but leaves my background white instead of alpha-ed out. What would be the next step (before or after this tutorial) to achieve that alpha? Should I use keylight 1.2 (and how from that stage) - or is there a simpler way?
The tutorial just shows putting a shift channels effect on the footage, moving the reds and blues to green, and then dropping in the same footage on top (composited to lighten - or maybe it was screen, not sure).
I didn't see an option to "suppress alpha channels" - what would I have to do now that I've gotten the spill suppressor on there via the above mentioned method?