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Karma Refraction Black Faces on thin Material

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JonasSorgenfrei
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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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jsmack
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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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JonasSorgenfrei
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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 - 2023年3月26日 20:41:46
Jonas Sorgenfrei
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mtf
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I'm not sure if this question makes sense, but: is there a way to detect "trapped" rays and "make" them escape? IE, if a ray hits the reflect limit to continue it to the environment light, sample that color, and return?
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jsmack
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mtf
I'm not sure if this question makes sense, but: is there a way to detect "trapped" rays and "make" them escape? IE, if a ray hits the reflect limit to continue it to the environment light, sample that color, and return?

Mantra has this feature but it has not be implemented in Karma.
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ScottKeating
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I believe enabling 'thin walled' under the 'Geometry' tab of the MTLX standard surface or on the Quick Surface LOP would be the fix for 'thin' refractive objects. Not sure if that is what you're looking for though.
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jsmack
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ScottKeating
I believe enabling 'thin walled' under the 'Geometry' tab of the MTLX standard surface or on the Quick Surface LOP would be the fix for 'thin' refractive objects. Not sure if that is what you're looking for though.

I think the question is more about physically thin materials, which would be 'thick' materials as far as raytracing is concerned.

Mantra has the render property vm_raylimiteval, which allows for either trapping rays at the ray limit or leaking them to the lighting: "black" or "direct lighting" for the background. I couldn't find the equivalent property for Karma.
Edited by jsmack - 2024年7月15日 14:12:59
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gunjack
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Since my Arnold subscription has just expired I was about to ditch Arnold for Karma. I did some tests and Karma really looks impressive. However I hit the wall already on the first project.

1. I am getting black edges even on most basic refractive object regardless of the amount of refraction limit setting. There really should be an option for rays to "exit into background" when limit is reached otherwise the glass looks odd. Thin walled option is not a solution to this issue.
2. Transmission scatter should be implemented as well, it's very important parameter for many transmissive objects.
3. Karma needs proper shadow matte material. Background plate has some limitations.
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