Apple Motion 4 - Manual (Page 1165)

You can feather mask edges either outward or inward. The direction in which you feather
a mask is dictated by the subject you’re masking. When rotoscoping a subject, a
combination of masks using both inward and outward feathered masks may be necessary
for the best results.
Feathered in (color channel) Feathered in (alpha channel)
Feathered out (color channel) Feathered out (alpha channel)
To feather the edge of a mask
1 Select a mask.
2 Open the Mask tab in the Inspector.
3 Do one of the following:
• Drag the Feather value slider to the right for a positive value to feather the mask
outward.
• Drag the Feather value slider to the left for a negative value to feather the mask inward.
Applying Filters to Masks
As with shapes, you can apply filters to masks, which remain editable even after the
application. Although uses for this technique might not be immediately obvious, it’s
actually an extremely powerful way to further manipulate the alpha channel created by
one or more masks.
1165 Chapter 17 Using Shapes, Masks, and Paint Strokes