there’s a material setting in maya which doesn’t seem to be very well known but is extremely powerful when it comes to creating passes with render layers. i’ll explain this feature by going through a practical example: i have a traffic light in my upcoming short which needs some nice glowing effects on its lights. because i’m rendering with renderman i can’t use the standard glow special effect, so i have to do that in post. but: how can i render out just the the parts of the lights which are covered by the metal surrounding? here’s the solution:
first, i created a render layer and added the green and the red light including the metal surrounding. now switch to the hypershade and create a lambert material, make it completely black and jump to ‘matte opacity’ and switch the matte opacity mode to ‘black hole’. now i selected the metal surrounding and put this new material on it.
if you want you could also throw a white lambert shader with diffusion set to 1 to all other objects if you don’t need the color information.
now i can just render out this render layer separately and use it in a compositing program like after effects or combustion using a specific blending mode like screen or overlay and blur it to have a nice, subtle light glow effect. here’s my final result:
sure, it would also be possible to render out a glow pass with the maya software renderer, but since i have attached some renderman attributes like the ‘subdiv scheme’ to my geometry, i’d run into troubles as the software renderer doesn’t understand those attributes at all (therefore rendering out a lowres geo). the black hole (i like the name) gives you a lot of flexibility and combined with the render layers it’s a little door opener for interesting render workflows.