my little groups are
a footage,then add to it
-shift
set alpha as lightness(the checkboard will reveal transparency.)
-then below it, level, individual controls, with alpha input black and input white
then below that layer, another footage with the above set as alpha matte. anything you add to this lower footage now will be masked by the matte. I ususaly add sliders to the input bw / 100.
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you can also precomp half the levels then the input b-w won’t cross over each other and snap the alpha level backwards. also, you can soften the alpha layer by changing how much alpha output(makes softer transitional matte) and what effects are you trying to limit? are you testing my shadow program?
https://technicolorsoftware.hostzi.com/