Connecting a ramp attribute to the basecolor/emmision color of a material creates darker colors than the result should be on XPU vs CPU.
In the example bellow you can see the huge difference between one and the other one. This is just a ramp applied to an mtlx_unlit surface.
I have seen post that XPU seems to limit the albedo but this should not be it right? as the difference does not exist if I just use the color input of the mtlx_unlit surface node rather than connecting the ramp. PS using latest production build and studio driver.

can you post a repro scene? (or better yet, file a bug)
thanks lots!

brians
can you post a repro scene? (or better yet, file a bug)
thanks lots!

Sorry for the late reply, I was occupied with other tasks, here I've attached a sample Hip.
I've already posted the bug, though I am still not sure if this is a bug or some energy conservation issue with XPU I need to understand.
I will update this post as soon as I know the bug report number.

Thanks,
Nico.

Thanks for posting!
Given the proximity to Christmas, it may be a while before it gets looked at. Hope thats ok.

brians
Thanks for posting!
Given the proximity to Christmas, it may be a while before it gets looked at. Hope thats ok.

No problem. Merry Christmas and enjoy your holidays!

Seems a 590 driver bug.
A short-term workaround seems to be to set this envvar
KARMA_XPU_OPTIX_FORCE_OPTIXIR=1

brians
Seems a 590 driver bug.
A short-term workaround seems to be to set this envvar
KARMA_XPU_OPTIX_FORCE_OPTIXIR=1

Thanks will give it a try soon enough. A bit busy in another project now, But will definitely come up soon enough.

We've confirmed its a 590 driver bug.
It turns out the KARMA_XPU_OPTIX_FORCE_OPTIXIR envvar may not solve the issue for you after all.
The good news is that I've committed a workaround in code which should work for you, its out in 21.0.594
cheers