Adobe Premiere Pro CS3 - Manual (Page 373)

ADOBE PREMIERE PRO CS3
User Guide
367
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 an Adobe Premiere Pro Monitor panel,
white indicates complete opacity, black indicates complete transparency, and shades of gray indicate partial trans-
parency.
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 PSD (from Adobe Photoshop), ElectricImage, FLV, F4V,
TGA, TIFF, EPS, PDF, and AI (from Adobe Illustrator). AVI and QuickTime files (saved at a bit depth of Millions
Of Colors+), also can contain alpha channels, depending upon the codecs used. For AI, EPS, and PDF files, After
Effects automatically converts empty areas to an alpha channel.
A
B C
April 1, 2008