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• Recording lossy video stream

  27 janvier 2014, par tomas

  I'm trying to record live video stream to a file.

  I tried with VLC using

  vlc INPUT —sout '#stdaccess=file,mux=ts,dst=file.mp4'

  I tried with ffmpeg using

  ffmpeg INPUT -vcodec copy -acodec copy file.mp4

  Both records just fine, but the source tends to loose connection for 1-2seconds, and then recording just stops >.< leaving me with half finished recording :(

  I want the recording to continue, and the recording app try to reconnect.

  What can you recommend ?

• 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 :

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

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  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


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  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.

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  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."

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  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 :

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  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


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

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

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  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.

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• how can I slow down a video (I lower pitch by a semitone) with FFMPEG ?

  8 mai 2022, par patrick

  I have an old black and white film with audio that was recorded on a phonograph and the playback speed of the audio is a semi-tone higher than it should be. How can I process this video with ffmpeg and slow down BOTH the video and audio together, bringing the audio down a half step (semi-tone) while keeping the video in sync ?

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  I have tried filters like asetrate, setpts, atempo, etc and although I could change the pitch of the audio, the video became badly out of sync with the audio.

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