set correct-downscaling=yes leads to Ortho (vert) and (horiz) downscaling (bicubic) even when no downscaling is needed

[danbezerra]

Hello, as mentioned in the title, I'm trying to understand why correct-downscaling=yes triggers Ortho downscaling. Some details:

1. This is only happening for HDR titles (all HDR 4K rips I tried). 4K 709 and 1080p rips don't show the Ortho downscaling in the stats.
2. I'm using a 4K TV and confirmed that no scaling is needed (via stats, native and sized resolution are the same 3840x2160, scale: 1)
3. If I set correct-downscaling=no, I don't see the Ortho downscaling message any longer. But my understanding from the manual is that correct-downscaling may increase quality so I always left it active.
4. I'm using gpu-next, Windows, NVidia.
5. using the last MPV build from https://github.com/shinchiro/mpv-winbuild-cmake/releases/tag/20230627

Attaching my config file as FYI.

Is this normal? Why only on HDR titles? What is it actually downscaling here?

mpv.txt

[lavilao]

I might be wrong but I think that the scaling done there (the pic) is on chroma aka the colors, maybe hdr does by default some cheap chroma downscaling and with correct-downscaling does the good one?

[danbezerra]

Yes, I also believe it's chroma downscaling. But why is the big question. If anything, it should be upscaling from 4:2:0 used in UHD movies via the KrigBilateral shader.

[haasn]

The downscaling you see there is the HDR feature extraction pass, used for HDR contrast recovery. If you set --no-correct-downscaling, it is forcibly disabled.

This is all working as intended, except for the part where I haven't hooked up the contrast recovery options to mpv yet.