Apple Motion 3 - Manual (Page 1106)

1106 Chapter 12 Using Shapes and Masks
 Open the Image Mask tab in the Inspector, then drag the layer you want to use as
the mask into the Mask Source well.
 Drag the layer you want to use as the mask into the Mask Source well in the HUD.
Important: Be sure to click and drag in one movement to successfully drop a layer in
an image well. If you select the layer to use as the source, then release the mouse
button, you lose the selection.
In this example, text is used as the mask layer.
4 Choose the channel you want to use to create transparency from the Source Channel
pop-up menu, as well as any other options that are necessary to create the required
transparency.
Image Mask Parameters
Once you’ve added an image mask to a layer, the following parameters become
available in the Image Mask tab of the Inspector.
Mask Source: An image well that assigns a layer (a shape, text, an image, a movie,
particle emitter, numbered image sequence, and so on) to use to mask a layer.
Mask Source well