After Effects 7 for Windows and Macintosh: Visual Quickpro Guide

Chapter 10. Mask Essentials

A mask is a shape, or path, that you create in a layer. You can draw a mask manually with a tool, define it numerically using the Mask Shape dialog box, or copy it from Adobe Illustrator or Photoshop. A mask can be a closed shape (such as a circle) or an open path (such as a curved line). Masks are essential to compositing images and to creating a number of other effects. As the name suggests, the most common use for a mask is to mask, or preserve, parts of a layer’s image while cropping out the rest. A closed mask modifies a layer’s alpha channel—which, as you recall, defines the opaque and transparent areas of an image. Even if a footage item lacks an alpha channel of its own, you can define areas of transparency ...