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.
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. By itself, an open path can’t define areas of ...
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