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Photoshop® CS4 Bible by Dan Moughamian, Simon Abrams, Stacy Cates

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10.3. Understanding Channels

Many people use "alpha channels" and "channels" synonymously when referring to Photoshop; however, it's important to realize that there are actually three types of channels: color channels; alpha channels; and spot color channels. All of them have one thing in common, and that is that they are all really just grayscale images hiding beneath your canvas. There are few topics that seem to strike fear into the hearts of new Photoshop users as much as channels do, but rest assured they are actually not complicated.

Once you get past the initial hesitation, you will see that channels are an important (if somewhat obscured) part of the digital workflow and not hard to leverage at all. At worst, they take a few extra steps to create, but the benefits can be well worthwhile.

10.3.1. Alpha channels

An alpha channel is nothing more than a selection that is displayed as a grayscale (sometimes pure black and white) image when you activate it from the Channels panel. "Marching ant" selections, layer masks, and alpha channels are effectively all the same beast, they are just viewed and manipulated in different ways and at times are used for different purposes.

For alpha channels, the pure black areas represent the parts of the image that are not selected, while the pure white areas represent the region that is completely selected. Gray indicates areas that are partially selected (such as feathered areas or those that were modified with the Refine Edge's Radius ...

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