I'm trying to convert an rgba png to prores 4444 with an alpha. When I run the following command it will create prores file with an alpha. But, the alpha is full white, it doesn't retain the values I put in.
ffmpeg -start_number 1001 -r 24.000 - "pngWithAlpha_%06d.png" -c:v prores_ks -profile:v 4 -alpha_bits 16 -y "proresWithAlpha.mov"
I've tried this with both FFMPEG 4.2.2 and 6.0.0 on a linux system, I'm not sure if I'm doing it wrong or if it is a bug.
Looking at this post it seems it should be possible (they're complaining about quality not a fully missing transfer of the alpha).
I would expect it to pass the alpha straight through but it doesn't. I used alpha_extract to copy the alpha in to the rgb to confirm I'm able to read the alpha from the png, which worked. But the alpha was still full white.