Adobe 65021048 User Guide - Page 374

ADOBE PREMIERE PRO CS3 368 User Guide 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. Adobe 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. Defining transparent areas with keys Apply a key to a clip A key effect defines transparent areas in a clip based on values such as color or brightness. Use color-based keys to knock out a background, brightness keys to add texture or special effects, alpha channel keys to modify a clip's alpha channel, and matte keys to add traveling mattes or apply other clips as mattes. 1 In the Effects panel, click the triangle to expand the Video Effects bin and then click the triangle to expand the Keying bin. 2 Drag a key to a clip in the Timeline panel. April 1, 2008