Mapping the transparency channel of VRayMtl screws up double sided rendering

[vazap]

Hi all,

I keep bumping into this very annoying problem. I want a surface object (without thickness) to render double sided, using a VRayMtl shader. It works fine until I map transparency (either refraction color or amount yield the same result). As soon as transparency is mapped, the object renders single sided only... Of course I've checked my object's normals, the shape's rendering attribute being set to double sided, and the VRayMtl double sided option being turned on as well. I've also played with IOR without success.

An image speaks a thousand words, so here's a screen grab with a lambert shader, color and transparency mapped. This produces the result I would expect: double sided.



Here's the same object with a VRayMtl, mapped with the same color and transparency maps. As you can see it renders single sided. (This is my first post, I hope the images are attached correctly...)



Is this a known bug? Is there any workaround for this? I tried using a VRayMtl2Sided shader with the same VRayMtl shader attached to both inputs, no luck, transparency is then ignored...

Help!

Vania

[vlado]

This works fine for me on a simple test. Can you post a scene where it doesn't?

Best regards,
Vlado

[vazap]

That's very strange.

I'm trying to upload 2 simple .mb files to you but your uploader won't let me... I also tried a .rar file, same thing... How can I send you files other than jpg's?

[vlado]

.zip files will work (.mb and .rar won't); you can also email me at vlado@chaosgroup.com

Best regards,
Vlado

[vazap]

Ok here are 2 scenes. One shows the issue with a ramp mapped to the refraction color (VRay_transpNormalBug.mb). I later noticed that even if this not mapped, as long as it is non zero the object will render single sided as well (VRay_transpNormalBug_noMap.mb).

scenes.zip

I also join the renders:

[vlado]

Ah, I thought you used the opacity slot of the material, and not refractions. Refractions will indeed turn off the double-sidedness of the material, for various reasons. Depending on what exactly you need, you can either use the opacity slot, or else if you really need refractions, you can use a vrayblendmtl material where the base layer is refractive, and the first coat is a diffuse material with mapped opacity.

Best regards,
Vlado

[vazap]

I did mean to map the refraction color... I'll try and do with mapped opacity only for now, but eventually I will need refraction, so I'll give it a try with your tip.

Thanks,

Vania