Re: AE: Clipping mask from alpha footage

There are no less than three ways, possible more.
One is using the keyed layer as an Alpha Matte for another. This method requires that the "punching layer" sits immediately above in the timline from the "punched" layer. Then (if needed) press the Modes/Switches button at the bottom of the layer panel and you'll get a column that reads "Trk Mat". In that column set the mode for the lower layer to "Alpha Matte". It will be punched by the layer above, which will be turned off.

Second method: Apply the Set Matte effect (Effect > Channels > Set Matte) you want to be punched. You have a menu to choose which layer to use as a matte. Choose the keyed/alpha layer. The "Use for Matte" menu defaults to alpha, so you're good to go. The advantage of the method is that it doesn't require any specific order in the timeline for the layers you use.

Last method: In the Modes panel, set the alpha/keyed layer to the "Stencil" blending mode. The advantage or drawback (depending on your needs) is that every layer located below that one in that timeline will only show through the alpha layer.

Adolfo Rozenfeld · Adobe