Unsharp Masking

Unsharp masking is derived from a conventional photographic technique, and, despite its name, is used to sharpen images. The technique was first applied to images made on 4 x 5 and larger sheet film. In the darkroom, a film positive is made from the original film negative, a sort of reversed negative in which all the parts of the image that were originally black are black, and all the parts that were white are white. This positive is slightly blurred, which causes the image to spread a bit is documentiscreated withtrialversionofCHM2PDF Pilot2.16.100.andwiched together and used to expose yet another image, the light areas of the positive correspond very closely to the dark areas of the negative, and vice versa, canceling each other out to a certain extent. However, at the edges of the image, the blurring in the positive produces areas that don't cancel out, resulting in lighter and darker lines on either side of the edges. This extra emphasis on the edges of the image adds the appearance of sharpness. Figure 2.25 might help you visualize how this works.

Figure 2.25. A blurred positive and negative image (left) are combined to produce a new, sharpened negative that can be used, in turn, to print an image like the one at right.

Figure 2.25. A blurred positive and negative image (left) are combined to produce a new, sharpened negative that can be used, in turn, to print an image like the one at right.

It's fairly easy for Photoshop to create the blurry positive "mask" and then match it with a negative image of the original picture. As a bonus, you end up with greater control over the amount of blur in the mask, the distance around the edges that are masked, and a threshold level (relative brightness) at which the effect begins to be applied. The Unsharp Mask filter is similar in many ways to the Sharpen Edges filter, but with this enhanced control. There are three slider controls:

• The Amount slider controls the degree of edge enhancement applied. You can vary the sharpening effect from 1 percent to 500 percent, and view the results in the Preview window as you work.

• The Radius slider determines the width of the edge that will be operated on, measured in pixels, with valid values from .1 (very narrow) to 250 pixels (very wide). You can preview the results visually, but use a few rules of thumb to decide how much to move this control. The main thing to keep in mind is the original resolution of your image. Low-resolution images (under 100 dpi) can't benefit from much more than one- to three-pixels worth of edge sharpening, whereas higher resolution images (300 dpi and up) can accommodate values of 10 or more. You'll know right away if you have set your values too high. You'll see thick, poster-like edges that aren't realistic, accompanied by a high degree of contrast. You may, in fact, actually like the weird appearance, but you've left the realm of sharpening and ventured into special effects at this point.

• The Threshold slider sets the amount of contrast that must exist between adjacent pixels before the edge is sharpened. Sharpness is actually determined by how much the contrast varies between pixels in an area, as shown in Figure 2.26, a super-enlargement of the clock face in the tower that holds Big Ben (which itself is actually a bell) in London. Low contrast equals a blurry, soft image, whereas high contrast tends to mean a sharp, hard image. You can see that the pixels on either side of the enlargement are exactly the same size, but that the contrast between them is greater on the left ("sharper") side.

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