Setting the thickness of the edges

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The Unsharp Mask filter works by identifying edges and increasing the contrast around those edges. The Radius value tells Photoshop how thick you want your edges. Large values produce thicker edges than small values.

The ideal Radius value depends on the resolution of your image and the quality of its edges:

* When creating screen images — such as Web graphics — use a very low Radius value such as 0.5. This results in terrific hairline edges that look so crisp, you'll think you washed your bifocals.

* If a low Radius value brings out weird little imperfections — such as grain, scan lines, or JPEG compression artifacts — raise the value to 1.0 or higher. If that doesn't help, don't fret. I include two different sure-fire image-fixing techniques later in this chapter, one designed to sharpen grainy old photos, and another that accommodates compressed images.

* When printing an image at a moderate resolution — anywhere from 120 to 180 ppi — use a Radius value of 1.0. The edges will look a little thick on-screen, but they'll print fine.

* For high-resolution images — around 300 ppi — try a Radius of 2.0. Because Photoshop prints more pixels per inch, the edges have to be thicker to remain nice and visible.

Tip If you're looking for a simple formula, I recommend 0.1 of Radius for every 15

ppi of final image resolution. That means 75 ppi warrants a Radius of 0.5, 120 ppi warrants 0.8, 180 ppi warrants 1.2, and so on. If you have a calculator, just divide the intended resolution by 150 to get the ideal Radius value.

You can of course enter higher Radius values — as high as 250, in fact. Higher values produce heightened contrast effects, almost as if the image had been photocopied too many times, generally useful for producing special effects.

But don't take my word for it; you be the judge. Figure 10-10 demonstrates the results of specific Radius values. In each case, the Amount and Threshold values remain constant at 100 percent and 0, respectively.

Figure 10-10: The results of applying eight different Radius values, ranging from precise edges to very gooey.

Figure 10-10: The results of applying eight different Radius values, ranging from precise edges to very gooey.

Figure 10-11 shows the results of combining different Amount and Radius values. You can see that a large Amount value helps to offset the softening of a high Radius value. For example, when the Amount is set to 200 percent, as in the first row, the Radius value appears to mainly enhance contrast when raised from 0.5 to 2.0. However, when the Amount value is lowered to 50 percent, the higher Radius value does more to distribute the effect than boost contrast.

200%, 0.5 200%, 2.0 200%, 10.0

Figure 10-11: The effects of combining different Amount and Radius settings. The Threshold value for each image was set to 0, the default setting.

For those few folks who are thinking, "By gum, I wonder what would happen if you applied an unusually high Radius value to each color channel independently," you have only to consult Color Plate 10-4. In this figure, I again applied the Unsharp Mask filter to each channel and each pair of channels in the RGB chess image independently. But I changed the Amount value to 250 percent, raised the Radius value to a whopping 20.0 pixels, and left the Threshold at 0. To make the splash more apparent, I applied the filter twice to each image. The colors now bound out from the king, queen, and knight, bleeding into the gray background by as much as 20 pixels, the Radius value. Notice how the color fades away from the pieces, almost as if I had selected and feathered them? A high Radius value spreads the sharpening effect and, in doing so, allows colors to bleed. Because you normally apply the filter to all channels simultaneously, the colors bleed uniformly to create thick edges and high-contrast effects.

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