Apple Motion 3 - Manual (Page 308)

308 Chapter 3 Basic Compositing
Silhouette Alpha
Silhouette Alpha is the reverse of the Stencil Alpha blend mode and is useful for
cutting holes in objects underneath.
Silhouette Luma
Silhouette Luma is the reverse of Stencil Luma.
Object used for silhouette End result Object underneath
Limiting the Effect of Stencil and Silhouette Blend Modes
Whenever you use the Stencil or Silhouette blend modes in a group that is set to the
Pass Through blend mode, the resulting effect carries down through every object in
every group that lies underneath it in the Layers list, unless the group that contains it
is rasterized. This is a powerful, but not always desired effect, because it prevents you
from placing a background group to fill the transparent area.
You can limit the Stencil or Silhouette blend mode to affect only those objects that
are within the same enclosing group by setting the group’s blend mode to anything
other than Pass Through.
For example, if you set the enclosing group of the two objects in the Silhouette Alpha
example to Normal, then add a group underneath containing additional objects,
those objects show through the transparent areas created by the silhouetted group.