Adobe After Effects - Manual (Page 405)

None
Add
Subtract
Intersect
Lighten
Darken
Difference
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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
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.
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.
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.
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.
The mask is added to the masks above it in the stacking order. Where multiple masks intersect, the highest transparency value is used.
The mask is added to the masks above it in the stacking order. Where multiple masks intersect, the lowest transparency value is used.
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.
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.
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