Adobe After Effects CS3 - Manual (Page 449)

AFTER EFFECTS CS3
User Guide
444
Difference Matte Key effect
A. Original image B. Background image C. New background image D. Final composite image
See also
“Keying overview and tips” on page 268
“Matte Choker effect” on page 450
“Simple Choker effect” on page 451
Use the Difference Matte effect
1 Select a motion footage layer as the source layer.
2 In the source layer, find a frame that consists only of background, and save the background frame as an image file.
(See “Export a single frame of a composition” on page 607.)
3 Import the image file into After Effects, and add it to the composition.
The imported image becomes the difference layer. Make sure that its duration is at least as long as that of the source
layer.
Note: If the shot doesn’t contain a full background frame, you may be able to assemble the full background by combining
parts of several frames in After Effects or Photoshop. For example, you can use the Clone Stamp tool to take a sample of
the background in one frame, and then paint the sample over part of the background in another frame.
4 Turn off the display of the difference layer by clicking the Video switch in the Timeline panel.
5 Make sure that the original source layer is selected, and then choose Effect > Keying > Difference Matte.
6 In the Effect Controls panel, choose Final Output or Matte Only from the View menu. (Use the Matte Only view
to check for holes in the transparency. To fill undesired holes after you complete the keying process, see “Close a hole
in a matte” on page 271.)
7 Select the background file from the Difference Layer menu.
8 If the difference layer isn’t the same size as the source layer, choose one of the following controls from the If Layer
Sizes Differ menu:
Center Places the difference layer in the center of the source layer. If the difference layer is smaller than the source
layer, the rest of the layer fills with black.
Stretch To Fit Stretches or shrinks the difference layer to the size of the source layer. Background images may become
distorted.
A B
C D