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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. For information on using the keys included with After Effects, see Keying effects.
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.
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