Very strange flickering of bruteforce gi on vraymesh/vrayproxy objects

[re:FORM]

We have a scene that is showing very strange behaviour.

The setup :
GI set to BF/BF.
Multiple instances of vraymesh trees.
A single vraymesh light.

The flickering(not grain) is on a per-tree basis and does not affect other scene objects( buildings, ground, etc).

At first I was convinced it might have been LC being re-enabled on the farm for some reason as the way it flickered was very light-cache-ey. BUT, I remembered that I had a similar problem in a previous job with forest-pack-pro with scattered VRay proxies in 3dsmax... as memory serves, I ended up rebuilding the forest-pack objects and it seemed to fix itself.

The only explanation I can come up with is that the GI is being sampled at the root node of the proxy object and then being applied across the entire geo of the proxy, thus turning the normal grain of BG into an entire object flickering brighter and darker from frame to frame.

Has anyone else had this issue, and if so, how do you fix it?

Unfortunately I can't share the scene.

Thanks
p.

[super gnu]

i know its unlikely, but is it possible forestpack is placing duplicate trees on top of each other? double faces can flicker like that. do you have collisions enabled in forestpack? definitely nothing funky in your proxies?

[vlado]

The only explanation I can come up with is that the GI is being sampled at the root node of the proxy object and then being applied across the entire geo of the proxy, thus turning the normal grain of BG into an entire object flickering brighter and darker from frame to frame.

This is not technically possible, as there is no code in V-Ray that can do that. So it must be something else, but without a scene, I don't know what it is.

Best regards,
Vlado

[re:FORM]

Thanks Vlado, good to know... I had a feeling my theory was a bit of pub-science!

I think we solved it in the max scene(forest pack) by upping the secondary ray bias... we are going to test that in this scene (Maya scene with simple instances of vray proxies; no forest pack) too so will get back to you tomorrow with our results.

Vlado, I assume by your response that you haven't ever come across a scene with BF GI that had objects flicker (ie each instance flickering brighter or darker) rather than normal BF style grain?

Super Gnu, I've not been working on the scene so I've not personally checked for coplanar surfaces but now you mention it I should double check!

Thanks
p.

[super gnu]

if upping the secondary ray bias fixes it, id say coplanar faces are strongly suggested. either in your proxy, or caused by duplicate objects in forestpack.

only other source of flickering like that i can think of would be interpolated glossies or sss2, but a) youd notice a prepass b) secondary ray bias unlikely to affect it

[re:FORM]

Super Super Gnu.... you were bang on the money....
There were three copies of the geo in each vrayProxy....

[re:FORM]

Ok, that's even more strange.

The original tree mesh is clean with no duplicate faces. But when we convert it to vrayproxy and then back to a mesh, the duplicate meshes occur for some reason.

Vlado, can you think of anything we might be doing that would cause multple copies of the mesh to be created during the proxy / deproxy process?

[re:FORM]

We've isolated the problem to it being when we create the proxies by commandline rather than manually using the gui.
I don't think we're using any unusual arguments.
Any ideas what we might be doing wrong?

[re:FORM]

We think this might be a bug in the vrayProxy exporter in Maya.
If you select the transform and shape nodes of an object, you will end up with two copies of the geo in the proxy.
Can someone verify this?

[yolov]

Yes, you're right. That's a bug. Selecting the transform and shape duplicates the object within the proxy file, created both with the GUI or with a script command.
I'll make a note in our bug-tracker for this.

Thanks.