I know this should be easy. I used to composite for academy award winning vfx companies with cineon, inferno, shake... Yeah... long time ago. But I am just fumbling my way through after effects and can't figure this stuff out to save my life out.
I am comping 3d elements on a bg plate; Whales on water with people in front of the water standing on rocks.
I used keylight to pull a key on the bg plate water and sky. works pretty great.
I have a second travel mask I created for an inside garbage matte on the rocks and foreground people I want to keep around so my whales are behind them all without any chatter and anomalies.
THE ONLY WAY I can think to make this work is to copy the ENTIRE RGBA background plate and paste it above the whales layer. And ashamed to admit....ADD ANOTHER RGB BG plate with the garbage matte on top of THAT one.
SO MY QUESTION:
How do I just combine the KEYLIGHT and my INSIDE garbage matte and then subtract them from the Alpha Channel of the 3d element so I only need ONE BASE BG plate, as it should be?
I know the answer is somewhere in channel combiner or something like that. So I need some specifics. I'm sort of a doofus artist type who once learned these things but has forgotten most everything. SO just assume that I know very very little outside of concepts.
Figured out my own thing. So now I tell you so others can know. Maybe it is right. Maybe not. But it works because I got what I wanted. No duplicated RGB.
1) Precomped a keylight matte with a careful internal garbage matte ADD in the garbage to the Keylight.
2) Precomped all my whales
3) Put my foreground mask later (precomp from (1) on top olayer and turn it off.
4) Set the track matte for Whale Precomp (2) to Alpha inverse of the FG mask.