Adobe After Effects CS4 - Manual (Page 384)

378 USING AFTER EFFECTS CS4
Transparency, opacity, and compositing
Last updated 12/21/2009
Compound masks that result when different modes are applied to the circle mask. The masks in this illustration have different Mask Opacity
values.
A. Original masks B. None C. Add D. Subtract E. Intersect F. Lighten G. Darken H. Difference
None The mask has no direct influence on the alpha channel of the layer. This option is useful when you are only using
the path of the mask for an effect such as Stroke or Fill, or if you are using the mask path as the basis for a shape path.
Add The mask is added to the masks above it in the stacking order. The influence of the mask is cumulative with the
masks above it.
Subtract The influence of the mask is subtracted from the masks above it. This option is useful when you want to
create the appearance of a hole in the center of another mask.
Intersect The mask is added to the masks above it in the stacking order. In areas where the mask overlaps the masks
above it, the influence of the mask is cumulative with the masks above it. In areas where the mask does not overlap
with the masks above it, the result is complete opacity.
Lighten The mask is added to the masks above it in the stacking order. Where multiple masks intersect, the highest
transparency value is used.
Darken The mask is added to the masks above it in the stacking order. Where multiple masks intersect, the lowest
transparency value is used.
A B
C D
E F
G H