Triangles showing up in overlapping transparent materials

[Evox_Productions]

I am rendering an extremely large number of car based animations. Almost 700. All of the animations are identical but each has a different car model as the subject. Only the camera is animated. In about 100 of the animations I am having a strange rendering issue. On some rendered frames of the animations, the windows of the car will have little black triangles scattered about them. The problem only occurs when the windows overlap, for example when you can see the windshield through the drivers side window. I am having trouble diagnosing the issue because the problem seems random. Sometimes a re-render of the frame eliminates the problem. Sometimes a re-render will just give a different distribution of triangles.

The model is being subdivided by the "render viewport subdivision". I have tried different combinations of the subdivision settings such as turning off viewport dependency and lowering the edge length. Ive tried turning off GI to see if the irradiance map was the issue. Also all of the car models have the same specifications. There is no difference between the models without the issue and the ones that have it, other than shape. Normals do not seem to be the problem either.

Attached is three sequential frames of an animation showing the problem. You will see how the issue is different in each frame. If anyone has seen this problem please let me know.

Thanks for any help.

[vlado]

Which V-Ray build is this? Have you tried a newer build?

Best regards,
Vlado

[Evox_Productions]

Our renderfarm is running the first version of 2.0. I do have a workstation with an updated build. I will test on that machine.

[vlado]

Ok, if you still get the issue, can you send me a scene with just the problem geometry to vlado@chaosgroup.com ?

Best regards,
Vlado

[Evox_Productions]

Thanks Vlado,

Since upgrading our nodes I am now getting this error. Warning: Performance Loss:dynamic memory limit too low. I was not seeing this error before on these scenes. Is there something I need to change in the render settings due to the update?

[vlado]

Just increase the dynamic memory limit option in the system tab of the V-Ray settings. The default is 500 MB I think but this is way too low.

So did this solve the triangle issues?

Best regards,
Vlado

[Evox_Productions]

Hi,

So far so good with the triangles. I did see one little triangle on a recent render. However the geometry on this particular car window model had an extrusion of less than 1 mm. The car models are built to scale so this was probably too thin for the subdivision? I gave the window a little more thickness and it went away.

On a side note. We have a script that will automatically swap maya shaders for vray shaders. This helps us quickly convert maya render files to vray shaded files from our backlog of car models. Since the stable version upgrade our script is giving us a few errors:

File "/xserve1/LIBRARY/cg_production/00_resources/production_scripts/shaderUtils.py", line 193, in px2vr
vraymat = cmds.ls(type='VRayMtl')
RuntimeError: Unknown object type: VRayMtl



TypeError: Object Car_Paint.flake_color is invalid
pure virtual method called
terminate called without an active exception


I know this is out of context and you probably need a lot more info, but if you see anything here that rings a bell please let me know. Are there no longer object types known as VRayMtl?

Thanks for the help.

[Evox_Productions]

Hi,

Our script problem is fixed. After the new vray install we did not throw out our preferences before working on files. Those files then gave an error when the script was run. After resaving the files with new maya preferences everything worked fine.

[vlado]

Ok then, good to know.

Best regards,
Vlado