Adobe After Effects - Manual (Page 406)

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2. Drag the underlined value for Mask Expansion.
Todd Kopriva provides a visual aid and further explanation regarding mask expansion—and why it creates rounded corners—on his blog on the
Adobe website .
Soften (feather) the edges of a mask
Feathering softens the edges of a mask by fading it from more transparent to less transparent over a user-defined distance. Using the Mask
Feather property, you make mask edges hard-edged or soft-edged (feathered). By default, the feather width straddles the mask edge, half inside,
and half outside. For example, if you set the feather width to 25, the feathering extends 12.5 pixels inside the mask edge and 12.5 pixels outside it.
Results for different Mask Feather values
A. Masked layer with 5-pixel feather B. Masked layer with 40-pixel feather C. Result with 5-pixel feather D. Result with 40-pixel feather
You can also extend or contract the mask edges using the Mask Expansion property to control where the mask feathering appears. (See Expand
or contract the edges of a mask .)
Mask feathering takes place only within the dimensions of the layer. Therefore, the path of a feathered mask should always be slightly smaller than
the layer area and should never move to the very edge of the layer. If a mask feather extends beyond the layer area, the feathered edge ends
abruptly.
1. To display the Mask Feather property for selected layers, press F.
2. (Optional) To constrain horizontal and vertical feather amounts to change proportionally, select the Constrain Proportions switch next to
the Mask Feather property.
3. Modify the Mask Feather property as you would any other property—by dragging the underlined value or clicking the underlined value and
entering an amount in the text input field.
Because the mask feather causes the opacity values to vary according to a Gaussian distribution, the area influenced by the feather
actually extends beyond the number of pixels specified. The magnitude of the feather’s influence beyond the specified feather range is very small.
This gradual, Gaussian fall-off appears more natural than would a linear fall-off.
Variable-width mask feathering | CC, CS6
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