2sided and vray fur problem

[windowlicker]

I've previously used vray fur and 2sided material to create quick and easy grass. After installing the latest vray (3.20.01), I noticed a very strange behaviour. When using 2sided, the vray fur object renders transparent, and very bright.
I have attached an image, where the left part is rendered using a normal vray shader and the right side is rendered with that same shader, wrapped in a 2sided material with the translucency set to 0. They should render the same (afaik) but they cleary do not.

It makes hardly any difference if you set the translucency higher, uncheck "force single-sided.." or check "multiply by front.."

It also takes forever to render...

(I haven't used this method in a while, so it could be that it was the same on other vray 3.x versions.)

Max file: vrayfurgrass3.20.01.zip

Render:

[vlado]

It would be best to use the VRayHairMtl material instead.

Best regards,
Vlado

[windowlicker]

Hm. Too bad. A rather significant change to the renderer... What is the reason behind this? Was it mentioned in the change notes? Any chance you'll bring it back the way it was?

HairMtl seems to work ok. Only problem now is that I think it's very hard to control reflection. The reflection pass seems to be tinted by the transmission color.
I guess it might just be a matter of tweaking though.

[vlado]

What is the reason behind this?

The hair is a "volumetric" primitive, meaning that it can receive light from any direction. VRay2SidedMtl expects the hit point to be a surface that can receive light only from one side normally, so the 2sided material must manually compute the lighting on the other side. The two concepts fight each other and I couldn't figure out a clean way to make both scenarios work.

Best regards,
Vlado