Transparency Overview; About Transparency; About Alpha Channels And Mattes; Chapter 12: Masks, Transparency, And Keying - Adobe AFTER EFFECTS 7.0 Manual

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Chapter 12: Masks, transparency, and
keying

Transparency overview

About transparency

To create a composite from multiple images, parts of one or more of the images must be transparent. You can use
alpha channels, masks, mattes, and keying to define which parts of an image are transparent and which parts of an
image will be used to obscure parts of another image. By manipulating transparency and choosing blending modes,
you can create a variety of visual effects.

About alpha channels and mattes

Color information is contained in three channels: red, green, and blue. In addition, an image can include an invisible
fourth channel, called an alpha channel, that contains transparency information.
A B
C
Channels at a glance
A. Separated color channels B. Alpha channel C. All channels viewed together
An alpha channel provides a way to store both images and their transparency information in a single file without
disturbing the color channels.
When you view the alpha channel in the After Effects Composition panel or the Adobe Premiere Pro Monitor panel,
white indicates complete opacity, black indicates complete transparency, and shades of gray indicate partial trans-
parency.
A matte is a layer (or any of its channels) that defines the transparent areas of that layer or another layer. White areas
define what is opaque, and black areas define what is transparent. An alpha channel is often used as a matte, but you
can use a matte other than the alpha channel when you have a channel or layer that defines the desired area of trans-
parency better than the alpha channel does, or in cases where the source image does not include an alpha channel.
Many file formats can include an alpha channel, including Adobe Photoshop, ElectricImage, TGA, TIFF, EPS, PDF,
QuickTime (saved at a bit depth of Millions Of Colors+), and Adobe Illustrator. For Adobe Illustrator EPS and PDF
files, After Effects automatically converts empty areas to an alpha channel.
About straight and premultiplied channels
Files with alpha channels fall into two categories: straight and premultiplied. Although the alpha channels are the
same, the color channels differ.