Apple Motion 4 - Manual (Page 1163)

Whatever the reason, you can easily combine multiple masks for any object by adjusting
each mask’s Mask Blend Mode parameter in the Inspector to create any combination.
Mask Blend Mode
pop-up menu
Each mask’s blend mode determines whether the mask adds to, subtracts from, or replaces
a layer’s previously existing alpha channel. Additionally, blend modes affect how masks
interact with one another. Ultimately, every mask applied to a layer combines according
to the specified blend modes to create that layer’s final alpha channel. Mask Blend Modes
can be changed at any time.
Blend mode operations are also determined by the order in which each mask appears in
the Layers tab, particularly with the Replace and Intersect modes. For example, a mask
set to the Intersect mode masks out all other regions of a layer that do not overlap it. A
mask set to Intersect affects all masks nested underneath it. Masks added above it are
unaffected by this behavior.
Masks That Add and Subtract
New masks always default to the Add blend mode, allowing you to add to an existing
alpha channel without cropping what’s already there. Once you’ve drawn the first mask,
you may need to use additional masks set to Add or Subtract to refine it.
Masks set to Add create white areas in the alpha channel, making those parts of the object
opaque. This can be useful when masking several unconnected subjects in a single image.
Result of first mask Additional masks added to the first
Note: To display the rest of a masked layer so you can trace it to create a second mask,
turn off the first mask in either the Layers tab or Timeline. The outline of the first mask
remains visible, even though it’s no longer affecting the layer’s transparency.
1163 Chapter 17 Using Shapes, Masks, and Paint Strokes