Using straight or premultiplied alpha channelsFootage 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. |