Apple Motion 2 - Manual (Page 186)

186 Chapter 2 Creating and Managing Projects
• 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.
Note: PDF files with transparent backgrounds do not have the Alpha Type or Invert
Alpha parameters.
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.
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