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• FFmpeg force_key_frames expression variables

  16 février 2014, par user1704620

  I am writing a Bash script, and I am trying to figure out a way to get FFmpeg to recognize a global variable in the -force_key_frames option. The -force_key_frames option can take a regular expression as an argument, allowing functionality such as forcing a key frame every 5 seconds :

  -force_key_frames 'expr:gte(t,n_forced*5)'

  This works fine for forcing a key frame every 5 seconds, but I don't know how to force a key frame every x seconds, x being an input variable from the user gotten by the rest of the script. The exact FFmpeg command that I'm trying is :

  ffmpeg -i "video.mp4" -vcodec: libx264 -b:v 500k \
  
  -force_key_frames 'expr:gte(t,n_forced*${SEG_TIME})' -s:v 640x480 \
  
  -r 29.97 -pix_fmt yuv420p -map 0 -f segment -segment_time ${SEG_TIME} \
  
  -reset_timestamps 1 -y "output%01d.mp4"

  The variable $SEG_TIME is set to 5 by the script, but the regular expression in the -force_key_frames option doesn't seem to like the $SEG_TIME variable.

• ffmpeg xfade custom transition expression - Fetch pixel value at specifc x, y from vid1, vid2 ?

  25 janvier 2021, par jsaluja

  I am using xfade with transition=custom, and trying to build expression.
I am referring to official documentation https://ffmpeg.org/ffprobe-all.html#xfade

  


  Per my understanding, expression should return pixel value of merged video

  


  I built a few custom transitions using expressions that return some combination of A, B

  


  For eg.,

  


  "custom:expr='if(gt(P, abs(X/W - 0.5)), A, B)'"
"custom:expr='lerp(A, B, 1 - P)'"


  


  For the next set of transitions, I want to fetch the pixel values from source, target video at specific positions, and combine them in my expressions

  


  I tried using a0(X, Y), b0(X, Y) etc inside of

  


  xfade=transition=custom:expr='r....


  


  I ran into exceptions with ffmpeg complaining it doesn't understand the expression.

  


  Can someone clarify, how i can fetch pixel values as specific position from source, target video and combine them in expression ?

  


• How to fetch pixel value at specifc x, y from vid1, vid2 in ffmpeg xfade custom transition expression

  25 janvier 2021, par jsaluja

  I am using xfade with transition=custom, and trying to build expression.
I am referring to official documentation https://ffmpeg.org/ffprobe-all.html#xfade

  


  Per my understanding, expression should return pixel value of merged video

  


  I built a few custom transitions using expressions that return some combination of A, B

  


  For eg.,

  


  "custom:expr='if(gt(P, abs(X/W - 0.5)), A, B)'"
"custom:expr='lerp(A, B, 1 - P)'"


  


  For the next set of transitions, I want to fetch the pixel values from source, target video at specific positions, and combine them in my expressions

  


  I tried using a0(X, Y), b0(X, Y) etc inside of

  


  xfade=transition=custom:expr='r....


  


  I ran into exceptions with ffmpeg complaining it doesn't understand the expression.

  


  Can someone clarify, how i can fetch pixel values as specific position from source, target video and combine them in expression ?