Unsharp Masking or USM is an image sharpening technique often available in digital image processing software. It 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. The word “unsharp” is derived from the fact that the technique uses a blurred or “unsharp” positive image in order to create a mask of the original image. This unsharped mask will them be combined with the negative image, thus creating an image that is less blurry than the original. However, even though the output is clearer it might show a less accurate representation of the image’s subject.
Unsharp Masking is the standard tool of choice in sharpening images, since sharpness makes edges clearer and distinct.
How does it work?
The sharpening filters emphasize the edges of an image as well as the difference between the adjacent light and dark sample points in such image. The Unsharp Mask provides the necessary greater control in sharpening photographs. This technique involves making the pixels on the light side of the edge even lighter and the dark side of the edge even darker in order to increase edge contrast.
This technique is a wonderful magic but excessive and improper use of its parameters may produce artificial images with bizarre problems. Some of the problems that improper use of Unsharp Masking may cause are edges that look like halos around objects, overly contrasting images, jagged edges and speckled or mottled areas, which include faces with bad complexions.