The Unsharp Mask tool lets you sharpen the edges and other fine details in the input image.

Unsharp masking provides more control on the sharpening process and will generally produce better results than the Laplacian filter, but at a higher computational cost.

Unsharp masking works by removing the low frequency spatial information from the image and emphasizing the high frequency details. The algorithm basically uses a Gaussian filter to produce a smooth version of the original image, called unsharp mask. The unsharp mask is then subtracted from the original image, removing low frequencies.

The Unsharp Mask tool has the following parameters:

• Unsharp Mask Radius X, Radius Y, and Link Control the radius of the blur (just like in the Blur tool). By default, the X and Y radii are linked and set to 1.20; otherwise, their range is the same as that of the Blur tool.
• Strength Controls the strength of the effect of masking that is added. Basically, the output image is computed as I+s*(I-M) where I is the original image, M is the unsharp mask, and s is the strength parameter (in 0 to 500 percent; default 100%).
• Threshold Defines how large the difference between the original image and unsharp mask must be before the original pixels are changed. A pixel is changed if the difference in any of its components is larger than or equal to the threshold (in 0 to 100%; default is 0%).