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Difference Matte (PB only)
This matte creates transparency by comparing a source layer with a difference layer, and
then keying out pixels in the source layer that match both the position and color in the
difference layer. Typically, it is used to key out a static background behind a moving object,
which is then placed on a different background. Often the difference layer is simply a
frame of background footage (before the moving object has entered the scene). For this
reason, the Difference Matte Key is best used for scenes that have been shot with a
stationary camera.
A
A. The difference layer is typically a static frame of the movie before the subject enters the scene. B.
The background is keyed out by comparing the static frame (difference layer) with the source
layer. C. The source layer is composited onto a new background.
To apply the Difference Matte:
1 Select a motion footage layer as the source layer.
2 Find a frame in the source layer that consists only of background.
3 Save this background frame as an image file; then import it into After Effects and add it
to the composition. This is the difference layer. For more information, see "Exporting a
single frame of a composition" on page 292. Make sure that the duration of the difference
layer is at least as long as that of the source layer.
Note: If there is no full-background frame in the shot, you may be able to assemble the full
background by combining parts of several frames in After Effects or Adobe Photoshop. For
example, you can use the Photoshop rubber stamp tool to take a sample of the
background in one frame, and then paint the sample over part of the background in
another frame.
4 Turn off the display of the difference layer by clicking the Video switch in the Timeline
window.
5 Make sure that the original source layer is still selected, and then choose Effect > Keying
> Difference Matte.
6 In the Effect Controls window, choose Final Output or Matte Only from the View menu.
Use the Matte Only view to check for holes in the transparency. To fill undesired holes after
you complete the keying process, see "Matte Choker (PB only)" on page 211.
7 Select the background file from the Difference Layer menu.
8 If the difference layer is not the same size as the source layer, choose one of the
following controls from the If Layer Sizes Differ menu:
Center places the difference layer in the center of the source layer. If the difference layer
•
is smaller than the source layer, the rest of the layer is filled with black.
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