Inside Photoshop® CS

Alpha Channels: Storage Space for Special Information

So far, we've played with what an image can provide—a number of color channels that together make up a color image. CMYK images have four channels, and Duotone images have two color channels. CMYK is a necessary color space because printing inks cannot capture RGB colors completely with fidelity. So cyan, magenta, yellow, and black simulate computer art colors. And Duotone is a color mode that uses a color plate in addition to a grayscale plate to make the gray tones look more rich. Truth be known, black ink doesn't provide total coverage on paper of the design you create. But Duotones are another story, covered later in this book.

In addition to the channel information that makes up an image, ...