Apple Motion 4 - Manual (Page 341)

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 tab, 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.
Stencil Alpha
The Stencil Alpha blend mode uses the alpha channel of the affected object to crop out
all non-overlapping parts of objects and groups underneath it in the Layers tab.
Object used for stencil End result Object underneath
Stencil Luma
The Stencil Luma blend mode does the same thing as the Stencil Alpha blend mode, but
uses the affected object’s luma value to define transparency. Stencil Luma is useful if the
object you want to use for cropping has no alpha channel of its own.
341 Chapter 7 Basic Compositing