Black Crush & Purple Shadow Tinting Caused by Forced PQ EOTF

[leeson865]

Hey Rayek,
Thanks for putting together the LeGo2BrightnessFix plugin and the associated display profile. It does an incredible job of unlocking the panel's full 1100-nit HDR peak brightness by forcing the PQ transfer function.

However, running eotf = gamescope.eotf.pq globally introduces a significant side effect for SDR content and HDR dark/greyscale scenes. Because Gamescope forces the entire Game Mode session into an HDR/PQ container, standard SDR content (and games with deep shadows) suffers from severe near-black EOTF conversion errors. This results in heavy black crush and a distinct purple/magenta color shift in dark gradients (making scenes like dungeon doorways in games look blocky and tinted).The issue becomes extremely obvious when running engine-level lighting mods (like RenoDX) or playing dark-toned games where the lower grayscale limits are heavily stressed.

Pictures here with an example from Crimson Desert, night time scene with RenoDX enabled: https://imgur.com/a/ccs94FN

Steps to Reproduce:

1. Install the display profile utilizing gamescope.eotf.pq to enable full peak brightness.
2. Launch into Game Mode and open a game with dark, low-lit environments (e.g., Crimson Desert with RenoDX mod enabled).
3. Observe the purple tint, color banding, and crushed black levels in shadow gradients.

Considerations:

• While switching the profile to gamescope.eotf.gamma22 completely fixes the black levels and color accuracy, it hard-caps the hardware highlights down to ~475 nits, losing the intended PQ peak behavior.
• Is there any architectural path forward or workaround within the Gamescope display profile configuration to allow dynamic EOTF handling, or a way to mitigate the near-black PQ conversion artifact without sacrificing the 1100-nit peak highlights?

Thanks for your work on this project!

[leeson865]

An update: RunningLilium's HDR black floor fix shader via Reshade improves this dramatically. Before and after pictures here:

https://imgur.com/a/D18KUhF

This is probably a bandaid fix rather than a permanent fix though.

Perhaps if you can find a way to modify the LUA file to fix the gamma/black floor without hurting brightness (similar to how this fix works?)

[Rayekkk]

Hey, I have been looking into this since yesterday when I saw the report.

Unfortunately, from what I have been able to work out, this cannot be fixed from the plugin or from the gamescope display definition while the panel stays in PQ.

PQ is an absolute transfer function: a code value maps to a fixed luminance, so the panel ignores its hardware brightness control completely and writes to /sys/class/backlight/amdgpu_bl0 do nothing. The brightness slider can therefore only scale the content signal down. White drops, but the panel's near-black floor stays exactly where it is, so contrast collapses, shadows crush and the darker greys merge together. The only real fix at this point is changing the EOTF from PQ to gamma 2.2.

I have now run detailed tests of the panel in both PQ and gamma 2.2, and the best solution I could come up with is to extend the plugin with a choice of how the panel is configured:

• Hybrid [default, recommended] - switches the panel between PQ and gamma 2.2 automatically, depending on whether the content on screen is SDR or HDR.
  Downside: the moment the panel changes mode you get 1-2 seconds of black screen, much like switching inputs or toggling HDR on a monitor. In exchange you get the best of both worlds: SDR keeps normal brightness control with no black crush and no color banding, and HDR reaches its full potential, at the cost of a roughly one second of blank screen when HDR turns on or off in a game.

• PQ - what the plugin already does today.
  Downside: it is affected by the low brightness SDR problem this whole issue is about.

• Gamma 2.2 - the least problematic EOTF.
  Downside: it is not the Go 2's native mode for HDR, and HDR peak brightness may (and probably will) be clipped.

I already have update 1.1.0 ready, but I could use a test and some feedback from someone other than me. Would you be willing to try the plugin in Hybrid mode and let me know whether everything is fine, in SDR as well as HDR? If it holds up, this exact build will become the official 1.1.0 release.

Download 1.1.0:

LeGo2BrightnessFix-1.1.0-rc.zip

A few screenshots:

First-time setup:

Plugin settings after the first setup:

[leeson865]

Hey mate

Thanks for this! I can confirm this works brilliantly and switches seamlessly - the blank screen is very minor and similar to how consoles behave anyway. I also used a nits measurement device on the screen and confirmed that it switches properly and peaks are 1100nits in PQ mode and tone mapped to ~500 nits in Gamma 2.2 mode as expected. Switching to the gamma 2.2 on the fly also fully resolves the gamma issue with SDR games. Games tested:

Working perfectly:
Crimson Desert
Resident Evil 3
Resident Evil 7
Spiderman Miles Morales
The Darkness II (this game is a great test as its SDR, very dark hues and requires Gamma 2.2 to look right).

Only one game bugged out with it:
Horizon Zero Dawn (not remastered version) - this game showed the loading bar for a second then would have a black screen. To fix it, I had to force the gamma out of hybrid and into PQ mode, then the game launched. I suspect that the game launches for a second as SDR and then changes itself to HDR quick enough that the hybrid mode couldn't detect it. It's an older game though back when PC HDR was a bit of a shitshow.

The only fix I could think of is if you have an optional toggle for per game profiles - giving the user the ability to override the display's hybrid mode on games of their choosing.

Thanks for the effort put in!

[Rayekkk]

Thank you for testing the plugin so thoroughly! Your feedback helped me track down the real cause of the issue you encountered in Horizon Zero Dawn.
The problem was not specific to the game. I tested the same non-remastered version and could not reproduce it consistently. The actual cause was a timing issue: during very fast SDR/HDR transitions, the plugin could occasionally switch modes too aggressively and end up in the wrong state.
I have added transition stabilization to handle this case, and the fix is included in the 2.0.0 release.
Thanks again for your help!

[leeson865]

Hey, no problem. Thanks for the new release.

I tested version 2.0 on Horizon zero Dawn and unfortunately the issue remains. The panel stays in SDR mode it seems. That may be part of the issue as the game was definitely running with HDR enabled prior to enabling the brightness fix add-on.

Can you see if you can replicate it by enabling HDR in game while in PQ mode, closing the game, and then reopening it?

[Rayekkk]

Thanks for the feedback. I’ve tried several times to reproduce this bug using the same original, non-remastered version of Horizon Zero Dawn and the exact sequence you described, but it simply does not occur on my device. The panel switches to PQ correctly every time, which makes the problem difficult to diagnose reliably.
As a last resort, I will probably add an optional per-game override feature for cases like yours, but before doing that, however, could you please test this experimental build?

I found a potential race in XWayland handling: Hybrid advertised HDR capability to the XWayland servers that already existed, but a new per-game server created afterwards could miss that feedback. The game could then cache the output as SDR while creating its Vulkan surface and never trigger the switch to PQ. This RC detects late XWayland servers and restores the missing HDR feedback without forcing the panel into PQ.
If this resolves the problem, it should provide a global fix without requiring per-game profiles.

Please test it in Hybrid mode using the same sequence as before and let me know whether the panel switches to PQ when Horizon starts.

Download 2.0.1-rc.1

LeGo2BrightnessFix-2.0.1-rc.1.zip

[leeson865]

Thanks for this. Have just completed pretty extensive testing and the plot has thickened.

• I tried your 2.0.1 RC - same issue
• I then forced PQ (which is what made it work in the past) on 2.0.1 - but the issue persisted!
• I then downgraded to v2.0 and tested in hybrid - same issue
• I then put v2.0 in forced PQ (which fixed it in the past) - issue persisted
• I then downgraded down to 1.1.0 - issue persisted
• I then put v1.1.0 in forced PQ (which fixed it in the past) - issue persisted
• I then completely disabled the plugin and rebooted the LEGO2 - issue persisted!
• I then changed the proton version for Horizon Zero Dawn to forced compatibility version as Proton Experimental - issue solved
• I then upgraded back to v2.0 stable, left it in Hybrid mode with the Proton Experimental version enabled - issue solved

So in summary, it seems to be a HDR specific issue bug in SteamOS that requires a specific Proton Experimental version to work around for Horizon Zero Dawn. You are possibly already forcing your games to run as Proton Experimental globally which is why you didn't have the same issue?

Regardless - I apologise for the bum steer. It really did work for me consistently when I forced the panel into PQ so I am unsure why it is behaving this way now.

Notably, this might mean the changes you made since v1.0.1, namely:

• "Added transition stabilization to handle this case" (added in v2.0)
• "This RC detects late XWayland servers and restores the missing HDR feedback without forcing the panel into PQ. "(added in v2.0.1)

May both be unnecessary and have unintended side effects/complexity to the plugin. I'll leave it up to you if you think they should be removed and you revert back to the simplified v 1.0.1 with hybrid gamma shift only.