User Guide

CHAPTER 7
238
Working with Masks and Transparency
Using a footage item with an alpha channel
An alpha channel is a fourth 8- or 16-bit channel (see “Selecting 16-bpc color depth (PB
only)” on page 46 for information on working with 16-bit color in After Effects), included
in addition to the three 8- or 16-bit red, green, and blue color channels of video footage
or an RGB image. When a file contains an alpha channel, its image is defined as containing
a total of 32 or 64 bits, or using Millions of Colors + or Trillions of Colors +. An alpha
channel has the same function as a key in analog video compositing or a matte in optical
film compositing—it describes the areas of an image that are transparent. See “Importing
footage containing an alpha channel” on page 72.
Working with masks
Each layer in a composition can contain up to 127 masks. Create and view masks in either
the Composition or Layer window, and set interactions between mask properties in the
Timeline window or in the Layer menu. See “Creating masks” on page 238 for information
on creating masks. You can also animate the shape of the mask by animating individual
control points on a mask path. For more information on creating keyframes, see “Setting
keyframes” on page 163.
To view masks in the Composition window:
Select Layer Masks from the Composition Window menu.
To view masks in the Layer window:
Select Masks from the Layer Window menu.
To specify a mask in the Layer window as the target for all new mask shapes:
Select the mask name from the Target pop-up menu. For more information on the Target
menu, see “Specifying one mask as the target for animation” on page 253.
Creating masks
You can create one or more masks for each layer in a composition. Masks appear in the
Timeline window in the order you create them.
UG.book Page 238 Wednesday, February 21, 2001 12:05 PM