Adobe PREMIERE PRO 4.0 Help Manual page 215

Adobe Premiere Pro Help
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Superimposing and Compositing
Understanding transparency
Compositing is the process of creating a composite image by superimposing multiple
images. Because video frames are completely opaque by default, compositing requires
that parts of a video frame be transparent. When part of a clip is transparent, transparency
information is stored in the clip's alpha channel. You can combine partially transparent
clips using stacked tracks, and use a clip's color channel to create an effect in a clip on a
lower track.
Defining transparency terminology
To create composite clips, parts of each clip must be transparent. The terminology of
transparency varies by media and by software. You can apply any combination of opacity,
masks, mattes, and keying to modify the alpha channel, partially or totally hiding any or all
areas of a clip. Adobe Premiere Pro refers to transparent areas using the following terms:
Alpha channel A channel that defines transparent areas for the clip that contains the
channel. An alpha channel is an extra channel in addition to the visible color channels
(such as RGB). While the alpha channel indicates transparency, the channel itself is usually
hidden. With imported items, an alpha channel provides a way to store both the clip and
its transparency information in a single file without disturbing the footage item's color
channels. You can also ignore an existing alpha channel and use Adobe Premiere Pro
transparency effects to create a new one. When you view the alpha channel in the Monitor
window (see "Choosing a Display Mode setting" on page
opacity, black indicates transparency, and gray indicates partial transparency. Because an
alpha channel uses shades of gray to store transparency information, some effects can
apply a grayscale image (or the luminosity values of a color image) to an alpha channel.
Mask Sometimes used as another word for alpha channel; also describes the process of
modifying an alpha channel.
Matte A file or channel that defines or modifies the transparent areas of its clip or another
clip. You might use a matte when you have a channel or clip that defines the desired area
of transparency better than the alpha channel, or when a clip doesn't include an alpha
channel.
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Index
Index
Superimposing and Compositing
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