About alpha channels
Ordinary video clips and image files have three channels of color information: red, green, and blue. Many video and image file formats also support an additional alpha channel, which contains information defining areas of transparency. An alpha channel is a grayscale channel where white represents areas of 100 percent opacity (solid), gray regions represent translucent areas, and black represents 0 percent opacity (transparent).
When you import a QuickTime movie or an image file into a project, its alpha channel is immediately recognized by Motion. The alpha channel is then used to composite that object against other objects behind it in the Canvas.
There are two ways to embed alpha channel information into files. Motion attempts to determine which of these methods a media file uses:
Straight: Straight alpha channels are kept separate from the red, green, and blue channels of an image. Media files using straight alpha channels appear fine when used in a composition, but they can look odd when viewed in another application. Translucent effects such as volumetric lighting or lens flares in a computer-generated image can appear distorted until the clip is used in a composition.
Premultiplied: The transparency information is stored in the alpha channel as well as in the visible red, green, and blue channels, which are multiplied with a background color (generally black or white).
The only time it really matters which kind of alpha channel an object has is when Motion doesn’t correctly identify it. If a media item’s alpha channel is set to Straight in the Media list when it’s really premultiplied, the image can appear fringed with the premultiplied color around its edges. If this happens, select the problematic item in the Media list, then change its Alpha Type parameter in the Media Inspector.