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).