Matte 3D objects

[expdigital]

Hi everybody, my fist post on this forum!

Just getting into Nuke after years of Combustion. Love it! Had a quick question:
I rendered a 3D scene in Maya and comped it in Nuke, then exported the objects and camera from Maya as a fbx readGeo into Nuke. If I want to composite a card image of a live action man in the nuke scene that passes behind my 3D objects how do I assign a matte type material to all of the 3D objects so they "block out" the card element?

Thanks!

[bushpig]

the node you are looking for is called fillMat. the quickest way to call it it up is with Tab.
it is basically a texture to create a black hole in the alpha