OTOY Forums

I would say that depends on what you want to achieve.

The example with the curtains:
Try to think in real world ways. What is a curtain? A weave, a mesh.
So you probably need a transparency map reflecting the mesh. Or perhaps not, depending on how accurate you want it to be.
Plus the thread it is made of would be semi- transparent too maybe, so set the whole material to maybe 80%
I have a free 3d curtain somewhere and used it that way.
Have renders going right now so can't test.

As for the spec mat, I indeed think they need a volume, so your idea with the deformed cube seems reasonable.

The simplest setup achieving this effect is using a diffuse material with transmission enabled. Remember to make the reflection color ('diffuse' input) darker, because only the part of the light which is not reflected can go through the material.

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Roeland