Hi everyone,
I’m working on a lava cake simulation (MPM solver) and I’m struggling with persistent render noise in Karma XPU. The noise appears mostly on areas where I’m using a mask to blend different breakup/detail layers, so I’m not sure whether the mask itself is causing the problem or if my material network might be too complex (or if I simply need more samples).


Here’s what I’m using:

Renderer: Karma XPU

Denoiser: OIDN enabled

Samples: 250

Ray limits: (screenshot attached)

The noise seems to show up exactly where two different textures/material responses meet. I’m mixing multiple BSDFs based on a mask, and I suspect that might be contributing, but I’m not sure.



I’ve attached the render, and the sampling/ray-limit settings.

I’m not a rendering expert yet, so any pointers or recommended best practices would be super appreciated!

Thanks in advance!

That looks really great apart from obviously the slight flickering

Have you tried rendering without the denoiser just out of curiosity to see if that is the cause?


If so in many render engines there is an option for "static noise" which really helps when denoising is being used.

As far as I am aware Karma doesn't have this as an option on the interface BUT it can be achieved by setting
"husk -- lock-random 0" on the render command parameter of the USD render rop.


https://www.youtube.com/watch?v=0xEdN3q_f4I [www.youtube.com]

EZiniT
That looks really great apart from obviously the slight flickering

Have you tried rendering without the denoiser just out of curiosity to see if that is the cause?


If so in many render engines there is an option for "static noise" which really helps when denoising is being used.

As far as I am aware Karma doesn't have this as an option on the interface BUT it can be achieved by setting
"husk -- lock-random 0" on the render command parameter of the USD render rop.


https://www.youtube.com/watch?v=0xEdN3q_f4I [www.youtube.com]

Thanks for your awnser. In this render I've already tried to use "husk -- lock-random 0" but as you see it didn't help. As for trying to render it without denoising I have not tried this yet, but might be worth testing. Thank you

Ahhh damn, I was hoping the lock would be the solution for you.

Would definitely be interested in seeing a finial result, I think this is a great little sim and would
also like to know how the flickering is resolved.

The only other thing (on the rendering side) that comes to mind is 250 XPU samples could be a little low
especially when using denoise as it needs the detail to work with in the first place.

I think it would be fair to say getting very good results most people set samples between 512-1024

The Houdini channel on youtube posted about how to remove noise from XPU renders this week
https://www.youtube.com/watch?v=KTB-k1HABII&t=5s [www.youtube.com]

Although tbh there doesn't seem all that much to do, raise samples, lower secondary noise level and denoise were
the three things that really have any major impact with XPU.

also make sure that the noise is not cause by the geo itself as it can be the case with meshing etc...

since there are some randomly popping yellow details in your shader suggesting some unstable uvs or other issues coming from the geo itself

you can also try freezing one frame of the geo and have slow camera movement to see if the noise and jittering persists to better judge if it's really shading noise or not

So I've tried around a bit with rendering and this doesn't seem to cause the noise. I am not sure if the problem lies with the geo itself or from the shading. The UVs seem stable and The meshing seems fine as well(at least to me). Maybe the problem comes from the material. If you have anymore ideas it would be much appreciated!

You said "The noise appears mostly on areas where I’m using a mask to blend different breakup/detail layers"
I think you could be onto something!

We covered some renderer settings and that doesn't seem to be an issue.

Tomas gave a great tip freezing one frame to help identify what could be wrong.....did you try that?

Personally I would remove everything at this point and try step by step adding it back, process of elimination
with very basic default settings.....pretty obvious advice but without a hip it's hard to give an accurate answer.