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• Getting motion vector side data from ffmpeg from the compressed domain (without performing full decode)

  4 mai 2019, par John Allard

  The ffmepg source code comes with a file extract_mvs.c which is quite helpful in showing how to extract the motion vectors from an h264-encoded video file. Unfortunately, it performs a full decode before getting the motion vectors from the side data of the stream.

  One should be technically able to (as was confirmed by FFMPEG developer Carl Eugen in this thread https://ffmpeg.org/pipermail/libav-user/2016-December/009913.html) get the motion vectors from the compressed domain before performing a full decode step. This would obviously be less computationally intensive as one wouldn’t have to perform a full decode.

  Is it possible to do this with ffmpeg/libavcodec as is ? I know Carl said that it wasn’t possible but that was 3 years ago. If it is not currently possible, does anyone have any hints on how one would go about modifying the ffmpeg source code to do this ?

  Thanks

• Split a movie into 1000+ shots using PyAV in a single pass ?

  3 mai 2019, par Andrew Klaassen

  I need to split a 44 minute MP4 into 1000 shots (i.e. separate MP4s) with ffmpeg. I want to do it quickly (i.e. in a single pass rather than 1000 passes), I need perfect frame accuracy, and I need to do it in Windows.

  The Windows command-line length limit is stopping me from doing this, and I’m wondering if someone could show me an example of how to do this using a library like PyAV or Avpy. (Libraries like ffmpeg-python and ffmpy won’t help, since they simply construct an ffmpeg command line and run it, leading to the same Windows command-line length issue that I already have.)

  After much testing and gnashing of teeth, I’ve learned that the only way to get perfect frame accuracy from ffmpeg, 100% of the time, is to use the "select" filter. ("-ss" in the newest versions of ffmpeg is frame accurate 99% of the time ; unfortunately, that’s not good enough for this application.)

  There are two ways to use "select" for this. There’s the slow way, which I’m doing now, and which requires having ffmpeg open the file 1000 times :

  for (start, end, name) in shots:
  
      audio_start = start / frame_rate
  
      audio_end = end + 1 / frame_rate
  
      cmd = [
  
          path_to_ffmpeg,
  
          '-y',
  
          '-i', input_movie,
  
          '-vf', r'select=between(n\,%s\,%s),setpts=PTS-STARTPTS' % (start, end),
  
          '-af', 'atrim=%s:%s,asetpts=PTS-STARTPTS' % (audio_start, audio_end),
  
          '-c:v', 'libx264',
  
          '-c:a', 'aac',
  
          '-write_tmcd', '0',
  
          '-g', '1',
  
          '-r', str(frame_rate),
  
          name + '.mp4',
  
          '-af', 'atrim=%s:%s' % (audio_start, audio_end),
  
          name + '.wav',
  
      ]
  
      subprocess.call(cmd)

  And there’s the fast way, which causes the Windows command line to explode when there are too many shots. The long command line leads to a failure to run :

  cmd = [
  
      path_to_ffmpeg,
  
      '-y',
  
      '-i',
  
      input_movie,
  
  ]
  
  for (start, end, name) in shots:
  
      audio_start = start / frame_rate
  
      audio_end = end + 1 / frame_rate
  
      cmd.extend([
  
          '-vf', r'select=between(n\,%s\,%s),setpts=PTS-STARTPTS' % (start, end),
  
          '-af', 'atrim=%s:%s,asetpts=PTS-STARTPTS' % (audio_start, audio_end),
  
          '-c:v', 'libx264',
  
          '-c:a', 'aac',
  
          '-write_tmcd', '0',
  
          '-g', '1',
  
          '-r', str(frame_rate),
  
          name + '.mp4',
  
          '-af', 'atrim=%s:%s' % (audio_start, audio_end),
  
          name + '.wav',
  
      ]
  
  subprocess.call(cmd)

  I’ve looked through the documentation of PyAV and Avpy, but I haven’t been able to figure out whether the second form of my function is something I could do there, or how I’d go about doing it. If it is possible, would someone be able to write a function equivalent to my second function, using either library ?

• How to convert jquery animation to movie or gif format ?

  19 avril 2019, par Barış Demirdelen

  How to convert jquery animation to movie or gif format ?
  Animation and screenshot functions call or too slow.
  Help me please

  Animation Function

  function resim1(){
  
      $("#resim-1").animate({ left: leftx}, startEffectTime1);
  
      ......
  
  }

  ScreenShot Function but too slow

  function shot(){
  
            html2canvas($("#main"), {
  
                onrendered: function(canvas) {
  
                    theCanvas = canvas;
  
                    document.body.appendChild(canvas);
  
                    Canvas2Image.saveAsPNG(canvas); 
  
                    $("#img-out").append(canvas);
  
                }
  
            });
  
  }

  Call animation and screenshot functions

  $(document).ready(function() {
  
       resim1(); // Animate start
  
       setInterval(function(){ // Screenshot start
  
            shot();
  
       },500);
  
  
  
  });

  Bottom function call animation breaking :/

  $(document).ready(function() {
  
       resim1(); // Animate start
  
       setInterval(function(){ // Screenshot start
  
            shot();
  
       },500);
  
  
  
  });