Motion 5: Alpha type analysis of media with an alpha channel may not be determined correctly

When a media file with an alpha channel is imported into Motion, the alpha type of the alpha channel may not be determined correctly.

In the Media pane of the Inspector, choose the appropriate option (detailed below) from the Alpha Type pop-up menu to manually set the Alpha Type for the selected media.

  • 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.

 

More About Alpha Channels
Ordinary video clips and image files have three channels of information, one each for the red, green, and blue channels. Many video and image file formats also support an alpha channel which contains additional information that defines areas of transparency. 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).

When you import a QuickTime movie or an image file into your project, its alpha channel is immediately recognized by Motion. The alpha channel is then used to composite that object against any other objects that are behind it.
There are two different ways of embedding alpha channel information into files, and Motion attempts to automatically determine which type of alpha channel a particular object uses:

  • 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.
  • Premultiplied: 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. Most commonly, premultiplied alpha channels are multiplied against black or white.

The only time it really matters which kind of alpha channel an object has is when Motion doesn’t correctly determine it. If an object’s alpha channel has been set to Straight in the Media list when it’s really premultiplied, it may appear fringed with the premultiplied color around its edges. If this happens, you can select the problematic clip in the Media list, then change its Alpha Type parameter in the Media pane of the Inspector.

This document will be updated as more information becomes available.

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