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sharktacos
07-29-2011, 11:33 PM
I'm running into what likes a bug in mental ray, and am wondering if there are any know workarounds. Here is a simple example of the problem:

I have a sphere and two planes -- acting as a floor and a wall. The wall is blocking the light from hitting the sphere.

http://forums.cgsociety.org/attachment.php?attachmentid=163203&stc=1

The scene is lit using linear workflow: exr 16bit-half, with the color management for the render view display set to linear (profile) and sRGB (display). This is the resulting render:

http://forums.cgsociety.org/attachment.php?attachmentid=163202&stc=1

Notice that the sphere is lit by the reflections when it should be black since it is completely in shadow. Here is a close up:

http://forums.cgsociety.org/attachment.php?attachmentid=163204&stc=1

I tried this with both a (Maya native) Blinn material, and a (mental ray native) Mia material . Each had the exact problem. For the Mia material the error occurs regardless of whether the reflections are using BRDF or fresnel. The error appears to come from the linear workflow gamma correction (which I am applying in the render view). If I instead use the mia_exposure, I get the exact same problem. In fact, I get it if I render in Maya Software as well (not surprising).

In contrast, if I render this same scene in vray, it renders correctly (the sphere is unlit).
So it looks like mental ray is incorrectly calculating the reflection for linear workflow, and acting as if an object is not in shadow, giving it full reflections from the light. Does anyone know a fix for this bug (besides not using mental ray, that is)?

sharktacos
07-30-2011, 12:18 AM
Hey I found a solution!

Set the ray depth limit on the ray traced shadows of the light to 2 (default is 1).

Ask and ye shall receive :)

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07-30-2011, 12:18 AM
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