An optional "Unsharp Masking" mode can be set in the filter options dialog:
Unsharp masking is a filter technique where the output from a filter is combined with the input image:
Depending on the sign of the Unsharp Masking parameter (a) the filter combination will be as follows
, for positive values of a
, for negative values of a
In the special case where a equals -1.0 and the applied filter is a low pass filter the UM filter becomes a high pass filter:
High Pass = Original - Low Pass.
This effect can also be obtained automatically by setting the Subtract from Input check box in the Output Option in the Filter Settings.
When a > 0, more of the original image is added to the filtered result. When the filter type is of high pass type this restores partially the low-frequency components lost during the high-pass filtering operations.