Sharpening Images - Beginning Photo Retouching and Restoration Using GIMP - page 227

Figure 9-2. The setting options in the Unsharp Mask dialog
1. Radius. The slider and input boxes (0.1-120) determine how many pixels on
either side of an edge are affected by sharpening.
2. Amount. The slider and input boxes (0.00-5.00) determine the intensity of the
sharpening effect.
3. Threshold. This allows you to protect areas that are similar in tone from
sharpening. Areas of smooth tonal transition can be protected from sharpening,
minimizing the creation of unwanted blemishes or artifacts.
The main disadvantage of using the Unsharp Mask filter is that the effect can accentuate film grain
and digital noise and create unsightly artifacts around the edges in the image (Figure 9-3 ). This filter can
be somewhat destructive, so I recommend applying it with a light touch. Always work on a duplicate of the
background layer. If the sharpening effect is too strong, you can lower the layer's opacity to reduce the effect.