Remember about anti-aliasing blending the edges with intermediate tones? (usually only on angled or jagged edges). It is also shown greatly enlarged about 10X so the pixels can be seen well. This tiny image is a simple three tone graphic image containing two contrast edges, shown unsharpened on the left, and sharpened with the Unsharp Mask on the right. The programming details are rather technical, but it is a mighty fine sharpening tool. This creates the same type of sharpening shown below. The term UnSharp Mask comes from a photographic darkroom technique of exposing the print paper through an underexposed and blurred positive film copy of the negative sandwiched with the real negative. Mask is often a synonym, but it also implies a specific localized area is treated. In computer-talk, "filter" is a generic word which just means a tool or process where you "put something in, and get something out", that is, it separates or refines the object in some way, a philosophy not unlike a coffee filter or a color filter. Sharpening can be done as well at any later time, and in fact, if additional image processing is to be done, the USM is always best done last. Unlike Setting the Black and White Points, it is not of any particular advantage to apply the USM sharpening during the scan.
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