Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 275

• Once you have used a key to create transparency, use Matte effects to remove traces of key color and create clean
edges.
• Start with the highest-quality materials you can gather, such as film that you scan and digitize.
• Use uncompressed footage (or, at least, files with the least possible amount of compression). Many compression
algorithms, especially those used in DV and Motion JPEG, discard subtle variations in blue—which may be
necessary to create a good key from a bluescreen.
• Noise and compression artifacts can cause problems for keying, especially difference keying. Often, applying a
slight blur before keying can reduce noise and compression artifacts enough to improve keying results. For
example, blurring the blue channel for DV footage can smooth out noise in a bluescreen.
• Blurring the alpha channel after keying can soften the edges of the matte, which can improve compositing results.
• Light your greenscreen or bluescreen uniformly, and keep it free of wrinkles.
• Use a garbage matte to roughly outline your subject so that you don't have to waste time keying out parts of the
background far from the foreground subject.
• For evenly lit greenscreen or 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 subse-
quent frames. If lighting changes, you may need to adjust keying controls for other frames. Place keyframes for the
first set of keying properties at the start of the bluescreen scene. If you are setting keyframes for one property only,
use Linear interpolation. For footage that requires keyframes for multiple interacting properties, use Hold inter-
polation. If you set keyframes for keying properties, you may want to check the results frame by frame. Interme-
diate keying values may appear, producing unexpected results.
• To key well-lit footage shot against a bluescreen or greenscreen, start with the Color Difference Key. Add the Spill
Suppressor to remove traces of the key color, and then use one or more of the other Matte effects, if necessary. If
you are not satisfied with the results, try starting again with the Linear Color Key.
• To key well-lit footage shot against multiple colors or unevenly lit footage shot against a bluescreen or greenscreen,
start with the Color Range key. Add the Spill Suppressor and other effects to refine the matte. If you are not
completely satisfied with the results, try starting with or adding the Linear Color Key.
• To key dark areas or shadows, use the Extract Key on the Luminance channel.
• To make a static background scene transparent, use the Difference Matte Key. Add the Simple Choker and other
effects as needed to refine the matte.
See also
"Keying effects" on page 438
"Matte effects" on page 448
Use a garbage matte
A garbage matte removes unneeded portions of the scene, resulting in a rough area that contains only the subject that
you want to keep. When you are working with a poorly lit or uneven bluescreen or greenscreen, sketching a garbage
matte around the subject can greatly reduce the amount of work that you have to do in keying out the background.
However, if you spend a lot of time making a perfect garbage matte that exactly outlines the subject—essentially
rotoscoping—you lose the time-saving advantage of color keying.
Create a mask to roughly outline a subject.
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AFTER EFFECTS CS3 269
User Guide