The power of Render Selected
This is a feature that was newly introduced with Max 2009 and is worth mentioning. Along with the render region, crop, and blowup options is a render selected button. Unlike the other options, this option is only located in the render frame buffer window:
This option is a quick easy way to create a rendering of just certain objects while still being able to receive reflections lights and shadows from the scene. It’s useful for rendering dynamic animations of people moving, or that rug with tons of displacement. Unlike, Selected, with the box on, mental ray only renders what is selected.
Before you render, be sure to hit the Clear frame buffer button (the X icon on the top left), so you don’t have anything else in your frame.
This is a great way to create masks or passes and blend them in your favorite compositing program. A couple of things to note: If you want the light information to be seamless with your beauty pass, be sure to save your FG and GI maps first.