Glass and refraction insights.

[iarvi]

I've searched and read the forum several times now (3dcenter.ru thread anyone?) trying to figure out some things about this, and I have a few questions I hope somebody can answer and help me understand better the way glass and refractive materials work:

1. What's the difference between thin and solid option for materials? OK, thin is for one sided/single polygon surface and to get sun through windows and such, but why doesn't solid allow the sun to go through? isn't this physically incorrect? The material editor displays solid mode correctly, with light going through it. Using single polygons to represent glass isn't accurate also, so there must be something that is not physically correct in the way refractive materials get calculated, at least enclosed ones using solid mode. Am I missing some parameter or render setting?

2. Twosided mode doesn't use refraction override?

3. Maybe its a a bug (and already known): In twosided(THIN) mode: If refraction color is white 255, then using reflection level or color to dim reflection works ok. As I lower refraction color or level, there seems to be a point where you can't dim reflections anymore: say set refraction color to gray 200 or level to 0.8, and reflection level stays almost the same. Solid mode seems to work ok.

in attachments: left side is THIN mode, right is SOLID mode. don't mind the ugly model.
Didn't want to put this in bug reporting as I didn't know if it is a bug.

Thanks everyone.

[Ondra]

ad 1) Solid is darker sometimes, because there is bias added to prevent any fireflies by default. You can set Max sample Intensity to 0, eliminating the bias, but at cost of fireflies (which will eventually converge to the correct solution). Since there is no efficient way to render sun behind solid glass with path tracing, you would have to use for example bidir to get correct and fast results.

ad 2) no, currently not, it is treated as opacity

ad 3) this is probably a bug that was already fixed in daily builds and will be fixed in A5 release

[iarvi]

Thanks for the info Keymaster.
So twosided mode won't give correct refraction and onesided won't give correct shadows unless BiDir is used?
Also... absorption doesn't work with BiDir or twosided mode? I'm using tinted refraction in twosided instead of absorption.

[Ondra]

Thanks for the info Keymaster.
So twosided mode won't give correct refraction and onesided won't give correct shadows unless BiDir is used?
Also... absorption doesn't work with BiDir or twosided mode? I'm using tinted refraction in twosided instead of absorption.

Since the thickness of twosided glass is zero, the attenuation is always zero.