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Houdini Karma Render Transparent Material Issue

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sugnsu02
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I'm experiencing a recurring issue when rendering transparent materials with Houdini's Karma renderer. As shown in the first image, there are areas where refraction fails and instead appears black. Increasing the Reflection Limit and Refraction Limit doesn't seem to resolve the issue effectively.

The second image was rendered in Maya using Arnold, with both the Specular Limit and Transmission Limit set to 8.

Is there a way to achieve similarly clean and realistic transparent materials in Karma, like in Maya Arnold?
Edited by sugnsu02 - 2025年4月29日 15:44:35

Attachments:
houdini_karma_xpu_1.jpg (541.0 KB)
maya_arnold_1.jpg (294.1 KB)
test_1.hip (623.6 KB)
test_cam_1.abc (1.9 KB)
test_geo_v1.abc (473.9 KB)

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Heileif
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Try to also turn on Enable Internal Reflection on the glass mesh with a Render Geometry Settings node.
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sugnsu02
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Turning on Enable Internal Reflection is definitely helpful, but ultimately a high reflection limit is still necessary. This can sometimes be quite burdensome—can’t this be resolved with a lower limit, like in Arnold or V-Ray?
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dlee
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Hi. I might recommend a workaround in shader (eg use rayimport VOP to set IOR of glass to 1.0 after X number of bounces)
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sugnsu02
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wow!!! Thank you, dlee.
I tried what you suggested, and I got exactly the result I was looking for.
I set both the reflection and refraction limits to 8, and I was able to get a clean image with fast render times.
Thank you so much!

Attachments:
houdini_karma_xpu_2.jpg (510.4 KB)

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Tomasho
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dlee
Hi. I might recommend a workaround in shader (eg use rayimport VOP to set IOR of glass to 1.0 after X number of bounces)

Can you be more specific how to do this? please...
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peter_green
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sugnsu02
wow!!! Thank you, dlee.
I tried what you suggested, and I got exactly the result I was looking for.
I set both the reflection and refraction limits to 8, and I was able to get a clean image with fast render times.
Thank you so much!


Please could you please share how to do it? Like how to connect it?

Thanks
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eikonoklastes
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peter_green
Like how to connect it?
This should work:


That Mtlx If Greater node is a UX disaster, but it translates to this:
If the ray level (connected to 'value1') is greater than the specified value2 then use the Input 1 number, otherwise use the Input 2 number.

In my example it will default to an IOR value of 1.5 until the ray level goes above the threshold of 5 I have used, after which it will use a value of 1.
Edited by eikonoklastes - 2025年8月11日 06:54:31

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ray_level.png (244.4 KB)

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peter_green
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eikonoklastes
peter_green
Like how to connect it?
This should work:
Image Not Found


That Mtlx If Greater node is a UX disaster, but it translates to this:
If the ray level (connected to 'value1') is greater than the specified value2 then use the Input 1 number, otherwise use the Input 2 number.

In my example it will default to an IOR value of 1.5 until the ray level goes above the threshold of 5 I have used, after which it will use a value of 1.
Thank you very much, you are my savior! Will test tomorrow. Thanks !
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