Adobe Premiere Pro CS4 - Manual (Page 424)

418 USING ADOBE PREMIERE PRO CS4
Compositing
Last updated 11/6/2011
Transparency (masks, alpha channels)
About transparency
Before you can create a composite from multiple images, parts of one or more of the images must be transparent. You
can use alpha channels, masks, mattes, or keying to define which parts of an image are transparent and which parts of
an image can be used to obscure parts of another image. By manipulating transparency and choosing blending modes,
you can create a variety of visual effects.
About alpha channels and mattes
Color information is contained in three channels: red, green, and blue. In addition, an image can include an invisible
fourth channel, called an alpha channel, that contains transparency information.
Channels at a glance
A. Separated color channels B. Alpha channel C. All channels viewed together
An alpha channel provides a way to store images and their transparency information in a single file without disturbing
the color channels.
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, FLV, 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.
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.
A
B C