Using a footage item with an alpha channel
An alpha channel is a fourth 8- or 16-bit channel (see Selecting 16-bpc color depth (PB only) for information on working with 16-bit color in After Effects), included in addition to the three 8- or 16-bit red, green, and blue color channels of video footage or an RGB image. When a file contains an alpha channel, its image is defined as containing a total of 32 or 64 bits, or using Millions of Colors + or Trillions of Colors +. An alpha channel has the same function as a key in analog video compositing or a matte in optical film compositing--it describes the areas of an image that are transparent. See Importing footage containing an alpha channel.
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