Adobe After Effects CS4 - Manual (Page 386)

380 USING AFTER EFFECTS CS4
Transparency, opacity, and compositing
Last updated 12/21/2009
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 ” on page 379.)
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.
Note: Because the mask feather causes the opacity values to vary according to a Gaussian distribution, the area influenced
by the feather actually extends somewhat 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.
The PV Feather plug-in from RE:Vision Effects provides the ability to control the feather amount for each vertex of a
mask path independently. For information, see the
RE:Vision Effects website .
More Help topics
“ Creating shapes and masks ” on page 312
“ Managing and animating shape paths and masks ” on page 322
Track mattes and traveling mattes
When you want one layer to show through holes defined by another layer, set up a track matte. For example, you can
use a text layer as a track matte for a video layer to allow the video to only show through the shapes defined by the text
characters. The underlying layer (the fill layer) gets its transparency values from the values of certain channels in the
track matte layer—either its alpha channel or the luminance of its pixels.
Defining the transparency of a layer based on the luminance of the track matte’s pixels is useful when you want to
create a track matte using a layer without an alpha channel or a layer imported from a program that can’t create an
alpha channel. In both cases—using alpha channel mattes and using luminance mattes—pixels with higher values are
more transparent. In most cases, you use a high-contrast matte so that areas are either completely transparent or
completely opaque. Intermediate shades should appear only where you want partial or gradual transparency, such as
along a soft edge.