Karma Refraction Black Faces on thin Material

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Hey,

i'm struggeling getting a proper "thin" glass material working with Karma.
I tried to extrude a simple grid

and added a MaterialX Shader.



Ive added a simple Dome Light and enabled render Light Geometry for the test.

When rendering with Karma, it feels like there's kind of self shaddowing happening inside of the glass, which leads to the black sides.

When rendering the exact same setup with vray, it does the proper side refraction too:



Additionally when i increase the thickness of the extrusion to around 1, karma does proper refractions on the sides of my geo too.
Did i miss a specific setting or is this a bug in the renderer?

Attachments:
surfaceExtrude.png (156.3 KB)
mtlxSpec.png (164.9 KB)
mtlxTrans.png (171.9 KB)
karmaRenderin.png (2.0 MB)
vrayRenderin.png (3.6 MB)

Jonas Sorgenfrei
FX/PIPELINE TD | RISE Visual Effects Studios
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It looks like it's hitting the reflect limit. The sides look dark because the light bounces around inside a lot exiting when viewed at certain angles. Increase your reflect limit until the ray exits. You could also model the pane the way glass manufactures do to trap the light inside--round the edges and increase the roughness. Real glass is also not a perfect transmitter, which would make the glass appear darker on the edge anyway.

Attachments:
glass.hip (620.3 KB)

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Hey @jsmack,

thank you so much for your responds. Yeah fair point, cranking the values up helped.
I was just testing up to refract 10, reflect 10 cause I wasn't considering that the light would bounce that much.

That's a good point, and makes totattly sense now.
Think in a production scene i would probably overwrite the limits just on the object with a geometry settings node then.

Thanks for the help!

Cheers
Jonas
Edited by JonasSorgenfrei - March 26, 2023 20:41:46
Jonas Sorgenfrei
FX/PIPELINE TD | RISE Visual Effects Studios
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