Unsharp masking (USM)
From The Glossary of Digital Photography
A confusing name for a popular sharpening technique used on digital images to make them appear sharper when viewed. This apparent sharpness can be adjusted by the user to keep the image from looking over-sharpened. Unsharp masking software works by blurring a copy of the original and then comparing it to the original. The images are combined so that edges between elements in an image become more defined and have more contrast. This enhanced edge fools the eye into believing that the image is sharper than it is. A threshold value can be set so that small image details are not damaged or destroyed by the software. The amount of the sharpening can
also be controlled, along with sizes of edges to be affected (called radius in the software).
