ADOBE AFTER EFFECTS 7.0
User Guide
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2 Click inside the mask area in the Composition panel and drag the layer to a new position.
Keying
About keying
After Effects includes several 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.
After Effects uses 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.
Once you have used a key to create transparency, use Matte effects (Pro only) to remove traces of key color and create
clean edges.
Youoftenseecompositesmadewithkeyingtechniquesinmovies,forexample,whenanactorappearstodanglefrom
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.
Forsatisfactorykeyingresults,startwiththehighest-qualitymaterialsyoucangather,suchasfilmthatyouscanand
digitize. If appropriate for your footage, 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 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.
Adjusting keying controls on a single frame
For evenly lit bluescreen footage, adjust keying controls on only one frame. Choose the most intricate frame of the
scene, one involving fine detail such as hair and transparent or semitransparent objects, such as smoke or glass. If
the lighting is constant, the same settings you apply to the first frame are applied to all subsequent frames.
If lighting changes, you may need to adjust keying controls for other frames. Place keyframes for the first set of keying
propertiesatthestartofthebluescreenscene.Ifyouaresettingkeyframesforonepropertyonly,useLinearinterpo-
lation. For footage that requires keyframes for multiple interacting properties, use Hold interpolation.
If you set keyframes for keying properties, you may want to check the results frame by frame. Intermediate keying
values may appear, producing unexpected results.
See also
“Using keyframes” on page 192
“Interpolation methods” on page 220
Using a background color
To help you view transparency, temporarily change the background color of the composition, or include a
background layer behind the layer you are keying out. As you apply the key to the layer in the foreground, the compo-
sition background (or a background layer) shows through, making it easy to view transparent areas.
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