Adobe AFTER EFFECTS 5.5 Help Manual page 31

Adobe After Effects Help
Using Help | Contents
Using straight or premultiplied alpha channels
Footage files with alpha channels fall into two categories: straight and premultiplied.
Although the alpha channels are the same, the color channels differ.
With a straight alpha channel, a footage item keeps the transparency information in a
separate channel (the alpha channel only), not in any of the visible color channels. This
kind of alpha channel is also known as unmatted alpha. With a straight alpha channel, the
effects of transparency are not visible until the image is displayed in an application that
supports straight alpha.
With a premultiplied alpha channel, a footage item keeps the transparency information in
the alpha channel and also keeps the same information in the visible RGB channels, which
are modified, or multiplied, with a background color. A premultiplied alpha channel is
known as matted alpha with a background color. The colors of semitransparent areas, such
as feathered edges, are shifted toward the background color in proportion to their degree
of transparency.
Some software lets you specify the background color with which the alpha channel is
premultiplied; otherwise, the background color is usually black or white.
Straight alpha channels can produce better results when used in movies that require the
highest level of color precision. Premultiplied alpha channels are compatible with a wider
range of programs, such as Apple QuickTime Player. After Effects recognizes both types, so
for most projects, either type will produce satisfactory results.
Interpreting alpha channels as straight or premultiplied
When you import a footage file containing an alpha channel, After Effects determines if
it is straight or premultiplied. If the alpha channel is unlabeled, After Effects displays the
Interpret Footage dialog box and prompts you to select Ignore Alpha, Straight
(Unmatted), Premultiplied (Matted with Black), Premultiplied (Matted with White), or
Guess. When you select Guess, After Effects determines the interpretation method. For
information on straight and premultiplied alpha channels, see "Using straight or premulti-
plied alpha channels" on page 31. For information on interpretation methods, see
"Choosing an interpretation method for alpha channels" on page 31.
Correctly identifying the type of alpha channel when you import a file can prevent
problems, such as undesirable colors at the edge of an image or a loss of image quality at
the edges of the alpha channel. If a color inaccuracy, such as a halo, appears along the
edges of an alpha channel in a composition, try changing the interpretation method.
If a footage item doesn't have an alpha channel or transparency information, you can
create an alpha channel in After Effects using a layer mask. See "Working with masks" on
page 149.
Choosing an interpretation method for alpha channels
You can change the interpretation method for a file after you import the file. You can also
change the default alpha channel interpretation in the Import Preferences dialog box, in
the Interpret Unlabeled Alpha As pop-up menu. This is especially useful for importing
footage from an application that consistently uses one type of unlabeled alpha channel.
The interpretation methods include:
Ask User Displays a dialog box containing interpretation options each time you import
an unlabeled alpha channel.
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