Apple Motion 4 - Manual (Page 256)

Alpha Type: An object’s alpha channel contains information that defines areas of
transparency in the image or movie. When you import a QuickTime movie or an image
file, its alpha channel is immediately recognized by Motion. There are six different ways
to embed alpha channel information into files, which correspond to the options in this
pop-up menu. Motion automatically assigns one of these options based on an analysis
of the object when it is imported, but you can override this if necessary.
The six commands in this pop-up menu are:
• None/Ignore: The default setting for objects with no alpha channel. This option also
allows you to ignore an object’s existing alpha channel, so that the entire object is
solid.
• Straight: Straight alpha channels are kept completely separate from the red, green,
and blue channels of an image. Media files using straight alpha channels appear perfectly
fine when used in a composition, but they may look odd when viewed in another
application. Translucent effects such as volumetric lighting or lens flares in a
computer-generated image may appear distorted until the clip is used in a composition.
If Straight is chosen, but you see a black, white, or colored fringe around the object,
this parameter is incorrectly set and should be changed to one of the Premultiplied
options, depending on the color of the fringe.
• Premultiplied-Black: This type of alpha channel is multiplied with the clip’s red, green,
and blue channels. As a result, objects with premultiplied alpha channels always look
correct, even with translucent lighting effects, because the entire image is
precomposited against a solid color. This option interprets alpha channels that have
been precomposited against black.
• Premultiplied-White: This option interprets alpha channels that have been precomposited
against white.
• Premultiplied-Color: This option interprets files that have been precomposited against
another color.
• Guess Alpha Type: This option forces Motion to analyze the file in an attempt to
automatically figure out what kind of alpha channel it uses. If you’re unsure, use this
setting.
Invert Alpha: Ordinarily, an alpha channel is a grayscale channel, where white represents
areas of 100 percent opacity (solid), gray regions represent partially opaque areas, and
black represents 0 percent opacity (transparent). If an alpha channel has been incorrectly
generated in reverse, this checkbox inverts it.
Pixel Aspect Ratio: Defines whether the object was created using square or nonsquare
pixels. In general, objects created for computer display, film, and high definition video
use square pixels, while objects created for standard definition video formats use
nonsquare pixels. A text field to the right of this pop-up menu displays the numeric aspect
ratio, in case you need to manually change the ratio. By correctly identifying each object
you add to your project, you can mix and match both kinds of media.
256 Chapter 6 Creating and Managing Projects