question

Michael Wilde avatar image
Michael Wilde asked ·

GPU and CPU transmission / transparency discrepancy

Hi there,

I'm having a very annoying GPU rendering issue that I was wondering if anyone has any suggestions for? I'm doing some GPU renders and for some reason my iris (which is behind a transmissive cornea piece of geo) is coming out much darker than in CPU. I assumed it was a ray depth issue, but increasing the transmission and transparency depth seems to have no effect. The two look exactly the same when the glass shaded cornea ontop is removed which is what leads me to think it's something to do with GPU and transmission or transparency depth. Anyone ever had this or know what could be causing the difference? I've done loads of tests to see if it's a shader setting that is causing it as I'm using SSS but it is 100% to do with the glass shader infront of the eye as when I remove that the iris looks the same in both CPU and GPU.

I've checked every render setting, geo setting and shader setting I can think of with no luck. Unfortunately the GPU documentation for Arnold leaves much to be desired...

And no, it's nothing to do with the CPU render not being finished, I did plenty of tests where I let it run to completion that gave exactly the same discrepency.


error-01.jpg


UPDATE: I was asked if caustics were on, they were not and I've done some tests here that show that while it does help it's only putting a band-aid over the real underlying problem I've been unable to diagnose:


error-02.jpg

gpu
error-01.jpg (888.6 KiB)
error-02.jpg (338.9 KiB)
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Stephen Blair avatar image
Stephen Blair answered ·

Can we get a simplified scene with they eyeball setup?

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Lee Griggs avatar image
Lee Griggs answered ·

Is caustics enabled for the eye shader?

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While caustics helps, it's just putting a band-aid over the issue. As you can see here I changed the iris geo to a plain default aiStandard with green for colour to compare results. There is no difference when the glass outside is removed, and the same effect is had with or without the white around the edge. I'd be happy if I knew what the issue was and if it's a limitation of GPU then fine, but the lacking documentation makes it impossible to diagnose yet another issue with GPU rendering.

error-02.jpg

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