Photoshop CS5: The Missing Manual

The Mighty Alpha Channel

Photoshop has one other type of channel: alpha channels. Their job is to store your selections so you can use or edit them again later.

These channels get their name from a process called alpha compositing, which combines a partially-transparent image (one that has see-through areas) with another image; this is how filmmakers create special effects and fake backdrops. But the information about the shape of the transparent area and the pixels' level of transparency has to be stored somewhere, and that somewhere is an alpha channel.

This is powerful stuff because the same technology lets you save your selections. And, as you've learned, making selections can take a ton of time. Heck, you may not have the stamina to finish creating a particularly challenging selection in one sitting. And since clients change their minds occasionally—"Put the model in front of this bush. And change her hair color while you're at it!"—the ability to save selections so you can mess with them later is a lifesaver. As long as you save your document as a PSD file (Opening an Existing Document), that alpha channel will always be there for you to use. That ought to make you sleep better at night!

Tip

You can drag alpha channels between documents as long as the documents have the same pixel dimensions.

Folks sometimes refer to alpha channels as channel masks because, once you've made an alpha channel (as explained in the next section), you can use it to help you adjust certain portions of ...