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Last updated 12/21/2009
USING AFTER EFFECTS CS4
Transparency, opacity, and compositing
Some programs can store multiple alpha channels in one image, but After Effects only interprets the fourth channel as
an alpha channel.
Note: The term alpha channel technically refers to the fourth (A) channel in an RGBA image file, regardless of whether
that channel is used for communicating transparency information. However, since that fourth channel is used so often to
communicate transparency information, the terms alpha and transparency have become nearly synonymous in common
usage. It's important to remember, though, that this connection is essentially arbitrary. Some formats may use other
channels for transparency information, and other formats may use the fourth channel for something other than
transparency information.
The Knoll Unmult plug-in can be used to create an alpha channel from the dark areas of a layer. This works well for a
layer with a light effect (such as a lens flare or fire) that you want to composite on top of another layer. For information,
see the
Red Giant Software
website.
A matte is a layer (or any of its channels) that defines the transparent areas of that layer or another layer. White defines
opaque areas, and black defines transparent areas. An alpha channel is often used as a matte, but you can use a matte
other than the alpha channel if you have a channel or layer that defines the desired area of transparency better than the
alpha channel does, or in cases where the source image doesn't include an alpha channel.
More Help topics
"Alpha channel interpretation: premultiplied or
straight" on page 73
About masks
A mask in After Effects is a path that is used as a parameter to modify layer attributes, effects, and properties. The most
common use of a mask is the modification of an alpha channel of a layer, which determines the transparency of the
layer at each pixel. Another common use of a mask is as a path along which to animate text. (See
"Creating and
animating text on a
path" on page 363.)
For more information on paths in general, see
"About
paths" on page 307.
Default behavior for a drawn mask (left); same mask inverted (right)
Closed-path masks can create transparent areas for a layer. Open paths cannot create transparent areas for a layer but
are useful as parameters for an effect. Effects that can use an open or closed mask path as input include Stroke, Path
Text, Audio Waveform, Audio Spectrum, and Vegas. Effects that can use closed masks (but not open masks) as input
include Fill, Smear, Reshape, Particle Playground, and Inner/Outer Key.
A mask belongs to a specific layer. Each layer can contain multiple masks.
You can draw masks in common geometric shapes—including polygons, ellipses, and stars—with the shape tools, or
you can use the Pen tool to draw an arbitrary path.
In most ways, drawing mask paths is the same as drawing shape paths on shape layers, though the editing and
interpolation of mask paths have a few additional features. You can link a mask path to a shape path using expressions,
which allows you to bring the benefits of masks into shape layers, and vice versa. See
"Creating shapes and
masks" on
page 312 and
"Managing and animating shape paths and
masks" on page 322.
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