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Found in the same toolbox flyout as the eyedropper, the color sampler tool looks like the eyedropper with a little crosshair target. But where the eyedropper lifts foreground and background colors, the color sampler merely measures the colors of pixels so that you can monitor how the pixels react to various color changes.
Select the color sampler and click somewhere inside the image window. Photoshop adds a crosshair target to indicate the point you clicked. The program also brings up the Info palette (if it isn't displayed already) and adds a new color measurement item labeled #1. This item corresponds to the target in the image, which is likewise labeled #1. Click again and you add a second target and a corresponding item #2 in the Info palette. You can add up to four targets to an image, as demonstrated in Figure 4-
Figure 4-15: The color sampler tool lets you measure the colors of four points in your image. You can also measure a fifth point by merely moving the cursor around.
The color sampler is intended primarily for printers and technicians who want to monitor the effects of color corrections on specific points in an image. If you apply Image ® Adjustments ® Levels, for example, Photoshop constantly updates the items in the Info palette to reflect your changes (as I explain in more detail in Chapter 17). But you can also sample points in an image to monitor the effects of filters (Chapters 10 and Ц), blend modes (Chapter 13), and edit tools, such as dodge and burn (Chapter 5). The color sampler is just another way to monitor changes to an image.
Here are a few more techniques of interest when color sampling:
Photoshop limits you to four color targets. If you try to create a fifth one, the program generates an error message. If you want to measure a different point in the image, you can either hover your cursor over the point and note the top set of color values in the Info palette (as in Figure 4-15) or move one of the targets.
To move a target inside the image window, drag it with the color sampler tool. You can move a target also by pressing Ctrl (Win) or z (Mac) and dragging the target with the eyedropper tool. Hold down the Shift key to constrain the drag to a 45-degree angle.
To delete a target, Alt-click (Win) or Option-click (Mac) it. To delete all targets, click the Clear button in the Options bar.
The Info palette grows to more than twice its normal size when you start clicking with the color sampler. To hide the sampler information without deleting targets, click the Info palette's collapse box or choose Color Samplers from the palette menu. If you go the second route, you have to choose Color Samplers again to bring the samples back.
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127 Photoshop CS Bible @Team LiB
By default, the sampler items in the Info palette measure colors in the active color space. If you want to track a target in a different color space, click the item's eyedropper icon in the Info palette or right-click (Control-click on the Mac) the target in the image window. Either way, you get a pop-up menu of colorspace alternatives, including Grayscale, RGB, and several others that you may recall from previous explanations in this chapter.Tip
To select the color sampler, press I when the eyedropper is active or Alt-click (Option-click on the Mac) the eyedropper icon. Or press I repeatedly to cycle between the eyedropper, color sampler, and measure tool (add Shift if you activated the Use Shift Key for Tool Switch option in the Preferences dialog box). You can temporarily access the color sampler any time the eyedropper is active by pressing Shift. This little trick also works when a color correction dialog box such as Levels or Curves is open, as explained in Chapter 17. It's just the ticket when you're in the middle of an adjustment and you need to know how the adjustment is affecting specific portions of the image.
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7.1.3 Introducing Color Channels
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Introducing Color Channels
After I've droned on for pages about color in Photoshop, it might surprise you when I say that Photoshop is at its heart a grayscale editor. Oh sure, it offers an array of color-conversion features and displays and prints spectacular full-color images. But when it comes to editing the image, everything happens in grayscale.
This is because Photoshop approaches a typical full-color image not as a single collection of 24-bit pixels but as three or four bands of 8-bit (grayscale) pixels. An RGB file contains a band of red, a band of green, and a band of blue, each of which functions as a separate grayscale image. A Lab image likewise contains three bands, one corresponding to luminosity and the others to the variables a and b. A CMYK file contains four bands, one for each of the process-color inks. These bands are known as channels.
Channels frequently correspond to the structure of an input or output device. Each channel in a CMYK image, for example, corresponds to a different printer's plate when the document goes to press. The cyan plate is inked with cyan, the magenta plate is inked with magenta, and so on. Each channel in an RGB image corresponds to a pass of the red, green, or blue scanner sensor over the original photograph or artwork. Only the Lab mode is device independent, so its channels don't correspond to any piece of hardware. Why you should care