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With straight (or unmatted) channels, transparency information is only stored in the alpha channel, not in any of the
visible color channels. With straight channels, the effects of transparency are not visible until the image is displayed
in an application that supports straight channels.
With premultiplied (or matted) channels, transparency information is stored in the alpha channel and also in the
visible RGB channels, which are multiplied with a background color. The colors of semitransparent areas, such as
feathered edges, are shifted toward the background color in proportion to their degree of transparency.
Some software lets you specify the background color with which the channels are premultiplied; otherwise, the
background color is usually black or white.
Straight channels retain more accurate color information than premultiplied channels. Premultiplied channels are
compatible with a wider range of programs, such as Apple QuickTime Player. Often, the choice of whether to use
images with straight or premultiplied channels has been made for you when you receive the assets that you will be
editing and compositing. Fortunately, Adobe Premiere Pro and After Effects recognize both straight and premulti-
plied channels, so either type will produce satisfactory results for most projects.
About keying
Keying is defining transparency by a particular color value (with a color key or chroma key) or brightness value (with
a luminance key) in an image. When you key out a value, all pixels that have similar colors or luminance values
become transparent.
Keying makes it easy to replace a background of a consistent color or brightness with another image, which is
especially useful when working with objects too complex to mask easily. The technique of keying out a background
of a consistent color is often called bluescreening or greenscreening, although you do not have to use blue or green;
you can use any solid color for a background.
Difference keying defines transparency with respect to a particular baseline background image. Instead of keying out
a single-color screen, you can key out an arbitrary background.
Compositing video
About compositing
Compositing is the process of creating a compound image by superimposing one or more clips on another. You can
achieve this result in several ways: by applying keys, by using alpha channels (in clips that have them), by using the
opacity effect, and by simply scaling an image to reveal underlying images. All of these methods, except scaling,
require that part of a clip be transparent.
After Effects, another program in Production Studio, gives you a greatly expanded range of compositing tools. You
can easily import composites made in After Effects into Adobe Premiere Pro.
When part of a clip is transparent, transparency information is stored in its alpha channel. You can apply any combi-
nation of opacity, masks, mattes, and keying to modify the alpha channel. This partially or totally hides areas of a clip.
Adobe Premiere Pro refers to transparent areas using the following terms:
Defines transparent areas for the clip that contains the channel. An alpha channel is an extra channel
Alpha 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 trans-
parency 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
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