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Chapter 12 Using Shapes and Masks 1105
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.
Image masks can also be used to assign masks that were created in other applications.
For example, you can import an animated mask that was created in another application
and exported as a QuickTime movie into your Motion project, then use it as an image
mask.
When you use a layer as an image mask, you can choose which of the layer’s channels
to apply to create transparency via the Source Channel pop-up menu in the Image
Mask tab of the Inspector. The choices include:
 Red
 Green
 Blue
 Alpha
 Luminance
Because alpha channels are basically 8-bit grayscale images, you can use any single
color channel as an image mask. You can also use another layer’s alpha channel.
Luminance allows you to use the aggregate luminance from the red, green, and blue
channels of an image to create transparency. For all these options, color is ignored.
Assigning an image mask is a two-part process. First, you create a blank image mask
underneath a layer. Then you assign the image you want to use to create transparency.
To add an image mask to a layer:
1 Select the layer you want to mask.
2 Choose Object > Add Image Mask (or press Shift-Command-M).
The image mask appears underneath the layer in the Layers tab and Timeline.
3 With the image mask selected, do one of the following: