Adobe After Effects CS4 - Manual (Page 385)

379 USING AFTER EFFECTS CS4
Transparency, opacity, and compositing
Last updated 12/21/2009
Difference The mask is added to the masks above it in the stacking order. In areas where the mask does not overlap
the masks above it, the mask operates as it would alone on the layer. In areas where the mask overlaps the masks above
it, the influence of the mask is subtracted from the masks above it.
More Help topics
“ Creating shapes and masks ” on page 312
“ Managing and animating shape paths and masks ” on page 322
Expand or contract the edges of a mask
To expand or contract the area influenced by a mask, use the Mask Expansion property.
Mask expansion affects the alpha channel but not the underlying mask path; the mask expansion is essentially an offset
that determines how far, in pixels, from the mask path the influence of the mask on the alpha channel extends.
1 In the Timeline panel, expand the Mask properties of the layer you want to adjust.
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 .
More Help topics
“ Creating shapes and masks ” on page 312
“ Managing and animating shape paths and masks ” on page 322
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
C D
B A