stop with no error messages

  2) encode couldn’t start because of "uilibrium" instead of "./Equilibrium"

  3) = 1)

  4) = 2)

  ...

  last) correctly

• Anomalie #2749 (Fermé) : Problème de cookie

  21 mars 2013, par guytarr °

  il y a bien http://forum.spip.net/fr_248457.html mais rien trouvé dans forum ou sur les listes de "récent".