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 Pro

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.

To use the Four-Point Garbage Matte Filter
  1. Apply the Four-Point Garbage Matte to the clip being keyed. Open this clip in the Viewer, then click the Filters tab.

    It may be easier to see what you’re doing if you turn off the Chroma Keyer, Matte Choker, and Spill Suppressor filters first.

  2. In the Four-Point Garbage Matte filter, click the Point 1 point control.

    Figure. Four-Point Garbage Matte filter options.
  3. Move the pointer to the Canvas (the pointer turns into a crosshair pointer), then click to change the location of the matte corner defined by Point 1.

    Figure. Canvas windows showing the results of changing the first point in a four-point garbage matte.
  4. Repeat steps 2 and 3 for the Point 2, Point 3, and Point 4 point controls, until you’ve created a box that crops out everything outside the green screen surrounding the actor.

  5. To round the four corners of the matte, adjust the Smooth slider, if necessary.

    Figure. Canvas window showing a four-point garbage matte with rounded corners.
  6. If necessary, adjust the Choke and Feather sliders.

    The Choke slider adjusts the overall size of the matte, without changing the shape; the Feather slider blurs the edges of the matte.

    Figure. Canvas window showing the finished matte image of a woman composited against a desert background.

    The parameters of the Garbage Matte filters can be keyframed, in the same way as the parameters of any other filter. This can come in handy if you need to use one of the Garbage Matte filters to crop out the outside edge of a green screen in a shot where the camera is panning.

Example: Keyframing Garbage Mattes

In the following example, two sets of keyframes are created to compensate for a simple pan in the clip being keyed.

To keyframe Garbage Mattes
  1. With the playhead on the first frame of a clip, apply and adjust the Four-Point Garbage Matte filter.

    For more details about using this filter, see Example: Using the Four-Point Garbage Matte Filter.

  2. Once you’ve created the proper shape for the first frame of the clip, click the keyframe buttons for the Point 1, Point 2, Point 3, and Point 4 parameters.

    Figure. Four-Point Garbage Matte options showing the keygrame graph area for the four points of the matte.
  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.

    Figure. Four-Point Garbage matte options showing changes in the keyframe graph area for the 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.