5.5

849Motion User Guide
Add a mask to a 3D group
You can apply masks to the layers in a 3D group, but not the 3D group itself—unless you
flatten the 3D group).
1. Select a 3D group in your Motion project.
2. In the Group Inspector, select the Flatten checkbox.
3. Apply a mask to the group.
Combining multiple masks in Motion
Often you may find that it’s impossible to create the mask shape you need using only a
single mask. For example, when masking an image that has a hole in it, you need at least
two masks—one with its blend mode set to Add (to mask the overall object), and a second
one with its blend mode set to Subtract (to cut the necessary hole in the middle of the
image). In the following example, the only way to mask the space between the diver’s arms
and legs is to create three overlapping masks, two with blend modes set to Subtract.
Multiple masks are also useful when you need to mask an image with complex contours
or when you’re rotoscoping an object with multiple moving parts. See Rotoscope a mask’s
shape in Motion.
Whatever the reason, you can easily combine multiple masks for any object by adjusting
each mask’s Mask Blend Mode parameter in the Mask Inspector.
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.
Blend mode operations are also determined by the order of each mask in the Layers
list, particularly with the Replace and Intersect blend modes. For example, a mask set to
Intersect affects all masks nested underneath it in the Layers list, masking only the areas
of the layer where the masks overlap one another. Similarly, a mask set to Replace mode
effectively turns off any masks beneath it in the Layers list (including the image layer’s
built-in alpha channel, if one exists).
For more information on mask blend modes, see Mask controls in the Inspector in Motion.