Using Alpha Channels

Alpha channels are an implicit fourth channel in some graphics file formats that are used to reveal only certain parts of your still image. Alpha channels are useful when compositing a still image above another still or video clip in the Timeline. Make sure you save your still-image file with the alpha channel included if you want to use the still-image file for compositing. For more information about working with alpha channels, see Using Video and Graphics Clips with Alpha Channels.