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Chapter 4. Layers and Channels > Alpha Channels: Storage Space for Special Info...

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, you can add a channel of grayscale visual information to an image for purposes we'll discuss in a moment. The Targa format can hold one extra channel, as can the Macintosh PICT files. TIFF images can hold several extra alpha (information) channels, and Photoshop's native PSD file format can hold up to 32 channels (a ridiculously luxurious amount). Before Photoshop refined the Layers feature, alpha channels were a must, because this is the place you could store and retrieve selection areas you created with the Lasso tool or the Quick Mask tool. Suppose you were in the middle of some real intricate selection work, and Mom called you for dinner. No problem; you saved your work to an alpha channel.


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