About alpha channels and mattes

Color information in After Effects is contained in three channels: red (R), green (G), and blue (B). In addition, an image can include an invisible fourth channel, called an alpha channel, that contains transparency information. Sometimes, such an image is referred to as an RGBA image, indicating that it contains an alpha channel.

Channels at a glance

A.: Separated color channels
B.: Alpha channel represented as a grayscale image
C.: Composite using all four channels with a background showing through transparent areas

An alpha channel provides a way to store images and their transparency information in a single file without disturbing the color channels.

When you view an alpha channel in the After Effects Composition panel or Adobe Premiere Pro Monitor panel, white indicates complete opacity, black indicates complete transparency, and shades of gray indicate partial transparency.

Aharon Rabinowitz provides an introduction to alpha channels, “What is an Alpha Channel?”—part of the Multimedia 101 series on the Creative COW website.

Many file formats can include an alpha channel, including Adobe Photoshop, ElectricImage, FLV, F4V, 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 codec (encoder) used to generate the images stored in these containers. For Adobe Illustrator EPS and PDF files, After Effects automatically converts empty areas to an alpha channel.

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.