Apple Final Cut Express 4 - Manual (Page 830)

830 Part IX Effects and Color Correction
Changing a Clip’s Alpha Channel Type
When you import an image file, Final Cut Express identifies the type of alpha channel
and sets the imported clip’s Alpha property appropriately. You should usually not
change this setting, except in rare circumstances where Final Cut Express identifies an
image alpha channel as premultiplied black or premultiplied white and you are certain
that the alpha channel type is actually straight.
Important: During import, Final Cut Express identifies an alpha channel as straight when
the pixels do not meet the mathematical requirements to be a valid premultiplied image.
If you reassign this clip’s Alpha property Black or White, you will introduce unexpected
artifacts anywhere the alpha channel is neither fully transparent nor fully opaque. This
usually results in light or dark edges where the edges should be anti-aliased.
Also, some editing and compositing applications output alpha channels that are the
reverse of what Final Cut Express uses. When this happens, what’s supposed to be
transparent is solid, and what’s supposed to be solid is transparent.
In these cases, you can change or reverse the alpha channel setting.
To change a clip’s alpha channel type using the Item Properties window:
1 Do one of the following:
 Select a clip in the Browser, then choose Edit > Item Properties > Format (or press
Command-9).
 Control-click a clip in the Browser, then choose Item Properties > Format from the
shortcut menu.
The Item Properties window appears.
2 Control-click the Alpha row in the Clip column, then choose the alpha channel type
from the shortcut menu.
 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.
 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 multilayer sequence.