Apple Motion 3 - Manual (Page 1087)

Chapter 12 Using Shapes and Masks 1087
Masking Layers to Create Transparency
When you create a mask to define transparency within a layer, you’re drawing a shape
to use as that layer’s alpha channel. Because a layer’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 layers that already have an alpha channel, and can add to,
subtract from, intersect, or replace the layer’s original alpha channel.
Masking a Layer or Group
Use the mask tools to create different shapes with which to mask a layer or group.
There are five mask tools available that are analogous to the five shape drawing tools
covered earlier in this chapter:
 Rectangle Mask
 Circle Mask
 Freehand Mask (used like the Paint Stroke tool)
 Bezier Mask
 B-Spline Mask
Note: You can also use an Image Mask to mask a layer. For more information, see
“
Applying Image Masks to a Layer ” on page 1104.
Object alpha channel Masked object