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
” on page 410.