Backface cull in render?

[Jussing]

Hey all, I'm modelling these rooms and in the viewport I have backface culling so I can see what's going on inside them.

I'd love to render them the same way, and see the light come in from the windows and bounce around with the same culling as in the viewport, but the walls become solid.

Any ideas?

I've found some old threads on the same topic, and it seems the best options involved som pretty render heavy refraction on a 2-sided material, but I'm looking for something I can work with in the IPR.

(PS, it's not for a job, I'm literally just trying to arrange our new house )

[anthonyh]

Can you just create seperate walls/ceiling and just make the ones you want `not visible to camera` in object properties ? Works in Vray 3, not sure about newer versions.

[ThomasMiller]

Take a look at VRayClipper. That's what I'd use.

[Joelaff]

I think if you turn off double sided in VRayMtl options you can get a backface cull. Been a while since I tried, though.

[^Lele^]

here you go.

[fixeighted]

I wasn't sure what Lele's version was doing with this, so I thought I'd see what's what.
It is as simple as putting a vray colour black into the the opacity of a 2 sided mat, if that helps.

[^Lele^]

Don't do that unless it's unavoidable, it's *many* times slower to render than clipping, and you do not get the options the clipper has (affect lighting, cam rays only, matID and so on.).

[fixeighted]

Oh...honestly I looked at your scene but couldn't see what it was doing. Does it do exactly the same thing as what backface cull does?
I thought the clipper did only that, clipped geometry...I'll have another look

EDIT: I do now see that it does work...it's just that I looked at the problem from a different angle. I was trying to literally recreate the culling effect, so objects could be rotated and it would still work....I had in my mind rotating cards with letters on e.g. So horses for courses I guess. Good exercise to wake me up at least !

[^Lele^]

Attached: with "affect lights" active, the geometry is essentially removed from the scene, and the lights are aware of it.
As such, in what was a closed box, the dome from the outside lights the interior, while the red cast from the clipped walls does not show up.
With "affect lights" off, the geo is clipped for viewing only, the outside dome doesn't get through, and the red cast from the clipped walls is visible.
These take 20 secs to render, whereas making the double sided thing shot times to over a minute, and without any of those mentioned options.

[fixeighted]

Yeah it wasn't clear when I first looked...didn't notice that there were 2 pieces of geometry so that threw me lol....couldn't work out how the clipper would know what was a backface
Agreed it is quicker but lacks what I mentioned above - though a different exercise of course.

[Joelaff]

I like the solution, Lele!

It seems in reference to my post above that disabling double-sided in VRayMtl only affects lighting, and does not hide geometry (must be confusing it will LightWave materials, where it would hide the polygons). The same applies for the two-sided setting in a Standard material as well.

Oh, wait! Along Lele's idea of the VRay2SidedMtl I found that you can use a VRay2SidedMtl with Back Material turned on, but not connected and a Translucency of black (zero), and it renders how you want, but without the speed penalty! You can also use a standard Max Double Sided matrial with the back faces unconnected and it also is fast. (Did not time the two in comparison, but they both seemed about the same.)

Note that this does NOT allow light into the box through faces which are facing away form the camera (and thus invisible). For that you could use light exclusions are duplicate objects, etc.

Not sure if this is what you are after or not. I think this is what fixeighted was saying above too.

[fixeighted]

Hehe nice find, using only black trans and no need for a map
This is the way I'd use it due to the simplicity - based on what I imagined was the OP's requirement. With his room, he has windows, so light
coming in is not an issue.
Even if it was your average 'dungeon visualisation'....just stick a nice interrogation lamp in there

[Jussing]

Thanks guys, and that sounds like a great solution, Lele! Indeed, it would suit my needs better than the clipper, because I'd need to rotate the room and have all walls be transparent from the viewing side only.

Sorry for not checking by sooner, I know it's rude to ask a question and then split, but honestly I sometimes forget to go back to these forums because they see so little action.

Thanks again,
/Jonas

[Joelaff]

Thanks guys, and that sounds like a great solution, Lele! Indeed, it would suit my needs better than the clipper, because I'd need to rotate the room and have all walls be transparent from the viewing side only.

Sorry for not checking by sooner, I know it's rude to ask a question and then split, but honestly I sometimes forget to go back to these forums because they see so little action.

Where are you finding more action? These forums seem pretty busy, no?