Mental Ray - Custom Color pass with refracted objects?

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  02 February 2013
Mental Ray - Custom Color pass with refracted objects?

I was watching a tutorial on vray where the guy was writing a custom color pass that had an object in a glass of water with ice cubes. He was writing each object to the R G or B channel, and then he could use those as alphas in Nuke. As expected, each object was in its respective channel, but they were also showing up in reflections/refractions. So lets say the glass/ice is the green channel, and then there is a toothpick in the glass that is assigned to the blue channel. Where the toothpick is refracted through the glass, you get yellow, because it adds the green and blue, and you can adjust each channel separately.

So far I haven't been able to achieve this in Mental Ray. Is it possible? I'm just getting a solid color for whatever object is in the foreground, but nothing showing up through refractions.
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  02 February 2013
Currently mental ray renders to a matte channel whatever is in the primary ray. It's actually more of a shader limitation than anything. mental ray can render objects (occluded or not) in their entirety to a framebuffer if you wanted. (For example, a car behind a building would still get rendered completely even though it's behind something.) But the framebuffer system for now doesn't support that or the mattes through secondary rays.

It's something we'd like to see in the layering library but isn't available just yet in the Beta.
My opinions are always my own...and maybe a friend's, but never my employer's.
  02 February 2013
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