Adobe Photoshop CS5 One-on-One

2. Survey the color channels. Go to the Channels panel and
   click the individual channel names Red, Green, and Blue.
   Or press the keyboard shortcuts Ctrl+3, Ctrl+4, and Ctrl+5
   (�-3, -4, and -5 on the Mac). Now you can peruse the chan-
   nels and decide which ones are the best candidates for build-
   ing a mask. The three channels appear slightly colorized in
   Figure 10-18.

PeaRl Of WISDOm

You’re looking for the two channels that represent the biggest

extremes—that is, extreme contrast between shadows and highlights as

well as extreme contrast between each other. In our case, the differences

are subtle. But you can see that the skin tones are brightest in the Red

channel (as you can expect for human flesh tones in general) and the

hair is also darkest there. The hair seems to be the lightest in the Blue

channel, and as expected the blue background is lightest in the Blue

channel. So we’ll proceed with Red and Blue channels as our choices for

the Calculations command.

3. Return to the RGB composite view. Now that we’ve decided
   on our channels, click RGB at the top of the Channels list
   or press Ctrl+2 (�- 2 ) to make the full-color image active.


4. Choose the Calculations command. Go to the Image menu
   and choose Calculations to display the Calculations dialog
   box, as in Figure 10-19. Calculations lets you mix two chan-
   nels to form a new alpha channel using a blend mode and
   an Opacity value. In the days before layers, the Calculate
   commands (there were many of them back then) were your
   primary tools for mixing and merging images. Although to-
   day’s Calculations command is more powerful, it’s relegated
   almost exclusively to the creation of masks.

The dialog box is elaborate, so it’s a good idea to turn on the Preview

check box, if it’s not turned on already. With Preview on, you can

observe the results of your changes in the full-image window.

Unfortunately, the preview function can be a tad buggy, occasionally

refusing to update when you make a change. If this happens to you,

just turn the check box off and then back on to refresh the preview.

5. Select the desired Source channels. You can think of the
   Calculations dialog box as layering one channel on top of
   another. Source 2 is the background channel; Source 1 is the
   channel in front. Source 1 is in a position of emphasis and
   should therefore contain the channel with the highest contrast.

Red Channel

Green Channel

Blue Channel

Figure 10-18.

Figure 10-19.

352 Lesson 10: Creating and Applying Masks