Adobe After Effects CS3 - Manual (Page 258)

AFTER EFFECTS CS3
User Guide
253
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.
Many file formats can include an alpha channel, including Adobe Photoshop, ElectricImage, Adobe Flash Video
(FLV), TGA, TIFF, EPS, PDF, and Adobe Illustrator. AVI and QuickTime (saved at a bit depth of Millions Of
Colors+), also can contain alpha channels, depending upon the codecs used to generate these file types. For Adobe
Illustrator EPS and PDF files, After Effects automatically converts empty areas to an alpha channel.
About straight and premultiplied channels
Alpha channels store transparency information in files in one of two ways: straight or premultiplied. Although the
alpha channels are the same, the color channels differ.
With straight (or unmatted) channels, transparency information is stored only in the alpha channel, not in any of the
visible color channels. With straight channels, the effects of transparency aren’t visible until the image is displayed in
an application that supports straight channels.
With premultiplied (or matted) channels, transparency information is stored in the alpha channel and also in the
visible RGB channels, which are multiplied with a background color. The colors of semitransparent areas, such as
feathered edges, are shifted toward the background color in proportion to their degree of transparency.
Some software lets you specify the background color with which the channels are premultiplied; otherwise, the
background color is usually black or white.
Straight channels retain more accurate color information than premultiplied channels. Premultiplied channels are
compatible with a wider range of programs, such as Apple QuickTime Player. Often, the choice of whether to use
images with straight or premultiplied channels has been made before you receive the assets to edit and composite.
Adobe Premiere Pro and After Effects recognize both straight and premultiplied channels, but only the first alpha
channel they encounter in a file containing multiple alpha channels. Adobe Flash recognizes only premultiplied
alpha channels.
About keying
Keying is defining transparency by a particular color value (with a color key or chroma key) or brightness value (with
a luminance key) in an image. When you key out a value, all pixels that have similar colors or luminance values
become transparent.
Keying makes it easy to replace a background of a consistent color or brightness with another image, an especially
useful technique when you work with objects too complex to mask easily. The technique of keying out a background
of a consistent color is often called bluescreening or greenscreening, although you don’t have to use blue or green; you
can use any solid color for a background.
Difference keying defines transparency with respect to a particular baseline background image. Instead of keying out
a single-color screen, you can key out an arbitrary background.
About masks
A mask in After Effects is a path that is used as a parameter to modify layer effects and properties. The most common
use of a mask is the modification of a layer’s alpha channel, which determines the layer’s transparency 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 293.)