Apple Motion 5.1.1 - Manual (Page 894)

Chapter 20 Shapes, masks, and paint strokes 894
Feather the edge of a mask
1 Select a mask.
2 In the Mask Inspector, do one of the following:
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Drag the Feather value slider to the right for a positive value to feather the mask outward.
Feathered out (alpha channel) Feathered out (color channel)
•
Drag the Feather value slider to the left for a negative value to feather the mask inward.
Feathered in (alpha channel) Feathered in (color channel)
67% resize factor