Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 68

Specify alpha channel interpretation
Alpha channels store transparency information in one of two ways: straight or premultiplied. Although the alpha
channels are the same, the color channels differ. Straight (unmatted) channels store transparency information only
in the alpha channel, not in any of the visible color channels. Premultiplied (matted) channels store transparency
information in the alpha channel and also in the visible RGB channels, which are multiplied with a background color.
The colors of semitransparent areas, such as feathered mask edges, are shifted toward the background color in
proportion to their degree of transparency.
Setting the alpha channel interpretation correctly can prevent problems when you import a file, such as undesirable
colors at the edge of an image or a loss of image quality at the edges of the alpha channel. For example, if channels
are interpreted as straight when they are actually premultiplied, semitransparent areas will retain some of the
background color. If a color inaccuracy, such as a halo, appears along the semitransparent edges in a composition,
try changing the interpretation method.
A footage item with premultiplied channels (top) appears with a black halo when interpreted as Straight-Unmatted (bottom left). When the
footage item is interpreted as Premultiplied-Matted With Color and the background color is specified as black, the halo does not appear (bottom
right).
See also
"About straight and premultiplied channels" on page 253
Set the alpha channel interpretation for a footage item
1 In the Project panel, select a footage item.
Choose File > Interpret Footage > Main.
2
3 If you want to switch the opaque and transparent areas of the image, select Invert Alpha.
In the Alpha section, select an interpretation method:
4
Attempts to determine the type of channels used in the image. If After Effects cannot guess confidently, it
Guess
beeps.
Disregards transparency information contained in alpha channel.
Ignore
Interprets the channels as straight.
Straight - Unmatted
Premultiplied - Matted With Color
specify the color of the background with which the channels were premultiplied.
Interprets channels as premultiplied. Use the eyedropper or color picker to
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