Unsharp Mask

Image sharpening is accomplished by first creating a blurred version of the image and then comparing it pixel by pixel with the original. The appearance of greater sharpness is achieved by exaggerating the differences between the image and the blurred version. While the sharpened version of the image contains no more information than the original, details are more apparent. Unsharp Mask uses Gaussian Blur to create a blurred version of the image.

Amount

The amount control lets you control how much of the unsharp mask transformation is applied to the input image. You can apply a percentage of the transformation to the entire image, or you can specify an amount mask to restrict the effects of the transformation to only part of the input image.

Halo Limit

This double slider is useful for controlling light or dark halos that often occur around distinct edges. The white slider limits the maximum amount a pixel can be lightened when sharpening and the black slider limits how much it can be darkened. Setting them to zero suppresses all sharpening while setting them to 255 performs no halo suppression. Use the highest values that produces acceptable haloing – typically somewhere around 10%.

Radius

For best results, use a larger radius when sharpening softer images.

Threshold

The threshold is applied as follows: first, the blurred image is computed; next, pixel-by-pixel, the blurred image is compared to the input image. If the difference is less than the threshold, the blurred image replaces the input image; otherwise the original input image pixel is retained.

The effect of the threshold is to restrict blurring to those areas of the input image that are already relatively soft and to leave pixels that differ sharply from others around them alone. This can be very helpful, for example, in smoothing out noise in a clear sky while leaving the adjacent tree line unblurred. A threshold setting of 100% enables full blurring; relatively small values (say less than 10%) are good for selective blurring.

Preview

A 1:1 preview of the sharpen effect is displayed in the right-hand side of the dialog box. Clicking on the input image display centers the preview on the location you clicked on, or you can click and drag to scroll the preview window.

If you enlarge the dialog box, you can make the sliders wider and the preview area narrower or vice versa by dragging the vertical separator line between them.

 Preview Button

Clicking the Preview button causes the full output image to be recalculated, not just the preview area. This can be useful if you want to view the entire image at once and not just a small section, but it can take longer to compute.

 Difference Button

Clicking the Difference button causes the preview to display the difference between the original image and the preview image. Differences are exaggerated by a factor of two to make them more apparent.

Tips

Using a large radius is a useful way to perform local contrast enhancement, but Unsharp Mask is not the best transformation to use if this is what you are trying to accomplish since it can lead to wide halos around dark objects on a light background or light objects on a dark background. Bilateral Sharpen with a large radius does a better job of suppressing these halos.

 Settings Menu

Sharpen Luminance Only

This toggles the Sharpen Luminance Only setting. If the menu item is checked, just the brightness information (luminance) is sharpened, while color information (chrominance) in the input image is preserved. This can be useful, for example, when sharpening areas with color noise without amplifying the noise, but generally the difference is very subtle.