To Key Out A Luminance Value With The Luma Key Effect - Adobe AFTER EFFECTS 7.0 Manual

Create additional open or closed masks to clean up other areas of the image, and then select them from the
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Cleanup Foreground or Cleanup Background menu. Cleanup Foreground masks increase the opacity along the
mask; Cleanup Background masks decrease the opacity along the mask. Use the Brush Radius and Brush Pressure
options to control the size and density of each stroke.
Note: You can select the Background (outer) mask as a Cleanup Background mask to clean up noise from the
background portions of the image.
5 Set Edge Thin to specify how much of the matte's border is affected by the key. A positive value moves the edge
away from the transparent region, increasing the transparent area; negative values move the edge toward the trans-
parent region and increase the size of the foreground area.
Increase the Edge Feather values to soften edges of the keyed area. High Edge Feather values take longer to render.
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Specify the Edge Threshold, which is a soft cutoff for removing low-opacity pixels that can cause unwanted noise
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in the image background.
Select Invert Extraction to reverse the foreground and background regions.
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Set Blend With Original to specify the degree to which the resulting extracted image blends with the original
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image.
See also
"Creating masks" on page 244
"Inner/Outer Key effect (Pro only)" on page 465

To key out a luminance value with the Luma Key effect

When the object you want to matte has a very different luminance value than its background, you can make the
background value transparent by keying it out. For example, if you want to create a matte for dark musical notes on
a white background, you can key out the brighter values; the dark musical notes become the only opaque area.
Select the layer, and choose Effect > Keying > Luma Key.
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2 Select a Key Type to specify the range to be keyed out.
Drag the Threshold slider in the Effect Controls panel to set the luminance value on which you want the matte to
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be based.
4 Drag the Tolerance slider to specify the range of values to be keyed out. Lower values key out a smaller range of
values near the threshold. Higher values key out a wider range of values.
Drag the Edge Thin slider to adjust the width of the keyed area's border. Positive values make the mask grow,
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increasing the transparent area. Negative values shrink the mask.
Drag the Edge Feather slider to specify the softness of the edge. Higher values create a softer edge but take longer
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to render.
See also
"Luma Key effect" on page 466
ADOBE AFTER EFFECTS 7.0
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User Guide