Might be related to this other ticket?

I've been doing some prores tests for both color and alpha. Usually this isn't a big deal since my alpha channels are solid with a 1-2pixel transition to black. However I did some tests with a very gradual change in alpha value over many pixels and noticed some pretty bad alpha channel quantization with the prores_ks encoder. The Apple prores spec says for 4444(XQ) the alpha channel is 16bit and lossless so this is quite different from that. I've attached a comparison image.

ffmpeg -f image2 -framerate 24.0 -i tif/rgba_16bit.%3d.tif -filter_complex "[0]colorspace=all=bt709:range=pc:format=yuv444p12:iall='bt601-6-625':irange=pc[main];[0]alphaextract,format=gray16le[alpha];[main][alpha]alphamerge" -c:v prores_ks -profile:v 5 -pix_fmt yuva444p10le -alpha_bits 16 prores4444XQ_yuva.mov

Image below is actually 8bit due to imgur limitations so the 16bit and 10bit examples aren't really correct but you get the point.

Also it looks like -alpha_bits 8 and -alpha_bits 16 produce the same look so maybe its getting bad upstream?

alpha quantization

  • It migth be that just because you use XQ profile.
    – user12174
    Aug 7, 2019 at 15:13
  • Just checked with profile:v 4 which is regular 4444 and it still looks like that :(
    – Shao Zhang
    Aug 7, 2019 at 17:27


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Browse other questions tagged or ask your own question.