vray 3.5 beta 1 - huge visual difference in computing displacement CPU vs GPU

[PIXELBOX_SRO]

Hi guys,

i ve tried converting a scene from CPU to GPU and i have found 2 major letdowns unfortunatelly.
Displacement on the GPU renders almost like it it badly mapped by UVs as opposed to a nice clean CPU render.
Secondly...i could not believe my eyes that the scene actually rendered faster on CPU than GPU although the GPU was still showing quite a lot of free memory.
Not to mention the load and render preparation times.

There must be some sort of glitch or is it possible that the displacement is computed differently?

Clearly this scene is not meant for RT but still...i woudl not expect this sort of issue in BETA at this stage of developement.

Any clues?

[savage309]

Well, unfortunately V-Ray GPU is a totally different render engine and there is no guarantee that it will match exactly the CPU render. This is valid both for Beta and stable builds.

We have a note about the displacement here. You can check the same page for other known differences.

Also, hard to tell why it was slower, not having the scene nor the hardware specifications.

Best,
Blago.

edit:these two guides can save some time (1), (2).

[kosso_olli]

I had similar issues, was never able to solve it, though. Displacement mostly looks worse on GPU. I somehow think that the bitmaps or procedurals for GPU are resized, even if I don't tell it to do that. Same with bump maps. But I am sure it will be solved someday

http://forums.chaosgroup.com/showthr...fferent-jagged

[PIXELBOX_SRO]

There is a difference and difference...
If it was slightly different then i would be happy but how can this happen on an unwrapped model with full float not resized texture is beond my understanding.
Sorry but comparing these two results you can clearly see the displacement on the gpu is corrupt... no way that it looks different.

[savage309]

It can happen if the used displacement type is not supported (just like the docs say). If you send the scene to the support with link to this thread, they will be able to check exactly which parameter is that.
The GPU render is completely different and some of the algorithms the CPU has implemented are not applicable (and vice versa). We don't try to match everything on the CPU 1:1, we just try to make the GPU engine as feature complete, stable and fast as possible. Making a scene for CPU, switching to GPU and expecting them to match perfectly is rarely the case (but often they look quite the same though).

Best,
Blago.

[PIXELBOX_SRO]

unfortunatelly i have a quite strict NDA on the project so i cant share anythign from the scene Blago,
however, perhaps this will help, i am using the general systém displacement from materiál editor slot and using the systém settings.
edge length doesnt play much difference, all settgin set to default.

[savage309]

Okay. I will make a note to check that just in case and if there are any unexpected things we find along the way I will post here.
Keep in mind that because of the holidays, we will be able to check that sometime next week.

Best,
Blago.

[a7az0th]

Displacement on GPU works differently than displacement on CPU but similar results can be achieved between the two. The reason why CPU displacement looks much better than GPU with default settings is that CPU would also add an additional coating of bump mapping based on the displacement texture. This further increases the perception of displacement. Unfortunately there is no way for us to enable this on GPU because of limitations of the GPU architecture. The algorithm the CPU uses is inapplicable to the GPU architecture. This algorithm was previously controlled by the Cache Normals checkbox and could be disabled for CPU if set to ON (thus the CPU would output the same results as the GPU does) but in the latest versions of V-Ray for Max this checkbox has been permanently disabled and the algorithm is always ON for CPU. Thus the only way to achieve such detailed results on GPU is to actually increase the tessellation for displacement.
Another thing to bear in mind is that 2d displacement is currently not supported for GPU. Standard displacement and Vector displacement should work fine.
It would still be helpful if you could recreate this problem on a simple scene that you could send over to our support team for investigation.
Also, a screen shot of the displacement settings on your problematic scene could also help (if possible)

[PIXELBOX_SRO]

thanks for the explanation.
it looks like i will have to send you the particuar problematic object for your expertise because it looks like the different approach to rendergina skinned and animated object.
The part where it has difficulties are the parts where the rig parts are. Strange that its not evident on the CPU side but the GPU renders this with error.
Will talk to client and let you know if it helps you bug hunting.
M