Image files with alpha channels store transparency
information in one of two ways: straight or premultiplied. Although
the alpha channels are the same, the color channels differ.
With straight (or unmatted)
channels, transparency information is stored only in the alpha channel,
not in any of the visible color channels. With straight channels, the
effects of transparency aren’t visible until the image is displayed
in an application that supports straight channels.
With premultiplied (or matted)
channels, transparency information is stored 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 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 channels are premultiplied;
otherwise, the background color is usually black or white.
Straight
channels retain more accurate color information than premultiplied channels.
Premultiplied channels are compatible with a wider range of programs, such
as Apple QuickTime Player. Often, the choice of whether to use images
with straight or premultiplied channels has been made before you
receive the assets to edit and composite. Adobe Premiere Pro and
After Effects recognize both straight and premultiplied channels,
but only the first alpha channel they encounter in a file containing
multiple alpha channels. Adobe Flash recognizes only premultiplied
alpha channels.
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
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 (lower-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 (lower-right).
PDF (adobe.com)
