Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 448

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.
Turn off the display of the difference layer by clicking the Video switch in the Timeline panel.
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5 Make sure that the original source layer is selected, and then choose Effect > Keying > Difference Matte.
In the Effect Controls panel, choose Final Output or Matte Only from the View menu. (Use the Matte Only view
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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.
If the difference layer isn't the same size as the source layer, choose one of the following controls from the If Layer
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Sizes Differ menu:
Places the difference layer in the center of the source layer. If the difference layer is smaller than the source
Center
layer, the rest of the layer fills with black.
Stretches or shrinks the difference layer to the size of the source layer. Background images may become
Stretch To Fit
distorted.
9 Adjust the Matching Tolerance slider to specify the amount of transparency based on how closely colors must
match between the layers. Lower values produce less transparency; higher values produce more transparency.
Adjust the Matching Softness slider to soften the edges between transparent and opaque areas. Higher values
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make matched pixels more transparent but don't increase the number of matching pixels.
11 If the matte still contains extraneous pixels, adjust the Blur Before Difference slider. This slider suppresses noise
by slightly blurring both layers before the comparison is made. Note that the blurring occurs only for the comparison
and doesn't blur final output.
Before closing the Effect Controls panel, make sure to select Final Output from the View menu to ensure that
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After Effects renders the transparency.
Extract effect
The Extract effect creates transparency by keying out a specified brightness range, based on a histogram of a
specified channel. It's best used to create transparency in an image shot against a black or white background or
against a background that is very dark or bright but consists of more than one color.
Note: The controls for this effect are similar to those of the Extract effect in Adobe Premiere Pro, but the purpose and
results of the effect are different.
This effect works with 8-bpc and 16-bpc color.
See also
"Keying overview" on page 268
"Matte Choker effect" on page 448
"Simple Choker effect" on page 448
AFTER EFFECTS CS3 442
User Guide