Multipass rendering in Maya and Mental Ray

This tutorial shows a workflow for rendering 3D scenes in separate passes and compositing these renderings for finer control. The 3D rendering will be done in Maya, using Mental Ray and its “mia_x_Passes” materials, that allow the output of all information used by the shader at render-time. The compositing of these renderings will be done in Nuke using OpenEXRs in a linear workflow.

http://www.cgchannel.com/2010/07/tutorial-multipass-rendering-in-maya-and-mental-ray/