I have 2 video files. One file is the main content and the other file has an alpha channel emulated with black and white (white = opacity 100 and black = opacity 0). Sometimes the emulated alpha channel file has only full black and full white frames, but I think some videos that I want to create will have complex alpha channel emulation with gray scales.

Is it possible to make with ffmpeg an vp8,vp9 -pix_fmt yuva420p ready .webm movie?

1 Answer 1


The alphaextract/alphamerge filters are meant for this.

Since your alpha reference is already greyscale, we only need the alphamerge filter

ffmpeg -i main.mp4 -i alpha.mp4 -filter_complex [0][1]alphamerge,format=yuva420p -c:v libvpx -auto-alt-ref 0 out.webm
  • Wonderfull! A perfect result with b/w and complex greyscale alphaemulated movies:). Now i can relaxed wait that my Videoediting and Animating Software implement this automatically. Apr 9, 2018 at 10:53

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