Apple Motion 4 - Manual (Page 803)

To use text to mask an image
1 In the Layers tab or Canvas, select the layer you want to use as the texture.
2 Choose Object > Add Image Mask (or press Command-Shift-M).
A blank image mask layer is added to the image.
3 Drag the text that you want to use as a mask to one of the following:
• The Mask Source well in the Image Mask HUD
• The Image Mask layer in the Layers tab
• The Image well in the Image Mask tab of the Inspector
803 Chapter 13 Creating and Editing Text