vray3 render comp elements

[corestudio]

I have a question about how to handle more complicated comp elements for live action plates. Imagine a shot of an interior, with a generally blue but patterned rug sitting on a somewhat reflective wood floor. You're going to put a red sphere on the rug and due to the camera angle, the sphere is reflected in the floor. Assuming you've got the lighting and camera worked out, what kind of passes do you output? For instance, it's easy to create a shader that kicks the GI contribution from rug back onto the sphere, but the sphere is also affecting the rug. Same with shadows and the reflection. I'm not asking about the shaders per se, but rather what is setup to comp these elements together?

I should add that I know about Mtl Wrapper. I can set up a scene with an image plane, use Mtl Wrappers and have it look nice. But what if I don't want to render the final with an imagePlane? What I'm curious about is how to get those elements (GI, shadow, etc) out so they can be comped resulting in the same image I see in the viewport. Simply creating Render Elements doesn't seem to do it, since the they don't have their own alphas.

I'm probably missing some basic concept there. Thanks.

[Svetlozar Draganov]

It would be better if you could post some examples (maya scene and nuke comp) to work with.
In general it is possible to reassemble the RGB pass from the render elements but we need to look more carefully your setup.