Shadow Passes

Shadow passes seem to have disappeared in this version of Mental Ray, and today I wanted to find some sort of work around, so here it is:


I have got an annoying Matte line around the edge where the plane was removed, this can be fixed by making the plane bigger than the frame in maya, or rotoing it out in nuke. 
Here I have rendered the two objects on a plane, then removed the it in Nuke and composited the ambient occlusion pass underneath to two objects, giving them some shadows and giving the render a lot more depth, here's the process.

 original shadows:

Plane removed:


replacement background:


Added Ambient Occlusion:

This worked pretty well for these objects right on top of the plane, however it didn't work for floating objects, and it wont work for any hard, directional shadows either, because that's not what ambient occlusion is for.
see here.

I think this is a really useful way of giving CG elements nice looking shadows in a different setting, but getting hard shadows onto a backplate is still something I want to learn.

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