user21917: 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?

user1871: 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


21917: <p>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.</p>

<p>Is it possible to make with ffmpeg an vp8,vp9 -pix_fmt yuva420p ready .webm movie?</p>
 1871: <p>The alphaextract/alphamerge filters are meant for this.</p>

<p>Since your alpha reference is already greyscale, we only need the alphamerge filter</p>

<pre><code>ffmpeg -i main.mp4 -i alpha.mp4 -filter_complex [0][1]alphamerge,format=yuva420p -c:v libvpx -auto-alt-ref 0 out.webm
</code></pre>
