Glass errors

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mcm
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Glass errors

Post by mcm » Sun Nov 22, 2009 7:45 am

Here's a sample of some odd artifacting that I've encountered trying to create glass bottle. The raw cube inside the bottle is just a backdrop to make the artifacting more visible.

I vaguely recalled that someone else had this same issue before, but I wasn't able to find how they solved it when I searched through the forums. The bottle was modeled in Silo and the only workable import into Blender 2.48a was a .dxf (.obj & .3ds didn't work). Using Blendigo/Indigo 1.1.18. Texture is just a standard glass-like Lasf9.nk that I've applied as a Specular with Transparent set, and very low gain of .05 and IOR of 1.54. I've checked normals, smoothed, unsmoothed and made solid, auto-smoothed, even edge-split and sub-surfed, and nothing seems to really help. The bottle is modeled as real bottle with a small, somewhat variable thickness throughout. It almost looks like there are some oddly-textured or doubled-up polys on the inside surface of the bottle. But I'm having a hard time determining for sure.

Could someone please jog my memory as to how to solve this? Thanks very much.
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Stur
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Re: Glass errors

Post by Stur » Sun Nov 22, 2009 8:03 am

It looks like an overlapping faces issue. How thick is your glass ?

mcm
Posts: 26
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Re: Glass errors

Post by mcm » Sun Nov 22, 2009 8:16 am

It is very, very thin (I'm not sure how to quantify it, otherwise). Before I exported the refined control mesh out of Silo, I double-checked to make sure that thickness was consistent enough so that the inside polys didn't go outside and vice-versa.

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PureSpider
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Re: Glass errors

Post by PureSpider » Sun Nov 22, 2009 9:47 am

Be sure your mesh thickness is above the ray nudge disatance at every point!

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CTZn
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Re: Glass errors

Post by CTZn » Sun Nov 22, 2009 1:23 pm

As PureSpider said, 0.2 mm is the strict minimal distance between one glass surface and any other type of surface, unless if you are explicitly using the precedence system (for materials using mediums).

But this kind of artifact may also have existed in older Indigo versions (not sure about 1.1.18) with one or the other scene tree building method (there are two), try forcing the other available one by changing bih_tri_threshold (as in the Indigo Technical pdf, page 16):

Given the final polycount for one mesh, a value below that count will enforce one method for the mesh, and vice-versa.

Dunno how it's called in blendigo 1.1.18... in blendigo at all in fact :oops:
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mcm
Posts: 26
Joined: Sun Apr 08, 2007 12:21 pm

Re: Glass errors

Post by mcm » Sun Nov 22, 2009 2:23 pm

Ah, thank you for the tip! :D Before I started redoing the mesh and completely changing the glass thickness, I did a quick experiment with the exponent for the nudge. That did the trick!
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pixie
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Re: Glass errors

Post by pixie » Mon Nov 23, 2009 5:15 am

CTZn wrote:As PureSpider said, 0.2 mm is the strict minimal distance between one glass surface and any other type of surface, unless if you are explicitly using the precedence system (for materials using mediums).

But this kind of artifact may also have existed in older Indigo versions (not sure about 1.1.18) with one or the other scene tree building method (there are two), try forcing the other available one by changing bih_tri_threshold (as in the Indigo Technical pdf, page 16):

Given the final polycount for one mesh, a value below that count will enforce one method for the mesh, and vice-versa.

Dunno how it's called in blendigo 1.1.18... in blendigo at all in fact :oops:
This img the glass has no contact wih any other material yet the fireflies, oh the fireflies... :(
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CTZn
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Re: Glass errors

Post by CTZn » Mon Nov 23, 2009 7:29 am

Hey pixie, you are illustrating one of the weak points of bidir as a technique, wich is -approximatively- when rays have a path similar to:

Code: Select all

light > diffuse > specular > camera
Not "fireflies", but slow convergence in this case.

That's why Indigo comes with a panel of techniques that you can combine to suit your needs :)
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Stur
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Re: Glass errors

Post by Stur » Mon Nov 23, 2009 10:48 pm

Why such a thin glass mcm ? It's unrealistic, don't you think ?
You should let the ray nudge distance at its default value and make your glass a little bigger, in my humble opinion.

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