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Color Plate 4-1 shows what is apparently a beluga whale in its natural habitat — the water. As you'd expect, this underwater image contains lots of greens and blues. These colors, along with yellow and red, cover the four corners of the color spectrum. Therefore, you can expect to see a good bit of variation between the images in the independent color channels.
RGB channels
Suppose that the beluga whale is an RGB image. Figure 4-17 compares a grayscale composite of this same image (created by choosing Image Mode Grayscale) with the contents of the red, green, and blue color channels from the original color image. The green channel is closest to the grayscale composite because green is such a dominant color in the image. The red channel differs the most from the grayscale composite, simply because the image doesn't have much red. The overall darkness of the red channel bears this out. The pixels in the blue channel are lightest in the water because — you guessed it — the water is rich with blue.
Figure 4-17: A grayscale composite of the original image from Color Plate 4-1 followed by the contents of the red, green, and blue color channels.
Notice how the channels in Figure 4-17 make interesting grayscale images in and of themselves? The red channel, for example, makes the whale look like he's much deeper underwater; we can see the sun streaming through, but it seems to be filtered through a great mass of water.I mentioned this as a tip earlier, but it bears a bit of casual drumming into the old noggin. When converting a color image to grayscale, you have the option of calculating a grayscale composite or simply retaining the image exactly as it appears in one of the channels. To create a grayscale composite, choose Image Mode Grayscale when viewing all colors in the image in the composite view, as usual. To retain a single channel only, switch to that channel and then choose Image Mode Grayscale. Instead of the usual Discard color information? message, Photoshop displays the message Discard other channels? If you click the OK button, Photoshop chucks the other channels into the electronic abyss.
When the warning dialog box appears, select the Do Not Show Again check box if you don't want Photoshop to ask for permission to dump color information or channels when you convert to grayscale. If you grow to miss the warning, click the Reset All Warning Dialogs button on the General panel of the Preferences dialog box.
CMYK channels
In the name of fair and unbiased coverage, CMYK" section earlier in this chapter, dark areas in the channels represent high color intensity. For that reason, the whale is dark in the cyan channel, whereas it's light in the blue channel back in Figure 4-17.
Figure 4-18: The contents of the cyan, magenta, yellow, and black channels from the original image shown in Color Plate 4-1.
Figure 4-19: The contents of the luminosity channel and the a and b color channels after converting the original image shown in Color Plate 4-1 to the Lab mode.
Notice that the cyan channel in Figure 4-18 is similar to its red counterpart in Figure 4-17. Same with the magenta and green channels, and the yellow and blue channels. The CMY channels have more contrast than their RGB pals, but the basic brightness distribution is the same. Here's another graphic demonstration of color theory. In a perfect world, the CMY channels would be identical to the RGB channels — one color model would simply be the other turned on its head. But because this is not a perfect world (you might have noticed that as you've traveled life's bitter highway), Photoshop has to boost the contrast of the CMY channels and throw in black to punch up those shadows.
Lab channels
To create Color Plate 4-1 to the Lab mode. The image in the luminosity channel looks similar to the grayscale composite in Color Plate 4-1. If I apply Image Adjustments Auto Levels (that's Ctrl+Shift+L on the PC and z -Shift-L on the Mac) to the b channel, the whale assumes a groovy mottled lemon-green and blue appearance. The bottom example in Color Plate 4-1 shows what happens when I combine the inverted a channel with the Auto Leveled b channel. There's only one word to describe this dude — orcadelic.
