Adobe After Effects CS3 User Manual
Page 274
AFTER EFFECTS CS3
User Guide
269
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 111.)
•
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.
The KeyerforDV animation preset available through the AE Enhancers forum automates the process of blurring the
blue and green channels before keying, which can be crucial for keying DV footage:
. This animation preset was written for an earlier version of the
Keylight effect, but you can simply drag and drop the Keylight (1.2) effect from the Effects & Presets panel to replace
the older effect, which may be reported as missing if you're using After Effects CS3.
•
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. (See “Use a garbage matte” on page 270.)
•
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.