User Guide

237
Chapter 7: Working with Masks
and Transparency
hen part of a layer is transparent, that transparency information is stored in the
layer’s alpha channel. If the alpha channel of a layer doesn’t meet your require-
ments, you can use any combination of masks, mattes, and keying to display or
hide different parts of a layer. You can also combine layers to achieve visual effects, apply
layer modes to control the intensity or transparency of colors or brightness values, and use
a color channel in one layer to create an effect in another layer. With a combination of
layers and layer masks, you can create an array of effects.
To create effects by combining images, parts of each image must be transparent. The
terminology of transparency varies by media and by software. After Effects refers to trans-
parent areas using the following terms:
Alpha channel An invisible channel that defines transparent areas for the footage item or
layer that contains the channel. With imported items, an alpha channel provides a way to
store both the footage and its transparency information in a single file without disturbing
the footage item’s color channels. Each After Effects layer can accommodate an alpha
channel included with a footage item. See “Importing footage containing an alpha
channel” on page 72.
Mask A path, or outline, that modifies a layer’s alpha channel. Use a mask when you want
to draw areas of transparency in After Effects. A mask belongs to a specific layer, but each
After Effects layer can contain multiple masks.
Matte A layer (or any of its channels) that defines the transparent areas of that layer or
another layer. Use a matte when you have a channel or layer that defines the desired area
of transparency better than the alpha channel (or in cases where footage does not include
an alpha channel).
Keying Defining transparency by a particular color (color key) or brightness value
(luminance key) in an image. Pixels matching the key color become transparent. Use
keying to remove a background with a uniform color, such as a blue screen.
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UG.book Page 237 Wednesday, February 21, 2001 12:05 PM