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After Effects CS4 for Windows and Macintosh: Visual QuickPro Guide by Antony Bolante

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Chapter 10. Mask Essentials

A mask is a shape, or path, that you create in a layer. Masks are essential to compositing images and to creating many other effects.

You can draw a mask manually with a tool, define it numerically using a dialog, copy and paste it from Adobe Illustrator or Photoshop, or use a menu command to create one from a text layer or other image. A mask can be a closed shape (such as a circle) or an open path (such as a curved line).

A closed mask modifies or creates an alpha channel—which, as you recall, defines the opaque and transparent areas of an image. The image within the masked area remains visible; the area outside the mask reveals the layers below. An open path, in contrast, consists of a curve, or path, with two endpoints. ...

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