6.7. Unsharp Mask

Overview

This filter is found in Image>Filters/Enhance/Unsharp Mask

Out-of-focus photographs but also most of digitized images need correction of sharpness. This is due to digitizing process that must chop a color continuum up in points with slightly different colors: elements thiner than sampling frequency will be averaged into an uniform color. So sharp borders are rendered a little blurred. The same phenomenon appears when printing color dots on paper.

The Unsharp Mask filter (what an odd name!) sharpens edges of the elements without increasing noise or blemish. It is the king of the sharpen filters.

Some scanners apply a sharpen filter while scanning. It's worth to disable it so that you keep control on your image.

Options

Parameters setting

  • Radius: slider and input boxes (0.1-120) allow you to set how many pixels on either side of an edge will be affected by sharpening. High resolution images allow higher radius. Y'd better to always sharpen an image at its final resolution.

  • Amount: slider and input boxes (0.00-5.00) allow you to set strength of sharpening.

  • Threshold: slider and input boxes (0-255) allow you to set the minimum difference in pixel values that indicates an edge where sharpen must be applied. So you can protect areas of smooth tonal transition from sharpening, and avoid creation of blemishes in face, sky or water surface.

More informations

To prevent color distorsion while sharpening, Decompose your image to HSV and work only on Value. Then Compose the image to HSV. This is well explained at gimpguru.org