4k 120hz over HDMI 2.1 working almost correctly on a Samsung S95B TV

I have a Samsung S95B TV which supports 4k 120hz over HDMI 2.1 to an RX 6900 XT gpu. Initially this seemed to work, and in practice it sort of does but there's this subtle strange phenomena with using 120hz or 100hz that does not happen on 60hz and down.

If you have a black background and draw a single colored pixel, then translate the content one pixel to the left or right it will dim. It's almost as if for every other pixel row the colored pixel will become grey and dimmed.

To illustrate this problem, I draw an image like this where each square represents a pixel

And here's a video recorded on my phone which shows the problem in motion. I drew the pixels in gimp, set the zoom level to 100% and drag the gimp window carefully around.

pixelissue

In practice this has a noticeable effect on colored graphics that have limited amount of pixels to work with. Like small colored text, pixel perfect gui, etc.

This does not happen on windows. It happens on both wayland and xorg, but looks fine when using 60hz.

I'm using Mesa 22.2.0-devel (git-7b74747 2022-06-16 focal-oibaf-ppa) according to glxinfo

I'm on the KDE Neon distro which is based on Ubuntu 20.04. The Linux kernel version is 5.13.0-51-generic

Some relevant xrandr --verbose output

xrandr.txt

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See #1417 for more background on HDMI 2.1 limitations on Linux.

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I tried various xrandr properties, the only thing that would slightly improve it was using a transform matrix and scaling x by 1.5 pixels or something. But obviously overall image quality becomes blurry and worse in other ways.

This is an old issue but I wanted to add some additional context to the HDMI 2.1 issue - full RGB output at 120hz 4K resolution is not supported, however 120hz is still possible using a different color encoding. The distortion you're noticing is because you're using YUV420. This encoding saves bandwidth by encoding 'chrominance' at half-horizontal resolution. Luminance is unaffected, so if you try your test with a black-and-white image you should notice it stays sharp.

This issue hasn't had any activity since 2022-11-15. The AMD driver stack changes rapidly and contains lots of shared code across products so it's possible that it has already been fixed. Please upgrade to a current stable kernel and userspace stack and try again. If you still experience this issue with the latest driver stack, please capture relevant logging and open a new issue referring back to this one.

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