something to do with layered textures, but I want to see someone elses answer. I've had a terrible time getting mental ray layered textures to work, so hopefully someone else has some hints. In the meantime, if you want to play around with JS_Layers (that's a custom MR shader for layering mental ray textures, kinda the equivalent of the Layered Shaders node in maya's native shader list) or some variant, be my guest and good luck!
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Peter Srinivasan
Producer