Does anybody know if there is a way to make an object invisible to the camera but still contribute to the global illumination solution etc in VfR?
I have an interior model that I want to render from above the ceiling, so I want to have the ceiling add to the light bounces but I don’t want it obscuring the camera view. Easy to do in Max… but can’t seem to find any options in VfR.
Rob.
make the diffuse layer transparent (white), add a refraction layer, IOR=1.0, turn off affect shadows
edit: this should work for light from point lights etc but light from the GI skylight will pass right through it.
edit: nevermind, I’m an idiot. this used to work in 1.05 but seems not to work now.
Thanks for trying Emil!
I’ve got a feeling I’ll have to do without the bounced light from the ceiling and will have to simulate the skylight from the windows using area lights, while keeping the environment as black.
Rob.
oooo.. just found a tick box in the materials options tab that says ‘Only in Secondary’ - will give this a try.
Yep, that’s done it!
Just had to make sure the normals of the ceiling were pointing down and double sided was un-checked. Seems to work a treat.
Rob.
Great! Anything else you had to do to make it invisible? Just transparent in the diffuse layer? Or is that even not necessary with this option checked?
This option doe’s the job for invisible walls. 
Excellent, this is just what I needed.
On further investigation you don’t have to un-check double sided or check which way your normals are pointing. Just click the ‘only in secondary’ box and that’s it. No transparency needed what-so-ever.
You don’t need to switch anything other, except you need it.
this is the best thing since sliced bread. thanks, Rob!
Am I correct that only the “old” materials can have this option?
If I create a material by Scene Material->V-Ray Material, I don’t see how to enable this setting.
You Can use SKP 2 sided material and let the back material empty. The back face of the surface has to be pointing to the camera.
Great, thanks, that will help!