Defining group memberships allows RenderMan to associate different objects and lights for special effects. For example, you can group a light with objects to prevent them from casting shadows from that specific light or even avoid reflecting something for artistic purposes.
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- Trace Subset - inclusive, only the ones specified here are traced
- Exclude Subset - exclusive, trace anything except what is specified here
These are set on the object you wish to affect, not the object doing the affecting.
To be more precise, in the examples below, let's say you wanted to keep the ball from reflecting in the mirror.
We would select the mirror object and say not to trace the ball.
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Without telling it to exclude the shadow subset, transmission rays block light sources.
This is only an issue if seen against a light indirectly, in this case, the dome light.
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