Apple Final Cut Express HD - Manual (Page 797)

Chapter 54 Compositing and Layering 797
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Types of Alpha Channels Recognized in Final Cut Express HD
Final Cut Express HD recognizes three kinds of alpha channels. A fourth state indicates
there’s no alpha channel available; you can choose “None/Ignore” to ignore an existing
alpha channel. You can determine how Final Cut Express HD interprets a clip’s alpha
channel in the Item Properties window or in the Browser. There is also a clip property
to reverse the available alpha channel.
 Straight: Typically generated by a 3D animation or compositing program, straight
alpha channels contain only the pixels rendered for an image with no background
(although the background appears black). Because of this, movies with translucent
effects, such as volumetric lighting or lens flares, appear distorted unless they’re
composited with something in the background. Movies or images with a straight
alpha channel are easy to composite and always have very clean edges when
composited in a multilayered sequence.
 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.
It’s important that graphics with premultiplied alpha channels be correctly identified.
If a graphics file with a premultiplied black alpha channel is incorrectly identified as
having a straight alpha channel, a black fringe appears around the edges of your
imported image. If this happens, you can simply change the way the alpha channel is
used in the clip’s Item Properties window.
 White: Works the same as the black alpha channel, except that the clip is
precomposited against white.
 None/Ignore: If a clip has no alpha channel, or if you want to disable the alpha
channel of a clip so that it’s no longer used, you can use the None/Ignore setting.
 Reverse Alpha: When this property is checked, the clip’s alpha channel is reversed.