Apple Final Cut Pro 6 - Manual (Page 1198)

Chapter 19 Compositing and Layering 413
II
 Black: Also known as premultiplied (with black). Most other alpha channels, especially
channels that are hand drawn in Photoshop, are premultiplied. This means that,
unlike a movie with a straight alpha channel that contains only the pixels of the
foreground image, movies with alpha channels premultiplied against black always
look right, even if they have translucent lighting effects. This is because the entire
image is precomposited against black.
 White: Also known as premultiplied with white. Works the same as the black alpha
channel, except that the clip is precomposited against white.
∏
Tip: If necessary, you can also Control-click the Reverse Alpha row in the Clip column
and choose No or Yes from the shortcut menu.
To change the alpha channel type of a clip using the Alpha Type menu item:
1 Select a clip in the Timeline or open a clip in the Viewer.
2 Choose Modify > Alpha Type, then choose a new alpha channel type from the submenu.
Control-click in the Alpha
row, then choose an
alpha channel option
from the shortcut menu.