Is it a glitch or a feature?

[Endy73]

Hi guys. I'd experimented with glass shader in cycles and found it behaves quite strange with low poly smooth shaded models. If theese models have concave areas, like inner part of a glass, cycles render it black instead of transparent, no matter how high i trying to set transmission bounses (refraction caustics in on, of course). When i increase number of polygons of the models (with Subdivision Modifier for example) or turn on flat shading it renders just fine. Is it a feature of cycles or a glith?

[Scre]

sounds like a normal problem, u need to flip the black ones or recalculate all (select all in edit and ctrl+n)

also did u use solidify mod on the glass ? u need some thickness

aswell environment might be a problem, glass will reflect stuff back if got nothing to reflect it reflects black

[Endy73]

Originally posted by Scre:

sounds like a normal problem, u need to flip the black ones or recalculate all (select all in edit and ctrl+n)

also did u use solidify mod on the glass ? u need some thickness

aswell environment might be a problem, glass will reflect stuff back if got nothing to reflect it reflects black

Thanks for the answer.

i've localized the problem. It is as it is. One can replicate it:

1) Create a cylinder with 32 verts.
2) Remove top poligon
3) Solidify the cyl
4) Apply glass bsdf shader and render. You'll see a clear glass.
5) Apply smooth shading and render, You'll se black
6) Aplly Subdiv Surface modifier with a few interations and support loops and render. You see a clear glass again )

I wonder is it a glith or just a cycles particular trait?

[noodlebox]

The problem is that smooth shading interpolates normals across each face, and you have some extreme angles at the top and bottom edges of your cylinder. Subdivision surface helps because the faces are smaller, so the extreme angles at the top and bottom affect a smaller area. This adds more geometry than you need though.

A better way to fix it is to add a couple "holding edges" near the top and bottom (You should be able to find some good tutorials about this). Extra edge loops around sharp corners will give you more accurate smooth shading.

Or, if you want a perfectly sharp edge without any extra geometry, mark the top and bottom edges of the cylinder as Sharp, then add an Edge Split modifier. This separates the faces, preventing the smoothing from crossing this boundary.

[Endy73]

Originally posted by noodlebox:

The problem is that smooth shading interpolates normals across each face, and you have some extreme angles at the top and bottom edges of your cylinder. Subdivision surface helps because the faces are smaller, so the extreme angles at the top and bottom affect a smaller area. This adds more geometry than you need though.

A better way to fix it is to add a couple "holding edges" near the top and bottom (You should be able to find some good tutorials about this). Extra edge loops around sharp corners will give you more accurate smooth shading.

Or, if you want a perfectly sharp edge without any extra geometry, mark the top and bottom edges of the cylinder as Sharp, then add an Edge Split modifier. This separates the faces, preventing the smoothing from crossing this boundary.

Sounds reasonable. Thanks for the answer)