Apple Motion 2 - Manual (Page 851)

Chapter 12 Using Shapes and Masks 851
Masking Objects to Create Transparency
When you create a mask to define transparency within an object, you’re drawing a
shape to use as that object’s alpha channel. Since an object’s alpha channel is a
grayscale channel, masks are grayscale objects, where white defines solid areas,
progressively darker levels of gray define decreasingly opaque areas, and black defines
areas of complete transparency. If you choose View > Channels > Alpha to set the
Canvas to display the alpha channel only, you can see the grayscale alpha channel
directly.
Masks can be used with objects that already have an alpha channel, and can add to,
subtract from, intersect, or replace the object’s original alpha channel.
Masking an Object
Use the mask tools to create different shapes with which to mask an object. There are
four mask tools available that are analogous to the four shape drawing tools covered
earlier in this chapter.
• Rectangle Mask
• Circle Mask
• Bezier Mask
• B-Spline Mask
Object alpha channel Masked object
01112.book Page 851 Sunday, March 13, 2005 10:36 PM