Using the Alpha Channel key

Images represented on video are composed of three grayscale images called channels--one red, one green, and one blue. Such images are called RGB images. An alpha channel is a fourth channel in an RGB image that defines what parts of the image are transparent or semitransparent. Many programs, such as Adobe Illustrator and Photoshop, use alpha channels so that you can specify transparent regions in an image. An alpha channel is either straight or premultiplied, depending on the program that created the image. When an image contains a straight alpha channel, the transparency information is stored only in the alpha channel. When an image contains a premultiplied alpha channel, the background color that is intended to be transparent is also blended into the edges of an image. Use the Alpha Channel key on clips that contain a straight alpha channel, such as images created in Adobe Photoshop, Adobe Illustrator, and Adobe After Effects.

When you use the Alpha Channel key with a clip to which you have applied or plan to apply motion, select the Use Clip's Alpha option in the Motion Settings dialog box. If this setting is not selected, the clip's bounding box will appear when the clip moves. See Specifying alpha channel and color options.

Superimposing and Compositing > Using keys to superimpose and create composites > Using the Alpha Channel key