I'm dinking around with the new rotobrush feature in AE CS5 with a short clip of me. The object is to remove the background. I was wondering after I render out my roto the way I want it and click "freeze" in order to render it out onto a new layer, how do I treat that layer as a mask? The reason I need to do so, is because I'm doing a lightsaber effect in which the saber passes behind me and I need to make a mask for it to look as if it really passed behind me.
You use the resulting alpha information as a track matte. Check the help files.