To understand how it works, consider how unsharp masking alters the low and high frequency content of an image. The unity image contains both high and low frequencies, but the unsharp mask has been stripped of high frequencies. When the unity image is multiplied by the contrast factor c, the strengths of the high and low frequencies are both multiplied by that factor. When the mask, containing only the low frequencies, is multiplied by -(c - 1), the low frequencies become negative. When the unity image and the mask are summed, the resulting image has the same low-frequency content as the original; but its high-frequency content has been multiplied by the contrast factor.
Looking ahead briefly to the spatial frequency realm (see Chapter 17), unsharp masking acts as a high-frequency enhancement filter. It retains the low spatial frequencies from the original image, but multiplies the detail-filled high spatial frequencies by the contrast factor.
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