Light doesn't seem to pass well through smooth-shaded surfaces?

I’m making a glass vase in Blender 2.74 and I noticed that there is very little transparency when I set the vase to smooth shading:


As opposed to flat shading, which yields this pleasant result:


I’d think it would be the other way around! :no: Does anyone know why this occurs? Or any ways to make the smooth version less opaque, if possible? I’ve tried several types of nodes setups already - simple Glass BSDF and a Refraction-Fresnel-Glossy setup. The vase reacts the same either way.

Thanks!

can you post a blend file with the vase? I think that there might be a problem with your geometry, not the shading…

Probably an issue with the engine trying to interpolate normals across a 90 degree angle.

Add an edge-split modifier to the glass and render again.

This is the blend file: http://www.pasteall.org/blend/36322

Here’s how it looks with an edge-split modifier (and smooth shading):



It’s perfect! I’ll consider this solved. Does edge-split have any drawbacks, perhaps mesh that is harder to work with or animate? I’m not really aware of what it does or what long-term effects it has.