I need a plane to act as a mask. I want objects in front of it to show up, objects behind the it to not show up, and where the plane would have rendered to be transparent pixels.
You can think of it as the plane being a green screen and then keying out the green. (I tried implementing this as a green screen in the scene, but this cause problems like green light bouncing off the plane and imperfections around the edges.)
In short, is there a way to cause an object to not effect light in the scene and all of it's visible pixels to cause transparent pixels on the render?