Adobe AFTER EFFECTS 5.5 Help Manual page 149

Adobe After Effects Help
Using Help | Contents
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 12 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 30.
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 149 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 98.
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 159.
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.
You can create a mask using any of the following methods:
Draw a path using the tools from the toolbox.
•
Specify the dimensions of the mask shape in the Mask Shape dialog box.
•
Paste a path copied from another layer or from Adobe Illustrator or Adobe Photoshop.
•
For more information, see "Importing masks from Adobe Illustrator and Adobe
Photoshop" on page 161.
Working with multiple masks in one layer
You can create multiple masks in a single layer using any of the methods listed in"Types of
masks" on page 150, "Creating masks from motion paths" on page 151, and "Importing
masks from Adobe Illustrator and Adobe Photoshop" on page 161.
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Working with Masks and Transparency
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