4K → 1080p downsampling and retaining chroma resolution in yuv420p

[fililip]

This might be a dumb question (apologies 🙏), but I'm trying to understand current mpv behavior related to YCbCr channel scaling.

How does mpv handle downsampling of 4K content (yuv420p[10le]) on a 1080p display? Is there a way to make mpv only scale down the Y plane, and not touch the Cb and Cr planes at all to maintain full color information at that resolution? Would it even be beneficial to do so (or are there some technical limitations of chroma subsampling in the first place)?

I've tried capturing my 4K screen showing the rtings chroma test pattern using both I444 and NV12 with the libx264 codec at crf=20 in OBS at 4K, then using cscale=nearest in mpv, but the results don't seem to look that much better compared to the default behavior; the yuv444p output looks good but the yuv420p one does not on my 1080p screen (especially with the last two lines in that pattern image, as they appear blurrier and less vibrant than the yuv444p version).

Examples:

(the yuv444p downsample)

(the yuv420p downsample)

My config file:

profile=gpu-hq
vo=gpu-next
hwdec=vulkan,nvdec-copy,vaapi
gpu-api=vulkan

hwdec-codecs=all
title=mpv
autofit=1600x900
screenshot-format=png
volume-max=200
audio-pitch-correction=no

[hdr]
profile-desc=HDR
profile-cond=p["video-out-params/max-luma"] > 203
target-colorspace-hint=yes

I'm running kwin_wayland on a 6600 XT (with a secondary Nvidia card) with Linux 6.17.8 and Mesa built 6 days ago from git.

Could I be misunderstanding something, missing a config option or perhaps encoding it wrong for yuv420p?

[atrxia1]

mpv doesn't downsample the luma plane directly. It downsamples the final RGB converted image after the chroma resolution has been upscaled to match the luma resolution.

https://github.com/mpv-player/mpv/wiki/Video-output---shader-stage-diagram

[hooke007]

Is there a way to make mpv only scale down the Y plane

Just use a luma downscaling shader.

less vibrant than the yuv444p version

But the problem you were talking may be caused by encoding...

[fililip]

use a luma downscaling shader

Would something like KrigBilateral or SSimDownscaler work here?

[hooke007]

KrigBilateral is used for upscaling chroma - but it's true it can make the color pixel vibrant again in this case.

[atrxia1]

SSimDownscaler is not a luma downscaler. It is designed to downscale RGB in conjunction with mpv's built-in catmull_rom scaler. What you're looking for is https://github.com/Artoriuz/glsl-luma-downscaler

[fililip]

SSimDownscaler is not a luma downscaler. It is designed to downscale RGB in conjunction with mpv's built-in catmull_rom scaler. What you're looking for is https://github.com/Artoriuz/glsl-luma-downscaler

Thanks! This looks exactly like what I was looking for.