To Use The Extract Effect - Adobe AFTER EFFECTS 7.0 Manual

If the difference layer is not 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 is filled with black.
Stretches or shrinks the difference layer to the size of the source layer but may distort background
Stretch To Fit
images.
Adjust the Matching Tolerance slider to specify the amount of transparency based on how closely colors must
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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 do not increase the number of matching pixels.
If there are still extraneous pixels in the matte, adjust the Blur Before Difference slider. This option suppresses
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noise by slightly blurring both layers before making the comparison. Note that the blurring occurs only for
comparison and does not blur final output.
Before closing the Effect Controls panel, make sure that you have selected Final Output from the View menu to
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ensure that After Effects renders the transparency.
See also
"Difference Matte effect (Pro only)" on page 464

To use the Extract effect

In the Effect Controls panel, the Extract effect displays a histogram for a channel specified in the Channel menu. The
histogram displays a representation of the brightness levels in the layer, showing the relative number of pixels at each
level. From left to right, the histogram extends from the darkest (a value of 0) to the lightest (a value of 255).
Using the transparency control bar beneath the histogram, you can adjust the range of pixels that are made trans-
parent. The position and shape of the bar in relation to the histogram determine transparency. Pixels corresponding
to the area covered by the bar remain opaque; pixels corresponding to the areas not covered by the bar are made
transparent.
Select the layer you want to make transparent, and then choose Effect > Keying > Extract.
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2 If you are keying out bright or dark areas, choose Luminance from the Channel menu. To create visual effects,
choose Red, Green, Blue, or Alpha.
Adjust the amount of transparency by dragging the transparency control bar in the following ways:
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•
Drag the upper right or upper left selection handles to adjust the length of the bar and to shorten or lengthen the
transparency range. You can also adjust the length by moving the White Point and Black Point slider bars. Values
above the white point and below the black point are made transparent.
•
Drag the lower right or lower left selection handles to taper the bar. Tapering the bar on the left affects the softness
of transparency in the darker areas of the image; tapering it on the right affects the softness in the lighter areas.
You can also adjust the softness levels by adjusting White Softness (lighter areas) and Black Softness (darker areas).
Note: To taper the edges of the transparency control bar, you must first shorten the transparency bar.
•
Drag the entire bar left or right to position it under the histogram.
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User Guide