Apple Final Cut Pro 5 - Manual (Page 1196)

Chapter 19 Keying, Mattes, and Masks 409
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4 Apply the Mask Feather filter to the clip you want to blur.
5 Open this clip into the Viewer, then click the Filters tab.
6 Move the Mask Feather filter so it appears below any other filters that may be
creating or modifying the clip’s alpha channel (such as the Image Mask filter that’s
already been applied).
7 Adjust the Softening slider to soften the edge of the clip’s alpha channel.
The clip’s image remains
untouched.