Understanding USM

Of all the tools available in digital image processing none causes more confusion than the use ofUnsharp Masking. I believe that this is in large measure because users don't understand what each of the three tools,Radius,AmountandThresholdactually does. The mystery is now revealed.
By: Dale Cotton
Defining Sharpening
Sharpening is about restoring accutance that has been lost previously along the image capture chain of events. Accutance = how precisely the edges of things are defined. Fuzzy edges = rub eyes, do I need new glasses?
But if the edges have been blurred during digitization how can the lost detail that defined the edges be recovered? It can't be; the first law of computing is garbage in — garbage out. So we resort to the old law thattwo wrongs make a right. We throw away even more information in the hopes that what is left gives the illusion of the original Accutance
The principle behind unsharp masking is exaggerating the contrast between the two sides of an edge. By ...