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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.

  


• Why is ffmpeg's hstack so much slower than overlay and pad ?

  27 janvier 2021, par cgenco

  I'm using ffmpeg to stitch together two videos of people chatting into a video with each of them side-by-side, like this :

  


  

  


  Here's the command I'm currently using to get this done, which runs at 2.5x on my 13" M1 MacBook Pro :

  


  ffmpeg -y -i left.mp4 -i right.mp4 -filter_complex "
  [0:v] crop=w=in_w/2 [croppedLeft];
  [1:v][1:v] overlay=x=overlay_w/4 [shiftedRight];
  [shiftedRight][croppedLeft] overlay [vout];
  [0:a][1:a] amix [aout]
" -map "[vout]" -map "[aout]" -ac 2 out.mp4


  


  This command crops the left video to half of its original width (cropping so the video is centered), then shifts the right video a quarter of its width to the right, then overlays the left video on the left half of the output merged with the shifted right video.

  


  One day on my weekly fun-time read-through the FFmpeg filters documentation I stumbled on a filter named hstack, which is described as being "faster than using overlay and pad filter to create same output."

  


  My ex wife can affirm that there are few higher priorities in my life than going faster, so I altered my ffmpeg script to use hstack instead of two overlays :

  


  ffmpeg -y -i left.mp4 -i right.mp4 -filter_complex "
  [0:v] crop=w=in_w/2 [croppedLeft];
  [1:v] crop=w=in_w/2 [croppedRight];
  [croppedLeft][croppedRight] vstack [vout];
  [0:a][1:a] amix [aout]
" -map "[vout]" -map "[aout]" -ac 2 out.mp4


  


  ...but that command runs painfully slowly, like 0.1x. It takes multiple minutes to render a single second.

  


  So uhhh what's going on here ? Why is hstack taking so long when it's supposed to be faster ?

  


  I've tried this on both the M1 native build from OSXExperts (version N-99816-g3da35b7) and the standard ffmpeg from brew and hstack is just as slow on each.

  


• Can the frame size be cropped during decoding in FFMPEG ?

  13 février 2013, par James491

  I've followed Dranger's tutorial for displaying video using FFMPEG. http://dranger.com/ffmpeg/

  avcodec_decode_video2 is the slowest process for decoding video. I will occasionally have two videos decoding simultaneously but only displaying half of each video side by side. In other words, half of each video will be off-screen.
  In order to speed up decoding, is there a way to only decode a portion of a frame ?