[Bug]: `scale_vaapi` behaves differently than `zscale` (ffmpeg)

[Anuskuss]

Which component impacted?

Video Processing

Is it regression? Good in old configuration?

None

What happened?

I'm usually do HDR-to-SDR tonemapping using the following command:

ffmpeg -i hdr.mp4 -vf zscale=transfer=linear,tonemap=mobius,zscale=transfer=bt709,format=yuv420p thumb-%06.png

but unfortunately I can't using zscale so I've tried to use VAAPI instead:

ffmpeg -i hdr.mp4 -vf hwupload,scale_vaapi=format=p010,hwdownload,format=p010,tonemap=mobius,hwupload,scale_vaapi=format=nv12,hwdownload,format=nv12 thumb-%06.png

This creates some vastly different colors though. I don't think I'm doing something wrong (it's almost the same exact command) so I decided to open this issue.

What's the usage scenario when you are seeing the problem?

Transcode for media delivery

What impacted?

No response

Debug Information

No response

Do you want to contribute a patch to fix the issue?

None

[nyanmisaka]

scale_vaapi only does scaling, while zscale does not only scaling, but also linearization, which is essential for the tonemap filter. So mixing scale_vaapi and tonemap is wrong.

[Anuskuss]

Can you suggest another filter that does this linearization? Because Plex does something similar

hwupload,scale_vaapi=w=1920:h=1080:format=p010,hwmap=derive_device=opencl,tonemap_opencl=tonemap=mobius:format=nv12:m=bt709:p=bt709:r=tv,hwmap=derive_device=vaapi:reverse=1,hwupload

[nyanmisaka]

AFAIK currently only zscale can do it, that's why zscale+tonemap is always used in combination.

Also if your GPU supports tonemap_vaapi, you can also try it. But it cannot be fine-tuned.

[Anuskuss]

tonemap_vaapi is even worse than no color conversion at all so in the end I decided to just go the Plex route and use OpenCL instead, even though the colors weren't great either (when compared to zscale):

ffmpeg -i hdr.mp4 -vf format=p010,hwupload,tonemap_opencl=tonemap=mobius:format=nv12:m=bt709:p=bt709:r=tv,hwdownload,format=nv12 thumb-%06d.png

AFAIK currently only zscale can do it

Okay so if scale_vaapi=format=p010,tonemap is wrong should I turn this issue into a feature request instead (asking for linear support)?

[nyanmisaka]

This is unlikely to happen in VA-API. It's beyond the capabilities of fixed-function hardware. It's better to implement it in OpenCL or Vulkan via compute kernels.

So as an alternative, you can use the libplacebo filter, which requires the Vulkan runtime.

[Anuskuss]

you can use the libplacebo filter

libplacebo sounds interesting but I unfortunately can't use that either.

This is unlikely to happen in VA-API.

Guess I'll close this then. Thanks for your time.