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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

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  • 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

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