This topic came up recently on CGTalk and I also covered it in the compositing course I taught at the Queensland College of Art. I do have a video screencast as well that I did that I will try and dig up as I think it is on my main PC which is in pieces at the moment.
Inspiring, no?
The question was: “How do I use ID passes (or RGB mattes) in my compositing?”
I guess the first place to start is to remember that compositing is all about colour channels. Pretty much the first thing you do when you get your plates is check the colour channels (red, green and blue) to see what kind of information is there. When you are working with straight CG images, it is a bit different. You will often have various black and white mattes rendered out for you to isolate various objects in a scene.
As we (hopefully) know by now, mattes are essentially greyscale (grayscale for you North American types) images that affect an area of your based on the intensity of the pixel colour value, usually with pure white affecting the change the most and black affecting the least. These can be things like images, patterns, keys or rotoshapes.
