View Full Version : Glass Table (Tempered) w\ MIA

09 September 2007, 04:57 PM
I've been observing some real world pictures of glass tables and the one in my apt. and all of them seem to have a dark, opaque outer edge. Upon further observation it only seems to be opaque on the outside, because as you look through top of the table, that same edge appears to be way more transparent from behind. Does this have something to do with the way it's perhaps sanded down so that the corners aren't sharp? I don't know much about how glass tables are made, but I was wondering if someone could give me some insight as to what I am seeing. Basically, I can't seem to re-produce the effect when using many variations of the thick glass preset in the MIA material.

I was also wondering if it's absolutely necessary to use GI to reproduce accurate glass? Right now i'm using FG multi-bounce only to light my scene, so there are no photons and thus no caustics.


09 September 2007, 05:07 PM
The effect you mentioned is caused by the light ray reflection and refraction laws when they enter a more or less dense media. The light rays change their angle when entering from one media into another (refraction). But if to angle between the ray and the materials edge is too small there is no refraction. There is a total reflection instead. So the light doesnt enter (or leave) the media (best example is an aquarium).
However, the longer a light ray travels in a media like glass the more energy it loses. The glass becomes darker (especially low grad glass like your table).
If we go further: a light ray might have a small angle most of the time so it is reflected many times in your glass table before it finds a surface to leave the table. So the border looks really dark.
So everything also depends on the viewing angle. Your eye rays may enter the media fast and leave it fast at one perspective, but on another perspective your eye rays travel a long time within the glass, thus they are dark.

Please forgive me, my english is not the best, but I hope it helps you a bit.

09 September 2007, 10:15 PM
Use the refraction falloff (distance and color) of the mia shader to get the effect you're talking about. I don't believe one needs GI or FG to produce photo-realistic results at all. It depends on the scene but glass has almost no diffuse property only reflection and refraction so GI wouldn't affect it much directly. It may however affect the accuracy of the reflections of environmental diffuse objects seen in the glass. Good refraction and fresnel reflectivity are key.

If you want caustics well then yes you need photons - but you don't need GI or FG.

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09 September 2007, 10:15 PM
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