In image processing, an edge enhancement process performed either photographically (when exposing color separation negatives), optically (by a scanner), or digitally (in image-processing programs such as Photoshop). The latter two methods derive from the photographic process, in which a blurry unsharp mask is used when exposing color separation negatives from a transparency. The photographic method is commonly employed for reasons of color correction, a side benefit being an exaggeration and enhancement of the edges of images. (See Unsharp Mask.)
Optical unsharp masking is performed during scanning utilizing two types of apertures: the small, primary signal aperture, which captures the individual red, green, and blue image signals, and a larger unsharp masking aperture, which captures a larger amount of the same signal. The two signals are combined to create a single exaggerated signal which causes an enhancement where image edges occur.
Digital unsharp masking utilizes the application software to analyze each set of adjacent pixels of a digital image, locate where the edges occur, and adjust the tonal values of the pixels on either side of the edge in opposite directions, increasing the contrast along the edge. Some advanced programs allow the user to specify the number of pixels to modify, allowing greater or lesser degrees of unsharp masking.
Unlike photographic USM, neither the optical nor the digital USM techniques can be used to color correct the image, and actually neither of them technically use "unsharp masks."