GPU no good result, looks grainy - what's wrong? (Maya VRay Next)

[BeneZ]

Hi,
I am testing Rendering with CPU and GPU (CUDA)
The result with CUDA is always grainy, I have spent an hour to find out what's wrong.
The result shows up after short time. It's a simple greyshader VRayMaterial!

Can anybody explain it?

Thanks

(Object uses also Normal Displacement, but this should not have huge effect here)

[Mihail.Djurev]

Might be a bug, but my simple tests don't reproduce the issue? Would you be able to send us a sample scene that reproduces the issues, along with the normal maps?

[dgruwier]

Doesn't look like sampling noise to me, it's too large, splotchy and smeared. Are you certain that the normal displacement has nothing to do with it?

[BeneZ]

Hi,
yes, it's an issue with displacement.
Without Displacement it is ok.
I made a closeup render.

Where should I send that file? This is a dropbox-link for it: http://wck.me/12GL
Thanks

[BeneZ]

Here is a dropbox link for the file
http://wck.me/12GL
Thanks

[yolov]

Thanks for the scene, it made things easier for me.
To fix the issue, you'll just need to increase the max subdivs for subdivision and displacement quality. You have added those settings under your geometry's shape node > extra VRay attributes > Subdivision and Displacement Quality. 32 seems to fix it for the scene as you've sent it, but since the settings are 'view-dependent', keep in mind that you might need to readjust this should you change the resolution and the camera angle.
The reason this is happening is that there's an option called 'cache normals' for displacement that speeds up shading on displaced objects. https://docs.chaosgroup.com/display/...cement+Control

The GPU has this enabled at all times, as it needs it. The problem is that this option can lead to such issues when the displcament quality settings are very low, like in your case with 8 subdivs.