Adobe Premiere Pro CC v.7.xx - Manual (Page 544)

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Note:
Alpha channels store transparency information in files in one of two ways: straight or premultiplied. Although the alpha channels are the same, the
color channels differ.
With straight (or unmatted) channels, transparency information is stored only in the alpha channel, not in any of the visible color channels. With
straight channels, the effects of transparency aren’t 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
before you receive the assets to edit and composite. Premiere Pro and After Effects recognize both straight and premultiplied channels, but only
the first alpha channel they encounter in a file containing multiple alpha channels. Adobe Flash recognizes only premultiplied alpha channels.
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, an especially useful technique when you
work 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 don’t 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.
Keying in Premiere Pro is performed with keying effects. For more information, see Keying effects.
Blend clips with a Garbage Matte effect
You can use a Garbage Matte effect to blend images. For more information about using the Garbage Matte effect, see Eight-Point, Four-Point,
and Sixteen-Point Garbage Matte effects.
Blend clips with a matte using the Track Matte Key effect
You can blend clips with a matte (typically, a black and white image, or an image from the Titler.) using the Track Matte Key effect. For more
information, see the article, Track Matte Key effect.
This video shows how to create a track matte effect.
Specify clip opacity in a Timeline panel
1. Expand the view of a track, if necessary, by clicking the triangle next to the track name to expand its options.
2. If necessary, click the Show Keyframes button , or the Hide Keyframes button , and choose Show Opacity Handles from the menu. A
horizontal opacity control rubber band appears in all the clips of the track.
If no keyframes exist on the track, the rubber band appears as a straight horizontal line across the entire track.
3. In a Timeline panel, do one of the following:
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