Apple Final Cut Express HD - Manual (Page 826)

826 Part IX Effects
3 Choose Luminance from the Channel pop-up menu.
Note: When using a clip’s luminance to define transparency, white is used for
100 percent solid, and black is used for 100 percent transparent.
You can further modify this new alpha channel, blurring it with the Mask Feather filter,
for example.
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 now has a border that
matches the Border.tif clip.
The clip’s image remains
untouched.