Adobe After Effects CS4 - Manual (Page 390)

384 USING AFTER EFFECTS CS4
Transparency, opacity, and compositing
Last updated 12/21/2009
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.
Note: 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.
Shooting and acquiring footage for keying
Keying footage that was not acquired correctly is much more difficult than is keying footage that was acquired with
keying in mind. For best results, use Adobe OnLocation™ to monitor color and lighting as you acquire footage for color
keying.
For tips on shooting footage so that color keying is easier and more successful, see Jonas Hummelstrand’s General
Specialist website .
Alex Lindsay provides an article on the ProVideo Coalition website about setting up and lighting a green background
in preparation for color keying work. In part two of his greenscreen primer on the ProVideo Coalition website , Alex
Lindsay provides detailed information about cameras, codecs, and capture systems to use when acquiring footage for
color keying.
• Light your color screen uniformly, and keep it free of wrinkles.
• 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 the algorithms used in DV, HDV, and Motion JPEG, discard subtle variations in blue—which
may be necessary to create a good key from a bluescreen. Use footage with the least color subsampling possible—
for example, 4:2:2 rather than 4:2:0 or 4:1:1. (For information about color subsampling, see the
Wikipedia website .)
Tips for keying with After Effects
• 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.
The KeyerforDV animation preset available through the AE Enhancers forum automates the process of blurring
the blue and green channels before keying DV footage.
• 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. (See “
Use a garbage matte ” on page 385.)
• 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 59.)