RGB channels

Suppose that the sunbathing woman is an RGB image. Figure 4-16 compares a grayscale composite of this same image (created by choosing Image ^ Mode ^ Grayscale) compared with the contents of the red, green, and blue color channels from the original color image. The green channel is quite similar to the grayscale composite because green is an ingredient in all colors in the image, except for the red of the raft. The red and blue channels differ more significantly. The pixels in the red channel are lightest in the swimsuit and raft because they contain the highest concentrations of red. The pixels in the blue channel are lightest in the sky and water because — you guessed it — the sky and water are rich with blue.

Figure 4-16: A grayscale composite of the image from Color Plate 4-2 followed by the contents of the red, green, and blue color channels.

Notice how the channels in Figure 4-16 make interesting grayscale images in and of themselves? The red channel, for example, looks like the sky is darkening above our bather, even though the sun is blazing down.

I mentioned this as a tip in the previous chapter, 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 miss the warning, click the Reset All Warning Dialogs button on the General panel of the Preferences dialog box.

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