Adobe After Effects - Manual (Page 407)

Note:
Earlier versions of After Effects had the ability to add a feathered edge to a closed mask, but the width (extent) of the feather was the same
around the mask. A new Mask Feather tool (available in the same tool menu as the Pen tool) has been added to let you define points along a
closed mask that should have varying widths.
This video by Todd Kopriva and video2brain introduces the variable-width mask feathering feature. Assign multiple feather points to an object to
show how those feather points interact to create interesting effects.
To create a feather point:
A selected feather point has a small black dot at its handle.
Feather points define both the outer feather boundary and inner feather boundary. If no feather extent handles are inside the mask, the inner
feather boundary is the mask path. The mask feather extends from the inner to the outer feather boundary.
To select multiple feather points:
Using the Selection or Mask Feather tool, Shift-click the feather points' extent handles.
To toggle the selection of a feather point:
Shift-click the feather point.
To move a feather point, do either of the following:
Using the Selection or Mask Feather tool, drag the feather extent handle.
You can move multiple selected feather points in unison by dragging one of them, and scale multiple feather extents by dragging one of the
feather extent handles.
You can "sweep" feather points around corner points on a mask.
With the Mask Feather tool active, press Left or Right Arrow to move the feather along the path, or the Up or Down Arrow to lengthen or
shorten the feather's width.
You can move multiple selected feather points, and move longer distances/widths by holding down the Shift key.
To snap a feather point to the mask path:
Drag the feather extent handle across the mask path. It will stop at the mask path.
To control the falloff of the feather:
Choose Layer > Mask > Feather Falloff, and then select one of the following:
Smooth (default)
Linear
To adjust the tension (smoothness or curvature) of a feather boundary through a feather point:
Hold down the Alt (Windows) or Option (MacOS) key while dragging from a feather extent handle to adjust the tension. The Info panel shows the
current tension for a handle.
To set Hold interpolation for a feather point (for constant feather radius up to the next feather point):
Hold down the Alt (Windows) or Option (MacOS) key while dragging from a feather extent handle to adjust the tension. The Info panel shows the
current tension for a handle.
To set Hold interpolation for a feather point (for constant feather radius up to the next feather point):
Enable the Hold option from the context menu above the feather point. The feather point handle changes to be pointy in the direction of constant
radius.
To quickly create a feathered edge for a specific mask segment:
Hold down the Shift key as you click the mask segment (between vertices, not above them). The pointer changes to indicate that you are in this
mode. Drag from the segment to adjust the extent. Note the following behavior:
If just two mask vertices are selected, the feathered edge will be the contiguous segments between them.
If both vertices of the clicked segment are selected (that is, the segment is selected), the feathered edge will expand to include contiguous
selected segments.
If all or no vertices on the mask are selected, or if the first two conditions don't apply, the feathered edge will be only the clicked segment.
To delete a feather point:
Using the Selection or Mask Feather tool, select a feather extent handle, then press Delete. You can delete multiple selected feather points.
The pointer changes to selection mode instead of delete vertex mode when over a mask vertex.
Video tutorial: Mask Feather tool
403