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• ffmpeg change framerate : Question about bitrate [closed]

  9 février, par schweigerson

  Thanks to the forum, I successfully can reduce the frame rate of a video.

  


  ffmpeg -i input_50fps.mp4 -filter:v fps=25 output_25fps.mp4

  


  Input and output have a similar bitrate as well as filzesize :

  


  




  


  


  


  


  


  



  


  


  


  


  


  


  


  


  


  


  File	bitrate	filesize
  input_50fps.mp4	2900kBit/s	1.9GB
  output_25fps.mp4	2528kBit/s	1.7GB

  


  Assumption : Divide the number of frames into half should both reduce the bitrate and the filesize accordingly (half the size).

  


  Question : Does it make sense to enforce the reduction of the bitrate to approx. half the original one, when I reduce the framerate by to 50% (e.g. with -b:v 1500k) ? IMHO, the video quality should be comparable to the original video then.

  


  Note : I may make according test runs, but I perhaps do not have the ability to recognize the quality difference as I'm not an advanced video expert.

  


  Would be happy to read your advice.

  


• ffmpeg : Split MP3 in 1 second chunks fast ?

  30 décembre 2019, par Niko

  I want to split mp3 into one second chunks fast. I have the mp3 already in memory (in crystal lang) and want to process it further (so I need it in memory after splitting). I got two possiblities working :

  • Saving the file to ram disk, calling ffmpeg -segment on it, reading all the segement files into memory.
  • Piping the file into ffprobe --show_frames and parsing the pkt_pos fields from the result.

  Both are slow about one second due to unnescessary overhead. Writing to disk and reading hundrets of files in the first case, outputting a lot of unneeded information about a lot of unneeded frames in the second.

  I see two possibilities to improve :

  • a custom filter for ffmpeg that outputs the segements with a separator to stdout.
  • a custom filter for ffprobe similar to --show_frames but with a framestep parameter (the existing framestep filter only seems to work with video files) which outputs only the byte offset of the frames.

  Perhaps I’m missing possibilites with the existing filters. Perhaps there is a tool better fitted for my needs than ffmpeg. I’d be happy for any hints.

• FFMPEG - why zoompan causes unexpected stretching ?

  14 septembre 2020, par Sarmad S.

  I have two images as input, both are 1600x1066. I am vertically stacking them. Then I am drawing a box and vertically stacking that box under both of the image. Inside of the box I write text, then I output a video that is 1080x1920. Everything works well, until I use zoompan to zoom in on the images, I get weird behavior. basically all input images including the box stretchs (shrink) vertically and no longer fit the entire height of the video which is 1920.

  


  The command (removed some drawtext commands from it) :

  


  -filter_complex 
"color=s=1600x1066:color=blue, drawtext=fontfile=font.otf: text='My Text':fontcolor=white: fontsize=30: x=50: y=50[box]; 
[0]scale=4000x4000,zoompan=z='min(zoom+0.0015,1.5)':x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':d=125:s=1600x1066[z0];
[1]scale=4000x4000,zoompan=z='min(zoom+0.0015,1.5)':x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':d=125:s=1600x1066[z1];
[z0][z1][box]vstack=inputs=3"


  


  How do I fix this ? I want to zoom in without stretching the images

  


  Before using zoompan this is how the video looks like (I want to keep it this way while zooming in the images) :


  


  After using zoompan this is how the video looks like :