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  • 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 :

    


    left.mp4 + right.mp4 = out.mp4

    


    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 ?

  • Can the frame size be cropped during decoding using libavcodec ?

    5 mai 2013, par James491

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

    avcodec_decode_video2 seems to be the slowest part of the video decoding process. 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 ?

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