Adobe 12040118 - After Effects Standard Tutorial page 337

Keying
Keying introduction and resources
Use a garbage matte
Use a hold-out matte
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Keying introduction and resources
Note: When a background is not of a consistent and distinctive color, you can't remove the background with keying effects. Under these
conditions, you may need to use rotoscoping—the manual drawing or painting on individual frames to isolate a foreground object from its
background. (See Rotoscoping introduction and resources.)
About keying: color keys, luminance keys, and difference keys
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.
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 scene with the actor is composited over a new background.
The technique of keying out a background of a consistent color is often called bluescreening or greenscreening, although you don't have to use a
blue or green screen; you can use any solid color for a background. Red screens are often used for shooting non-human objects, such as
miniature models of cars and space ships. Magenta screens have been used for keying work in some feature films renowned for their visual
effects. Other common terms for this kind of keying are color keying and chroma keying.
Difference keying works 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. Noise, grain, and other subtle variations can make difference keying very difficult to use in practice.
Keying effects, including Keylight
After Effects includes several built-in keying effects, as well as the Academy Award-winning Keylight effect, which excels at professional-quality
color keying. (See Keying effects and Matte effects.)
For information on the Keylight effect, see its documentation in the folder in which the Keylight plug-in is installed, or on the Foundry website.
Keylight is not included with the trial version of After Effects CS5. (See Third-party plug-ins included with After Effects.) Keylight is included in the
trial version of After Effects CS5.5 and later.
Note: Though the color keying effects built into After Effects can be useful for some purposes, you should try keying with Keylight before
attempting to use these built-in keying effects. Some keying effects—such as the Color Key effect and the Luma Key effect—have been
superseded by more modern effects like Keylight.
For a video tutorial on keying with Keylight, go to the Adobe website.
Mark Christiansen provides tips and techniques for using Keylight in an excerpt from his book After Effects Studio Techniques: Visual Effects and
Compositing on the Peachpit Press website. In an excerpt from the "Color Keying in After Effects" chapter of After Effects Studio Techniques, Mark
Christiansen provides detailed tips and techniques for color keying, including advice on which keying effects to avoid and how to overcome
common keying challenges.
For a step-by-step tutorial demonstrating the use of the Color Difference Key effect, the Matte Choker effect, the Spill Suppressor effect, and
garbage masks, see the "Keying in After Effects" chapter of the After Effects Classroom in a Book on the Peachpit Press website.
Jeff Foster provides free sample chapters from his book The Green Screen Handbook: Real World Production Techniques. The sample chapters
cover basic compositing, color keying, garbage mattes, hold-out mattes, and how to avoid common problems with greenscreen shots. For more
information, see the Adobe website.
Rich Young collects more tips and resources for keying on his After Effects Portal website.
Tips on color keying and compositing from experienced compositor, Chris Zwar.
Chris & Trish Meyer shares tips & resources for color keying with Keylight and other effects.
Note: Keep in mind that generating a high-quality key can require the application of multiple keying effects in sequence and careful modification