User Guide

CHAPTER 8
278
Applying Effects
Using keying effects
After Effects includes several different effects that key out, or make transparent, parts of
an image. Each effect is called a key, and the color specified for transparency is called the
key color. A key locates pixels in an image that match the specified key color and makes
them transparent or semitransparent, depending on the type of key. When you place a
layer over another layer using transparency, the result forms a composite, in which the
background is visible wherever the first layer is transparent, making the first layer appear
to be part of the background.
After Effects creates an alpha channel for identifying areas in an image that are partially or
completely transparent. The view of an image in its alpha channel is often called the matte
view. The matte represents opaque, transparent, and partially transparent areas as white,
black, and gray, respectively. For more information on alpha channels and mattes, see
“Importing footage containing an alpha channel” on page 72. For information on using
the keys included with After Effects, see online Help.
You often see composites in movies, for example, when an actor appears to dangle from a
helicopter or float in outer space. To create this effect, the actor is filmed in an appropriate
position against a color screen. The color screen is then keyed out and the actor’s scene is
composited over the background footage item.
For satisfactory keying results, start with the highest-quality materials you can gather,
such as film that you scan and digitize. Strive for lighting that is constant for the duration
of the color-screen scene. Use footage files with the least amount of compression. Files with
lossy compression, especially DV and Motion JPEG files, discard subtle differences in blue.
These differences may be necessary to create a good matte from a bluescreen.
Effects included with After Effects
After Effects includes an array of visual and audio effects. And, because of After Effects’
plug-in technology, you can use additional effects from other Adobe plug-in-compatible
applications, such as Adobe Photoshop.
When you register your copy of After Effects, you can download the following effects from
the After Effects product section on Adobe’s Web site: Card Dance, Card Wipe, Caustics,
Foam, and Wave World.
UG.book Page 278 Wednesday, February 21, 2001 12:05 PM