Sharpening Images - Beginning Photo Retouching and Restoration Using GIMP - page 226

Figure 9-1. Before and after example of a scanned photograph slightly sharpened
The Unsharp Mask Filter
This filter's name sounds counterintuitive to making an image look sharper, but it's derived from a
photographic process that uses a blurred, or “unsharp,” mask of the original positive image. This mask is
combined with the negative image, creating a result that has greater apparent sharpness.
The Unsharp Mask filter increases contrast between neighboring dark and light pixels, which in
actuality creates the illusion of increased sharpness.
The Unsharp Mask dialog is nested within the filters (Image Menu ➤ Filters ➤ Enhance ➤
Unsharp Mask). There are three setting options in the dialog: Radius, Amount, and Threshold (Figure 9-2 ).