Keying - Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual

The mask will have motion blur regardless of the setting of the layer's Motion Blur switch.
On
The mask will not have motion blur.
Off
See also
"Use motion blur" on page 202

Keying

Keying overview
Keying is defining transparency by a particular color value or luminance value in an image. When you key out a value,
all pixels that have colors or luminance values similar to that value become transparent. Keying makes it easy to
replace a background, which is especially useful when you work with objects too complex to mask easily. When you
place a keyed layer over another layer, the result forms a composite, in which the background is visible wherever the
keyed layer is transparent.
After Effects includes several built-in keying effects, as well as the Academy Award-winning Keylight effect, which
excels at professional-quality color keying. For information on the Keylight effect, see its documentation in this
folder: Adobe After Effects CS3/Additional Documentation/Keylight.
To see a video tutorial on keying with Keylight, visit the Adobe website at www.adobe.com/go/vid0229.
Adobe® Creative Suite® Production Premium Edition also includes Adobe Ultra®, which can quickly key out a poorly
lit background shot with a low-quality webcam. For more information on Adobe Ultra, visit the Adobe website at
www.adobe.com/go/learn_ae_ultra.
You often see composites made with keying techniques 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 solid-
color background screen. The background color is then keyed out and the actor's scene is composited over the
background footage item.
The technique of keying out a background of a consistent color is often called bluescreening or greenscreening,
although you don't have to use blue or green; you can use any solid color for a background. Other common terms for
this sort of keying are color keying and chroma keying.
Difference keying works somewhat differently from color keying. Difference keying defines transparency with
respect to a particular baseline background image. Instead of keying out a single-color screen, you can key out an
arbitrary background. To use difference keying, you must have at least one frame that contains only the background;
other frames are compared to this frame, and the background pixels are made transparent, leaving the foreground
objects.
Keep in mind that generating a high-quality key can require the application of multiple keying effects in sequence
and careful modification of their properties, especially if the footage was shot without considering the requirements
of the compositor.
Here are some tips to help you as you work with keying effects:
• 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 keying effect to the layer in the foreground,
the composition background (or a background layer) shows through, making it easy to view transparent areas.
(See "Set composition background color" on page 115.)
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