A brief history lesson about an image sharpening method called “unsharp mask.” Photoshop and other image manipulation software all have a feature with this name that will sharpen an image. So, why is it called unsharp mask?
The name comes from the original photographic procedure used to increase the apparent sharpness of a photograph on film. First the original negative was copied and turned into a positive (In a negative, black is white and white is black. In a positive, it’s the other way around.). During the copy, the positive was intentionally blurred. This is where “unsharp” comes from. Then the positive and negative were put in contact and exposed to light again. The blurry portion of the positive cancelled out (masked) the blurry portion of the negative.
Despite working with bits instead of film, digital unsharp mask works similarly by comparing the source image to a slightly blurred version and subtracting one from the other.