Apple Motion 4 - Manual (Page 322)

Preserve Opacity Option
The Preserve Opacity checkbox allows you to limit an object’s visibility to areas of the
Canvas where the object overlaps nontransparent regions of other objects. For example,
you might place two overlapping objects into your project, as shown below:
If you turn on Preserve Opacity for the Dolphin object in front, the result is this:
The only area of the Dolphin object that is visible is the area that overlaps the sea life
picture object behind it. At first, this may not appear to be very exciting, but the Preserve
Opacity checkbox can be used in combination with the opacity and blend mode to create
some very interesting effects.
Note: The object with the enabled Preserve Opacity parameter takes the opacity value
of the object beneath it in the composite stack.
322 Chapter 7 Basic Compositing