double sided material and interior lighting

[renevc]

In the previous version of vray i was able to make a 2-sided material or skp 2sided material with one side white and one side transparant. I used this material for the ceiling of an interior so from the top view you do not see the ceiling but it does give a shadow in the interior scene. With rhino 5 and the latest vray i cannot make this work. The material is double sides the way i want it but it does not cast a shadow. I use a sun as a light source and tried both double side skp and normal double sided.
Any suggestions about what i am doing wrong?

Regards, René

[Micha_cg]

The problem is, that there is a bug at 1.5 and 2sided material doesn't work like before. Joe was looking at it, but doesn't found a quick solution last. I'm not sure what the last state is. From my side I must say too, it's a hard problem for scene setups. I hope it can be fixed.

[renevc]

Micha, thanks a lot. I have been trying all possible settings, flipping normals and what not. Still a pain in the ass that it does not work but i guess i have to do without it for now.

[Micha_cg]

Maybe a trick helps in your case - you could use a black, invisible rectangular light to block the light from outside.

[renevc]

Alas, the light does not cast a shadow. At least not in my scene. Made it invisible and 2 sided but still no shadow.

[Micha_cg]

I tested it here - right, it block GI light only. So, maybe you can setup the scene so that an extra surface placed above the scene block the sun light and the invisible black light block the GI.

[fpedrogo]

Hi Rene and Micha
Maybe I'm not understanding what Rene want, but the material to hide the ceiling is not the Two sided material, it is SKp Two Sided. I made a test and seem that is working fine for me.
Again, maybe I'm not understanding the effect that you want to get.
Best

[Micha_cg]

In the past I used a material at the backside of a surface as background behind a train window (motion blur effect) and the lighting from outside comes in. Now at 1.5 it's blocked. If I would change back and front material and flip the normal, than material isn't visible from inside the room.

[joe_b]

Hi Micha,

The Skp2Sided material behavior you are talking about was a fluke related to the implementation of that material. That was why I ended up just giving you a recompiled version of the old plugin so you could use it for your project. There is not a way for me to implement this behavior without putting that bug back in, and that bug messed up reflections/refractions, and other things.

Here is why the Skp2Sided used to work the way you want it: in certain cases I accidentally picked the incorrect side to render. In your case, it was accidentally picking the transparent side regardless of whether I was shading the front or back. This had the side-effect of allowing camera rays in, and light rays to be traced back to the camera. However, now that this bug is fixed, it will always pick the correct side: meaning the outside will be shaded as transparent but from the inside it returns an opaque material. This has the effect of letting camera rays in, but does not allow light rays to get out ( because it is opaque on the interior side now, which is correct ). That is why this trick no longer works - if light can't get back to the camera it is dark.

You can get pretty much the same affect by just having a fully transparent double-sided material - because that is essentially what was going on in the old Skp2Sided material.

I hope that helps,
Joe

[violine]

How do you block for example sunlight casting unwanted shadows on interior scene? Is it possible with fully transparent double-sided material?

[Micha_cg]

@Violine: typical it should work if the surface normal (set per _dir command) shows in the room and you set a wall material to the front side. But I suppose so it's working in the current nightly build only, because "accidentally picking the transparent side". So, you need to wait for the next update, if I understand right.

@Joe: What do you mean with "fully transparent double-sided material". I need to see the background plane from inside the room, but need all lights goe's through the plane. How do I setup a "fully transparent double-sided material"? Is it visible from one side? It's no problem for me to setup a fresh material for my project files.

[renevc]

Unfortunately i cannot see your posted image because i do not have permission to see them and i do not know why. I indeed ment the skp double sided which used to work in v ray 1 but not in vray 1.5. Which version did you use?

[renevc]

Micha
that is a good way around but you have to put a vertical surface in the scene otherwise it will block your cameraview from above. In topview you do not see this surface at all but in perspective views you have to post remove it. Good idea!

[fooprobe]

If i understand what you want, both of these can be done with a regular Two-sided material (not Skp2side).

To make a wall transparent from one side only, but cast a shadow as if it is opaque:
On one side place a standard diffuse matl - this is the opaque matl. On the other side place a standard matl with NO diffuse layer, only refraction layer, IOR = 1.0, affect shadows = off - the surface will appear transparent from this side.

To make a wall transparent from one side only, AND cast a shadow as if it is transparent:
Same as above, but in the OPAQUE matl turn off "cast shadows" in the options panel below the diffuse layer panel.

[Micha_cg]

I need a visible surface from inside, but this surface need to be transparent for GI lighting. Nothing works here, always the lighting is blocked. I very miss the option to get a visible surface that cast no shadow for any light source.