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Adobe Premiere Pro CC - 2015 Manual

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Compositing
Last updated 11/30/2015
When you view an alpha channel in the After Effects Composition panel or a Premiere Pro Monitor panel, white
indicates complete opacity, black indicates complete transparency, and shades of gray indicate partial transparency.
A matte is a layer (or any of its channels) that defines the transparent areas of that layer or another layer. White defines
opaque areas, and black defines transparent areas. An alpha channel is often used as a matte, but you can use a matte
other than the alpha channel if you have a channel or layer that defines the desired area of transparency better than the
alpha channel does, or in cases where the source image doesn’t include an alpha channel.
Many file formats can include an alpha channel, including Adobe Photoshop, ElectricImage, TGA, TIFF, EPS, PDF, and
Adobe Illustrator. AVI and QuickTime (saved at a bit depth of Millions Of Colors+), also can contain alpha channels,
depending upon the codecs used to generate these file types.
For instructions on how to export video with an alpha channel (transparency), see this post.
About straight and premultiplied channels
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 .
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