Adobe Photoshop Unmasked: The Art and Science of Selections, Layers, and Paths

SOME IMAGES DEFY EVEN THE MOST DEXTEROUS USE of the Pen tool, the most subtle choking of a layer mask, the most stealthy use of adjustment layers. You know the type of image I'm talking about: the woman with the frizzy hair, the flower arrangement with the seemingly infinite number of detailed edges, the dog or cat with the wispy fur. For such images we need a different approach: a channel mask.

The premise of channel masking is the same as that for "selecting" with adjustment layers: somewhere in your image is a built-in mask waiting to be teased out. For a channel mask the clues are in the color channels. The process involves copying whichever color channel has the most contrast between subject and background. This duplicate channel becomes an alpha channelwhich you'll remember from Chapter 2 is a potential selection that can be loaded on an as-needed basis. Before loading the new alpha channel as a selection, use your Levels or Curves to heighten its contrast until subject and background are forced towards opaque white and black (or the other way around), with the white areas representing what is selected in the image and the black areas representing what is masked.

The reality isn't always so simple. Depending on how much contrast you begin with, teasing may be insufficient, and coercion may be necessary. Real world images rarely conform to average book examples. It's common for part of your subject to require the channel mask treatment, while other parts are better served by a more conventional selection approach. For example, the head of your curly haired subject may need a channel mask, but the body is sharply defined against the background. Mixing and matching is the name of the game.

Photoshop also has a special masking tool: Extract, found under the Filter menu or Command/Ctrl-Option/Alt-X, which you can use to cut out complex subjects from their backgrounds. And how come I haven't mentioned this until now? Well, because you can almost always get better results using conventional selection methods or channel masking.

Along with showing you my best practices for creating channel masks, this chapter will look at how, if you take a few extra steps, you can get good results with the Extract tool. For basic information about channels, see Chapter 2, "Channels."