The masks in After Effects are classified into three types: matte, shape, and alpha. The matte type of mask is the most common one. It is used to isolate a specific area of the image or video in order to apply certain effects or filters on that area only. The shape type of mask is used when we want to change the shape of an object or some aspect of its outline. The alpha type of mask is used for compositing two layers together - it can be a live action footage with a background scene or any other combination.
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