Expand or contract the edges of a mask

To expand or contract the area influenced by a mask, use the Mask Expansion property.

Mask expansion affects the alpha channel but not the underlying mask path; the mask expansion is essentially an offset that determines how far, in pixels, from the mask path the influence of the mask on the alpha channel extends.

  1. In the Timeline panel, expand the Mask properties of the layer you want to adjust.
  2. Drag the underlined value for Mask Expansion.

Todd Kopriva provides a visual aid and further explanation regarding mask expansion—and why it creates rounded corners—on his blog on the Adobe website.