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• When compressing a set of images with libx264, why does frame rate affect final output size ?

  3 avril 2018, par jd20

  I’m using ffmpeg to encode a set of images as a short timelapse video, using libx264 codec. My first attempt, I encoded it at 30 FPS, using :

  ffmpeg -r 30 -pattern_type glob -i "*.jpg" -vcodec libx264 -crf 30 -pix_fmt yuv420p output.mp4

  With 60 frames, that gives me a 163 KB file that’s 2 seconds long. Then I realized I needed it to be slower, so I re-ran the same command, but changed -r to 2. Now I have a file that’s 30 seconds long, but the size jumped to 891 KB ! The video quality looks perceptually the same.

  How do I encode at a slower frame rate, without the final file size ballooning ?

  ---

  Notes : Some theories I had, and things I checked. First, to make sure ffmpeg wasn’t duplicating frames in the longer verison, I check the I/P/B counts. The 30 FPS file had :

  [libx264 @ 0x7f9b26001c00] frame I:1     Avg QP:30.67  size: 44649
  
  [libx264 @ 0x7f9b26001c00] frame P:15    Avg QP:31.19  size:  5471
  
  [libx264 @ 0x7f9b26001c00] frame B:44    Avg QP:31.45  size:   767

  The 2 FPS file had :

  [libx264 @ 0x7fcd32842200] frame I:1     Avg QP:21.29  size: 90138
  
  [libx264 @ 0x7fcd32842200] frame P:15    Avg QP:22.48  size: 33686
  
  [libx264 @ 0x7fcd32842200] frame B:44    Avg QP:26.29  size:  6674

  So, the I/P/B counts are identical, but the QP is much lower for the 2 FPS file. To offset, I tried increasing -crf for the 2 FPS file, to get about the same target size, but that just gave me a very blurry video (had to go to crf=40). I tried messing with -minrate, -maxrate, -bt, none helped. I’m guessing there is some x264 codec setting which is frame rate dependent, but I’m at a loss trying to figure out which one (from what I understand, constant bitrate is affected by frame rate but CRF should not be, but maybe I’m misunderstanding it.

• When compressing a set of images with libx264, why does frame rate affect final output size ?

  3 avril 2018, par jd20

  I’m using ffmpeg to encode a set of images as a short timelapse video, using libx264 codec. My first attempt, I encoded it at 30 FPS, using :

  ffmpeg -r 30 -pattern_type glob -i "*.jpg" -vcodec libx264 -crf 30 -pix_fmt yuv420p output.mp4

  With 60 frames, that gives me a 163 KB file that’s 2 seconds long. Then I realized I needed it to be slower, so I re-ran the same command, but changed -r to 2. Now I have a file that’s 30 seconds long, but the size jumped to 891 KB ! The video quality looks perceptually the same.

  How do I encode at a slower frame rate, without the final file size ballooning ?

  ---

  Notes : Some theories I had, and things I checked. First, to make sure ffmpeg wasn’t duplicating frames in the longer verison, I check the I/P/B counts. The 30 FPS file had :

  [libx264 @ 0x7f9b26001c00] frame I:1     Avg QP:30.67  size: 44649
  
  [libx264 @ 0x7f9b26001c00] frame P:15    Avg QP:31.19  size:  5471
  
  [libx264 @ 0x7f9b26001c00] frame B:44    Avg QP:31.45  size:   767

  The 2 FPS file had :

  [libx264 @ 0x7fcd32842200] frame I:1     Avg QP:21.29  size: 90138
  
  [libx264 @ 0x7fcd32842200] frame P:15    Avg QP:22.48  size: 33686
  
  [libx264 @ 0x7fcd32842200] frame B:44    Avg QP:26.29  size:  6674

  So, the I/P/B counts are identical, but the QP is much lower for the 2 FPS file. To offset, I tried increasing -crf for the 2 FPS file, to get about the same target size, but that just gave me a very blurry video (had to go to crf=40). I tried messing with -minrate, -maxrate, -bt, none helped. I’m guessing there is some x264 codec setting which is frame rate dependent, but I’m at a loss trying to figure out which one (from what I understand, constant bitrate is affected by frame rate but CRF should not be, but maybe I’m misunderstanding it.

• How can I get the final size after transcripting a video to stream it with fluent-ffmpeg ?

  5 avril 2018, par G. Manukyan

  I am trying to stream a video using createReadStream and using pipe(res) in node.js and it works fine if the file doesn’t need transcoding (mp4, webm).

  With mkv files I am using fluent-ffmpeg to transcode it on the fly, but the problem is that I can’t go back and forward in the html video player.

  download = function(file, req, res) {   
  
  
  
      const range = req.headers.range
  
      const parts = range.replace(/bytes=/, "").split("-")
  
  
  
      const start = parseInt(parts[0], 10);
  
      const end = parts[1] ? parseInt(parts[1], 10) : file.length - 1
  
  
  
      res.setHeader('Content-Type', 'video/webm')
  
      res.setHeader('Accept-Ranges', 'bytes');
  
      res.setHeader('Content-Length', 1 + end - start);
  
      res.setHeader('Content-Range', `bytes ${start}-${end}/${file.length}`);
  
      res.statusCode = 206;
  
  
  
      var stream = file.createReadStream({start, end})
  
  
  
      ffmpeg(stream)
  
      .videoCodec('libvpx')
  
      .audioCodec('libvorbis')
  
      .videoBitrate('512k')
  
      .format('webm')
  
      .on('start', () => {
  
          console.log('transcoding...')
  
      })
  
      .on('error', (err, stdout, stderr) => {
  
          console.log(err.message, err, stderr);
  
      })
  
      .on('progress', function(progress) {
  
          console.log(progress);
  
      })
  
      .on('end', function(filenames) {
  
          console.log("Finished transcoding.");           
  
      })
  
      .pipe(res);
  
  
  
  }

  I think that comes from the fact that we don’t know in advance the size of the final transcoded file so the range we send in the headers is wrong and somehow make the video player "limited".

  What can be a workaround to this problem ?