Color Images

Color is a complex subject that you will learn more about in Chapter 2, "Working with Color." For now, let's start by taking a look at how color images are stored in computers. Color is represented digitally by combining a few basic or elementary colors to obtain others. For example, in the RGB color mode, there are three component colors: red, green, and blue. A large range of color can be produced by combining the elementary or component colors in differing amounts. Color is stored in channels that correspond to each of the components. Let's see how this works with an example.

1. Open the file StreetRGB.psd from the CD.

2. Choose Edit > Preferences > Display & Cursors to open the Preferences dialog box, as shown in Figure 1.14.

3. Check Color Channels In Color in the Display group, and then click OK. This setting will help you initially visualize the way channels work.

4. Click the Channels palette, and then click the Red channel. The other channels are turned off, and you see only the selected channel in the document window, displayed as shades of red.

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5. Zoom in to the full magnification of 1600% with the Zoom tool.

6. Using the Eyedropper tool, take a point sample of the red channel (see Figure 1.15). Notice that the color sample appears in gray in the foreground color swatch in the toolbox.

7. Click the foreground color swatch to open the Color Picker. Observe that the color sample you selected with the Eyedropper is a shade of gray.

Drag the Eyedropper around on the image, and verify that all the samples it takes are shades of gray, even though the image in the document window appears in shades of red.

Figure 1.14

The Preferences dialog box

Figure 1.15

Sampling the Red channel

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9. Close the Color Picker and double-click the Zoom tool in the toolbox to return to 100% magnification.

10. Choose Edit > Preferences > Display & Cursors again to open the Preferences dialog box. Now that you have seen how the data in the channels is actually grayscale, you can uncheck Color Channels In Color in the Display group and click OK.

11. In the Channels palette, click the green channel. The image in the document window changes.

12. Click the blue channel. Once again the image's tonality changes as you are now viewing only the blue data that the digital camera's blue sensor captured.

13. Finally, click the RGB channel to turn on all three color channels. Now you see the composite color image in the document window.

What this procedure should teach you is that although color is stored in channels, each channel by itself has only tonal information (light/dark) and can be represented as a grayscale image. Only when the channel data is reproduced with colored light does the true color image emerge.

A traditional cathode ray tube (CRT) monitor has three electron guns insideā€”a red gun, a green gun, and a blue gun. Each gun is fed the corresponding grayscale channel data by Photoshop. When the colored light emerges on the screen, it combines to form the color image that you perceive.

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