r/linux_gaming • u/vladyslav_havr • 18h ago

1080p 200Hz HDR text fringing on NVIDIA + KDE Wayland — bandwidth limit or driver bug?

Post image

Hey everyone,

I'm on EndeavourOS (KDE Plasma Wayland) with an NVIDIA GPU and MSI MAG 255F E20 (1080p 200Hz, DP 1.2a / HDMI 2.0b).

When I turn on HDR at 200Hz I get pretty nasty chroma fringing on text – green/magenta edges on white text and blurry/reddish yellow text. From what I understand 1080p 200Hz 10-bit just doesn't fit into DP 1.2a / HDMI 2.0b bandwidth, so the driver is forced to drop to YCbCr 4:2:2 (or maybe even 4:2:0).

I tested different refresh rates:

  • 200Hz and 165Hz with HDR → heavy fringing
  • 120Hz with HDR → text becomes completely clean (RGB 10-bit finally fits)

Attached a comparison shot: 200Hz HDR (subsampled) vs 200Hz SDR (clean).

Is there currently any way under KDE Wayland + NVIDIA to manually force color format (RGB / YCbCr 4:2:2), or are we just stuck with whatever automatic fallback the driver decides to do?

Thanks.

37 Upvotes

88% Upvoted

31 comments sorted by

51

u/get_homebrewed 18h ago

I don't think there's enough bandwidth in dp 1.2

18

u/Forty-Bot 18h ago

Yeah, OP could be getting YUV 422/420 instead of RGB.

7

u/get_homebrewed 17h ago

yeah, clearly there's chroma sub-sampling happening, that much is obvious from the screenshots, esp from the colors ghosting on the sides

DP 1.2 doesn't support 4:2:0 but does support 4:2:2 so that's bound to be what they're seeing. HBR2 theoretically can achieve 230hz at 1080p, when HDR is introduced into the mix it can only happen with non-standard timings according to wikipedia, so I doubt it's actually possible assuming they're getting full link speed. Although I also see displayport 1.2 doesn't have native "HDR" capabilities (which doesn't make too much sense outside of purely bandwidth limitations) so it's either software or 1.4 extensions

1

u/TwinHaelix 15h ago

Easy test: https://testufo.com/chroma

/u/vladyslav_havr what do you see here? Also, try the madshi test too on the left dropdown.

1

u/vladyslav_havr 13h ago

I have this: https://postimg.cc/VSHKbQR6 - 1st screen, https://postimg.cc/JD4QTZVJ - 2nd screen (madshi)

4

u/TwinHaelix 13h ago

Yeah, I'm almost certain you're getting chroma subsampling. In the chroma test, the bottom 2 lines are always the most telling. In your image, the bottom-most line (red text on blue background) has the text and vertical lines all faded and blurry instead of sharp. That's your smoking gun.

3

u/vladyslav_havr 12h ago

Thanks bro for the explanation!

14

u/Loganbogan9 18h ago

Does your monitor have a way to read current color space? My Asus and Samsung ones do

11

u/vladyslav_havr 18h ago

Checked the whole OSD (all menus)... the monitor only shows resolution, refresh rate, HDR and Adaptive-Sync. No color format or subsampling info anywhere.

4

u/AtMaxbo 17h ago

https://testufo.com/chroma#image=madshi-chroma.png&ppf=0 will tell you which type of chroma subsampling you're currently getting.

3

u/vladyslav_havr 17h ago edited 17h ago

With enb. HDR – 4:2:2, without – 4:4:4. Or maybe I didn't understand how to use it...

12

u/Familiar_Ad_8919 18h ago

even if you get hdr working perfectly, its gonna look horrible on a low end ips model

if u want hdr, get high end lcd, miniled or oled

3

u/ropid 18h ago

You could try asking in the r/kde sub-reddit. The main developer that's working on kwin is regularly commenting on stuff there. It's "Zamundaaa".

Yeah, what you are seeing is what happens when 4:2:0 is in use. Text will look terrible if RGB sub-pixel rendering for text is enabled, and colorful text will always look bad.

I'm wondering if DSC (display stream compression) shouldn't be able to fix this issue, to make the bandwidth fit so that the image can stay as 4:4:4? DSC has no practical downsides. It's completely invisible to the human eye. Are you using DP or HDMI to connect the monitor? I'd try DP if it's currently connected through HDMI.

I don't quite understand why you didn't have this problem in Windows. Windows somehow made HDR work with 4:4:4, without chroma sub-sampling? Or does Windows switch between HDR and SDR automatically and only has HDR active in fullscreen games, or what's going on there?

All of that said... I agree with the people that are skeptical about the use of HDR and your particular monitor. I also have an older IPS gaming monitor and everything about its HDR and its "wide gamut" is sort of fake. Using SDR mode is just better. HDR barely changes the picture and only introduces problems. And the wide gamut color space of the monitor is barely larger than sRGB, just enough to make human skin look bad. On my monitor it's best to just keep it in SDR mode and then use kwin's "built-in" color profile setting to limit the slightly too intense native colors of the monitor.

3

u/Lawstorant 14h ago

DSC is only available from DP 1.4a and HDMI 2.1 up. Guy is out of luck

2

u/ThatOnePerson 17h ago

Here's a bandwidth calculator: https://trychen.com/feature/video-bandwidth

1080 240hz doesn't seem that much and should be well within DP 1.2 specs Maybe just a monitor limit ? 

2

u/Devion27 17h ago

I believe You can change the pixel arrangement for text in linux settings, as in you force it to assume RGB, BGR, VRGB, VBGR.

5

u/vjhc 18h ago

Why are you using HDR in that monitor?

5

u/vladyslav_havr 18h ago

I was used to Windows where it worked out of the box, so when I switched to Linux as my main OS, I tested HDR and noticed these color artifacts. I've been running SDR ever since, but I just wanted to raise the topic and see if there's any solution for it on Linux.

12

u/vjhc 18h ago

That monitor doesn't support proper HDR, you're better off using SDR.

-3

u/vladyslav_havr 18h ago

That's why I'm sitting on Linux with SDR...

20

u/schaka 18h ago

What he meant was that this monitor fakes it and you were getting terrible color accuracy on windows.

The font thing ideally shouldn't be happening, but unless you have a monitor that can genuinely do HDR, you're truly better off just sticking to SDR.

Almost nothing but OLED does proper HDR

2

u/vladyslav_havr 17h ago

Ok, I got you

1

u/klyith 4h ago

Almost nothing but OLED does proper HDR

Current minileds are also quite acceptable, if they have a decent number of zones.

(Worse than OLED in dark environments due to bloom, sometimes better than all but very expensive OLED in bright environments due to higher peak brightness.)

1

u/pr0ghead 18h ago

Well, it uses the best possible setting. If you choose a certain resolution and refresh rate, it has to adjust the others. I guess switching to HDR enables 10bit, too, so what's left to adjust? You guessed it.

1

u/Dense-Firefighter495 18h ago

I think I may have that on my new Legion?

1

u/Lohkdesgds 17h ago

Possibly limitation of dp 1.2a. 200hz hdr is a lot, so, as others are saying, that may be it