Apple Final Cut Express 4 - Manual (Page 831)

Chapter 52 Compositing and Layering 831
IX
 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.
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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.
To change the alpha channel type of a clip in the Browser:
m Control-click in the Alpha column in the Browser, then choose a new alpha channel
type from the shortcut menu.
To reverse a clip’s alpha channel:
m Do one of the following:
 Select a clip in the Timeline or open a clip in the Viewer, then choose Modify >
Reverse Alpha.
 In the Browser, Control-click in the Reverse Alpha column next to the clip you want
to modify, then choose Yes from the shortcut menu.
If there is a checkmark by the Reverse Alpha menu item, the clip’s alpha channel is
being reversed. If there is no checkmark, the alpha channel is left alone.