Intersecting geometry with primary visibility off in alpha channel

[CerpinTaxt]

Hi,

I'm having an issue with intersecting geometry that shows up in the alpha channel although its primary visibility is turned off.
As you can see in the viewport screenshot there are two cubelike shapes flying over a ground plane.These two cubes have overlapping faces where they touch each other. For some reason this area shows up in the alpha channel.
I noticed that in my occlusion rendering but it also shows up if you throw a simple lambert on everything. It disappears as soon as you move the cubes slightly apart.
I attached a simple test scene. Any ideas what is going on?

I'm using Vray build 13582 from Dec 2 2010 on Windows 7 x64.

vray_primVis_prob.zip

Thanks in advance,
jan

[Zdravko]

This can be solved by using the Force back face culling option in the Global options roll-out.

[CerpinTaxt]

Whoa, that was right in time. Thanks a lot!

[Evox_Productions]

Is there a threshold setting for this or something? Im having the same problem and turning on "force backface culling" gets rid of most of my artifacts but not all.

[Zdravko]

If the Force Back Face culling option doesn't help, try to set Secondary ray bias to 0.001.
Usually we advise our customers to avoid situations with overlapping faces.