Render Layer Mask alternative but per material? (not object)

[Rev]

Hi There!

I am trying to generate a black and white image much like a render layer mask output, but based on materials not objects. I have a complex objects with many materials and would like some of these materials to be solid (white or black, like a 'matte' but without catching shadows). Is there a way to achieve this on a per material level?

[aoktar]

Take a look for material id. But it just generates different colors per materials.