An unsharp mask is simply another type of high-pass filter. First, a mask is formed by low-pass filtering an image. This mask is then subtracted from the original image. This has the effect of sharpening the image.
Usually the mask is scaled before it is subtracted; this allows the user a great deal of control in the amount of sharpening that is applied. The strength of the low-pass filter used can also be adjusted. A very strongly blurred mask can be used to remove large-scale brightness differences, while a slightly blurred mask will sharpen fine detail.
A variation of this technique uses a geometric mean mask. In this method, a logarithmic function is applied to the image before the mask is calculated. This technique is very useful for sharpening images that have faint details superimposed on large brightness variations. Among other things, this technique is very useful for revealing dust jets coming from the nucleus of a comet.
Another variation is called Digital Development Processing. This technique was invented by Dr. Kunihiko Okano. He noticed that standard film processing chemistry applied a hyperbolic stretch (gamma with an ”S” curve) and compensated for the flattened appearance by adding an edge enhancement effect (created by developer depletion in bright areas). He devised a simple and highly effective method to emulate this effect digitally, using a hyperbolic stretch and unsharp mask.