Generating Alpha with Keyers and Mattes
Alpha channels, which provide transparency information for After Effects layers, are an important part of the compositing process. Although source files may carry their own alpha channels, as may be the case with 3D renders and Photoshop artwork, other source files may lack alpha completely. For example, video footage, whether it arrives in the form of a self-contained movie or as an image sequence, carries no transparency information. This becomes problematic if a nonalpha layer needs only partially occluded lower layers.
As a solution, you can create alpha information in After Effects by borrowing ...
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