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Keying in After Effects CS3

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Does the idea of keying conjure up images of a meteorologist on the evening news, or a shooting technique used in motion pictures? With After Effects, even the simplest, least-expensive project can take advantage of keying. The Adobe Creative Team takes us through a step-by-step example of this technique.
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In this exercise, you'll learn how to do the following:

  • Create a garbage mask.
  • Use the Color Difference Key effect to key an image.
  • Check an image's alpha channel for errors in a key.
  • Use the Spill Suppressor effect to remove unwanted light spill.
  • Key out a background by using the Keylight effect.
  • Adjust the contrast of an image by using the Levels effect.

In this exercise, you'll build a promotional spot for a weekly news segment on a fictional television station. You'll begin by working with live-action footage of an actor captured on a greenscreen stage. You'll learn how to use keying effects to remove the green background and clean up the edge of the key to remove any lingering green spill. Then you'll add a title to the promo, and place a station ID into the composition to complete the project.

About Keying

Keying is defining transparency by a particular color value (with a color key or chroma key) or brightness value (with a luminance key) in an image. When you key out a value, all pixels that have similar colors or luminance values become transparent. Keying makes it easy to replace a background of a consistent color or brightness with another image, which is especially useful when working with objects that are too complex to mask easily. 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.

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.

Why might you use keying? There are a variety of reasons, including the following:

  • A stunt may be too dangerous for an actor to perform on location.
  • The location may be too hazardous for cast and crew to film live.
  • In the case of science-fiction movies, the location may not exist at all.
  • You might be working with a limited budget and cannot shoot any other way.

The best way to ensure a good key is to know what format works best. MiniDV is a popular format, and you'll use it in this exercise, but compression can cause MiniDV footage to have artifacts such as blocky regions or aliasing along the curves and diagonals of your subject. The greater the digital compression, the harder it is to pull a clean key. After Effects can help you solve these and other keying problems.

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