Uber Add Emission's "Two Sided Light" parameter broken ?

emu42

Hello,

I have been trying to create an iRay shader that emits light on both surface and backface using the Uber Add Emission brick in Shader Mixer. I thought it was simply a matter of switching the Two Sided Light input to true, but that seems to have no effect at all. I also attempted to use the Copy Surface To Backface brick, but that does not work either.

So, is the Two Sided Light parameter broken?

Is the Copy Surface To Backface brick supposed to copy emission properties as well?

Thanks in advance!

fastbike1

I don’t belive you need to use Shader Mixer. Select the surface in the surfaces tab, add the uber shader emission shader and select two sided in the light tab

 

emu42

fastbike1 said:

I don’t belive you need to use Shader Mixer. Select the surface in the surfaces tab, add the uber shader emission shader and select two sided in the light tab

 

I created my own emitting shader from scratch there for other reasons. Just wanted to make it emit on both sides. But I suppose I could look at how it's done in the uber shader and try to figure out how to apply this to my own.

EDIT. It's the Thin Walled parameter. That must be enabled, otherwise the backside does not emit light even with Two-Sided light set to true.

JonnyRay

emu42 said:

fastbike1 said:

I don’t belive you need to use Shader Mixer. Select the surface in the surfaces tab, add the uber shader emission shader and select two sided in the light tab

 

I created my own emitting shader from scratch there for other reasons. Just wanted to make it emit on both sides. But I suppose I could look at how it's done in the uber shader and try to figure out how to apply this to my own.

EDIT. It's the Thin Walled parameter. That must be enabled, otherwise the backside does not emit light even with Two-Sided light set to true.

If you care about the technical reason that this is true...

With Thin Walled turned OFF, Iray is expecting that you're providing a full geometry. So it thinks there should be another face that is part of the object which represents the "back" of the object. If all you have is a single plane, then it feels like it's missing the rest of the geometry and doesn't know which faces should be emitting light in the other direction.

With Thin Walled turned ON, Iray is basically treated the flat object as if it were actually a volume of infintesimallly small depth; so it can treat the back set of faces as if they were the back of the object.