Apple Motion 4 - Manual (Page 1174)

Roundness: Controls the roundness of the corners of a mask. This facilitates the creation
of rounded rectangle masks, as well as generally smoothing the edges of any mask.
Preserve Scale: This checkbox controls whether the Roundness setting is absolute or
relative to the overall mask size. When enabled, the roundness will remain at the same
approximate percentage of curvature as the object is scaled. When disabled, the curvature
will vary as the overall mask changes size.
Feather: A slider that feathers (softens) the edges of a mask. Positive feathering values
soften the edge of the mask from its edge outward. Negative feathering values soften
the edge of a mask inward from the edge. Feathering the edge of a mask can soften a
harsh rotoscoping job, making the masked object blend more easily with the background.
Mask Color: A pop-up menu that controls the color of the mask as displayed in the Canvas
when it is selected. This setting has no effect on the final output. Setting masks to different
colors may aid you in identifying which mask is which.
Control Points: Click the disclosure triangle to display the position parameters for the
mask control points. Use the value sliders to adjust the position of a control point. The
left value slider represents X and the right value slider represents Y.
Applying Image Masks to a Layer
Another way you can create transparency in a layer is by using image masks. An image
mask creates transparency in a layer by deriving an alpha channel from another layer,
such as a shape, text, movie, or still image.
Text layer Image layers Layers masked by text layer
Note: You can use masks and image masks together.
The power of image masks is that they do not have to be drawn or animated. Instead,
you can use virtually any image or movie clip to create transparency in another layer. By
default, movie clips create animated image masks, but you can also set an image mask
to use only a single frame.
1174 Chapter 17 Using Shapes, Masks, and Paint Strokes