Adjusting the tolerance

You may have heard the standard explanation for adjusting the Tolerance value: You can enter any number from 0 to 255 in the Tolerance option box. Enter a low number to select a small range of colors; increase the value to select a wider range of colors.

Nothing is wrong with this explanation — it's accurate, in its own small way — but it doesn't provide one iota of information you couldn't glean on your own. If you really want to understand this option, you have to dig a little deeper.

When you click a pixel with the magic wand tool, Photoshop first reads the brightness value that each color channel assigned to that pixel. If you're working with a grayscale image, Photoshop reads a single brightness value from the one channel only; if you're working with an RGB image, it reads three brightness values, one each from the red, green, and blue channels; and so on. Because each color channel permits 8 bits of data, brightness values range from 0 to 255.

Next, Photoshop applies the Tolerance value, or simply tolerance, to the pixel. The tolerance describes a range that extends in both directions — lighter and darker — from each brightness value.

Suppose you're editing a standard RGB image. The tolerance is set to 32 (as it is by default); you click with the magic wand on a turquoise pixel, whose brightness values are 40 red, 210 green, and 170 blue. Photoshop subtracts and adds 32 from each brightness value to calculate the magic wand range that, in this case, is 8 to 72 red, 178 to 242 green, and 138 to 202 blue. Photoshop selects any pixel that both falls inside this range and can be traced back to the original pixel through an uninterrupted line of other pixels, which also fall within the range.

From this information, you can draw the following basic conclusions about the magic wand tool:

* Clicking on midtones maintains a higher range: Because the tolerance range extends in two directions, you cut off the range when you click a light or dark pixel, as demonstrated in Figure 8-6. Consider the two middle gradations: In both cases, I selected the Contiguous check box and set the Tolerance value to 60. In the top gradation, I clicked on a pixel with a brightness of 140, so Photoshop calculated a range from 80 to 200. But when I clicked on a pixel with a brightness value of 10, as in the bottom gradation, the range shrank to 0 to 70. Clicking on a medium-brightness pixel, therefore, permits the most generous range.

* Selecting brightness ranges: Many people have the impression that the magic wand selects color ranges. The magic wand, in fact, selects brightness ranges within color channels. So if you want to select a flesh-colored region — regardless of shade — set against an orange or a red background that is roughly equivalent in terms of brightness values, you probably should use a different tool.

* Selecting from a single channel: If the magic wand repeatedly fails to select a region of color that appears unique from its background, try isolating that region inside a single-color channel. You'll probably have the most luck isolating a color on the channel that least resembles it. For example, to select the yellow Sasquatch Xing sign shown in Color Plate 8-1, I switched to the blue channel (Ctrl+3). Because yellow contains no blue and the brambly background contains quite a bit of blue — as demonstrated in the last example of Figure 8-7 — the magic wand can distinguish the two relatively easily. Experiment with this technique and it will prove even more useful over time.

Figure 8-6: Note the results of clicking on a pixel with a brightness value of 140 (top row) and a brightness value of 10 (bottom row) with the tolerance set to three different values.

Green

Blue

Green

Blue

Figure 8-7: Because the yellow Sasquatch sign contains almost no blue, it appears most clearly distinguished from its background in the blue channel. So the blue channel is the easiest channel in which to select the sign with the magic wand.

Note Here's one more twist to the Tolerance story: The magic wand is affected by the

Sample Size option that you select for the eyedropper tool. If you select Point Sample, the wand bases its selection solely on the single pixel that you click. But if you select 3 by 3 Average or 5 by 5 Average, the wand takes into account 15 or 25 pixels, respectively. As you can imagine, this option can have a noticeable impact on the extent of the selection that you get from the wand. Try clicking the same spot in your image using each of these Sample Size settings, using the same Tolerance value throughout, to see what I mean.

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