Manual

822 Part IX Effects
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, as well as with clips that have alpha channels created using a keying
filter, such as the Chroma Keyer.
Matte Filters Available in Final Cut Express HD
There are three matte filters used most often in conjunction with the Chroma Keyer.
 Eight-Point Garbage Matte: Use 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 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: This filter eliminates 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-screened 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 it 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 into 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 crosshair control.
Click the Point 1 crosshair
control to start changing
the matte.