Interpreting alpha channels as straight or premultiplied

When you import a footage file containing an alpha channel, After Effects determines whether 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, Straight - Unmatted, Premultiplied - Matted with Color, or Guess. When you select Guess, After Effects determines the interpretation method. For information on straight and premultiplied alpha channels, see Using straight or premultiplied alpha channels. For information on interpretation methods, see Choosing an interpretation method for alpha channels.


A footage file with a black alpha channel (left) appears with a black halo (center) when interpreted as Straight-Unmatted, or Premultiplied-Matted with Color (white selected). When the footage file is interpreted as Premultiplied-Matted with Color (black selected) the halo does not appear (right).

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 by using a layer mask. (See Working with masks.)