Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 276

Apply one or more keying effects to mask out the remainder of the background.
2
Apply Matte effects as necessary to fine-tune the matte.
3
See also
"Creating and importing masks" on page 254
"Matte effects" on page 448
Use a hold-out matte
Use a hold-out matte (also known as a hold-back matte) to patch a scene to which a keying effect has been applied.
A hold-out matte is a masked-out portion of a duplicate of a layer that you have keyed. The duplicate is masked to
include only the area of the image that contains the key color that you want to preserve as opaque. The hold-out matte
is then placed directly on top of the keyed layer.
A
C
Example of using a hold-out matte
A. Original bluescreen image. The number's background is also blue. B. After keying, the number's background is also transparent. C. Hold-
out matte containing the part of the image you want to remain opaque D. When the hold-out matte is placed on top of the keyed image, the
number's background is now opaque.
1 Duplicate the layer containing the color screen.
Note: Don't change Transform properties of only one of the layers after making the duplicate; keep the layers moving
together.
2 Apply keying effects and Matte effects to the original layer to create transparency.
On the duplicate layer, create masks to mask out everything in the image except the area that you want to preserve.
3
Make sure that the copy (the hold-out matte) is positioned directly on top of the keyed layer and contains exactly
4
the same keyframes with the same values. Then render the movie.
B
D
AFTER EFFECTS CS3 270
User Guide