[SOLVED] Different results on GPU and CPU in Cycles

Support Lighting and Rendering
#1

I have a one sided mesh which is not watertight and the normals pointing inwards. Basically a cube with “2D” walls with holes in them. My problem is it renders differently in Cycles depending on if I use the CPU or GPU.

With the CPU I get what I feel is a correct result:

cpu.jpg576×562 96.8 KB

With the GPU this is my result:

gpu.jpg563×563 94.7 KB

Normals:

opengl-normals.png542×557 7.28 KB

Now, I’m pretty sure this is because it is an open mesh with a strong light outside it, but shouldn’t Cycles renders be identical between CPU and GPU?

Closing this mesh will be a huge pain, but I guess I will end up doing it.

I have abandoned GPU renders before because the lighting looked different from the CPU renders, and now I just realized it was because I had several light sources that were “2D” mesh lights stacked behind each other.

Is this expected behavior? Should I just always have a completely closed mesh in any case?

#2

I presume the card you are using was a nvidia one? If so it’s interesting as I was experimenting with an amd/ati card and I got different results - light-levels were vastly different (lower) with gpu than cpu.

I thought it was because of current state of cycles with amd, even using beta drivers; maybe it’s a different reason?

I agree with you though that they should render exactly the same.

#3

Yes, Nvidia GTX-560M, CUDA render on an SVN build.

#4

There are defferent sub-modes as ADVANCE_SHADING vs BASIC, as compile time option. I suspect you have build that use ADVANCE for CPU and BASIC for GPU. Cannot talk for sure, I think pictures must be same, may be some hidden logic produce different result in one case. But you can check it if build Blender from sources and looking in kernel_types.h file.

#5

Hi storm, thanks for weighing in.

This actually happens on two different builds from different people/sources, and happens with both CUDA and OpenCL. I guess I will have to make my own build at some point and try those compile options.

For now I think I will just close my meshes properly and try again :slight_smile:

#6

Hi. or upload the file so we can test it on our systems.

Cheers, mib,

#7

Here is the blend file. You will need bmesh:

openbox.blend (74.5 KB)

#8

@storm_st

would you mind telling me where those are? i’m building on os x cmake->xcode. Never came across this.

#9

Hi monsterdog, the render looks similar with cpu and gpu.
My system specs are in my signature, gpu render is with GTX 550Ti.
Strange but it is not your file.
I bright one part of the image to view the difference.
Blender rev. 45785 from today.
Cheers, mib.

Attachments

gpucpu.jpg1496×776 387 KB

#10

Check out 2.6x in installation folder; find the: 2.62\scripts\addons\cycles\kernel\kernel_types.h, or whatever the mac equivalent is.

#11

How odd.

I also see some of these render artifacts with CPU rendering, and you seem to have none at all in your render.

EDIT: Same result with 45790.

Maybe my nvidia driver…

#12

Oh look, I’m an idiot :slight_smile:

This only happens in the viewport render, not final render, duh, sorry :frowning: Also closing the mesh did not help.

I guess storm_st is right, the basic shading is probably done in the viewport.

I’m still a little perturbed by the difference between CPU and GPU.

#13

thanks, found it.