Claim: For the photographic darkroom unsharp masking (an image sharpening technique often available in digital image processing software such as Adobe Photoshop and GIMP), a large-format glass plate negative is contact-copied onto a low-contrast film or plate to create a positive image; which is made with the copy material in contact with the back of the original, rather than emulsion-to-emulsion, so it is blurred.
Evidence: Unsharp masking (USM) is an image sharpening technique, often available in digital image processing software. For the photographic darkroom process, a large-format glass plate negative is contact-copied onto a low-contrast film or plate to create a positive image. However, the positive copy is made with the copy material in contact with the back of the original, rather than emulsion-to-emulsion, so it is blurred. The same differencing principle is used in the unsharp-masking tool in many digital-imaging software packages, such as Adobe Photoshop and GIMP.