Apple Final Cut Express 4 - Manual (Page 854)

854 Part IX Effects and Color Correction
Using Mattes to Add or Modify Alpha Channels
You can use the matte filters to create a new alpha channel or add to or subtract from a
clip’s existing alpha channel. Matte filters work equally well with clips that already have
an alpha channel and with clips that have alpha channels created using a key filter,
such as the Chroma Keyer.
Matte Filters Available in Final Cut Express
There are three matte filters used most often in conjunction with the Chroma Keyer:
 Eight-Point Garbage Matte: Use this filter to create an eight-point polygonal matte.
This is useful for cropping out unwanted elements surrounding a foreground
element with a complex shape.
 Four-Point Garbage Matte: Use this filter to create a simple four-point polygonal
matte. This simplifies the process when the shape you want to remove can be
encompassed in four points.
 Matte Choker: Use this filter to eliminate any faintly remaining blue or green fringing
or pixels surrounding the edge of your foreground subject. Instead of simply
shrinking the size of the blue-screen clip’s alpha channel, however, which might
result in the accidental elimination of part of your foreground subject, the Matte
Choker selectively shrinks only those parts of the layer’s alpha channel that have a
marginal degree of transparency, such as barely keyed-out blue or green fringing.
Note: Instead of relying solely on the feathering and edge thinning controls of the
Chroma Keyer filter, you should allow some fringing when you adjust the Chroma Keyer’s
settings, so that you can more precisely eliminate fringing with the Matte Choker.
Example: Using the Four-Point Garbage Matte Filter
The following example illustrates how to use the Four-Point Garbage Matte filter to
eliminate the outside edges of a clip being keyed.
1 Apply the Four-Point Garbage Matte to the clip being keyed. Open this clip in the
Viewer, then click the Filters tab.
∏
Tip: It may be easier to see what you’re doing if you disable the Chroma Keyer, Matte
Choker, and Spill Suppressor filters first.
2 In the Four-Point Garbage Matte filter, click the Point 1 point control.
Click the Point 1 point
control to start changing
the matte.