Apple Final Cut Pro 7 - Manual (Page 1245)

3 Move the playhead to the last frame of the clip, then readjust the Point 1, Point 2, Point
3, and Point 4 point controls to fit the new position of the green screen.
Click point controls
to make an adjustable
crosshair appear in
the Viewer or Canvas.
Create keyframes for all
four points.
Keyframes are created automatically when you make these adjustments. The change of
the matte’s shape and position is calculated for each frame in between the two sets of
keyframes you’ve created. If the motion of the camera is consistent, the position of the
matte should follow the position of the background screen. If the motion of the camera
is inconsistent, you can create more keyframes to adjust the shape and position of the
four matte points as necessary.
Using Masks to Replace or Modify Alpha Channels
The mask filters allow you to replace, contract, or expand a clip’s alpha channel. If a clip
has no alpha channel, a new one can be assigned to it using a still image or an alpha
channel copied from another clip.
Mask Filters Available in Final Cut Pro
There are several mask filters you can use to modify alpha channels in a clip:
• Mask Feather: Lets you selectively feather the edge of just the alpha channel of a clip,
making it softer, without blurring the image channels of the clip. The Mask Feather
filter works with other filters that generate alpha channel information, allowing you to
feather the edge of a mask created with the Mask Shape filter, for example.
• Image Mask: Uses the luma or alpha channel in one clip to create an alpha channel in
another. The clip used to create the alpha channel can be a still image or a QuickTime
movie. Using the Image Mask filter, you can create complex alpha channels; for example,
you can put the kind of rough border used in art photography around a video clip. The
alpha channel created with the Image Mask filter can be inverted, if necessary.
1245 Chapter 72 Keying, Mattes, and Masks