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CHAPTER 8 268
Superimposing and Compositing with Transparency
Using the Alpha Channel key
Images represented on video are composed of three grayscale images called channels —one red,
one green, one blue. Such images are called RGB images . An alpha channel is a fourth channel
in an RGB image that defines what parts of the image are transparent or semi-transparent.
Many programs, such as Adobe Illustrator and Photoshop, use alpha channels to let you specify
transparent regions in an image. An alpha channel is either
straight or premultiplied , depending
on the program that created the image. When an image contains a straight alpha channel, the
transparency information is stored only in the alpha channel.
Use the Alpha Channel key on clips that contain a straight alpha channel, such as images
created in Adobe Photoshop, Adobe Illustrator, and Adobe After Effects.
The Photoshop image (left) contains a separate alpha channel (center) which Premiere uses to
composite the foreground against another background (right).
Using Black Alpha Matte and White Alpha Matte keys
When an image contains a premultiplied alpha channel, the transparency information is
contained in all three color channels, in addition to the alpha channel. Typically, an image
that contains a premultiplied alpha channel also contains either a black or white background.
If you are compositing the image, you will likely need to key out the background.
To key out a black background, use the Black Alpha Matte; to key out a white background,
use the White Alpha Matte.
c00.book for PS Page 268 Tuesday, March 31, 1998 1:28 PM