The type of sharpening performed is typically called unsharp masking, and results from a weighted subtraction of a blurred version of the image from the original. This creates a circularly symmetric, Gaussian shapped kernel.
G is the orignal image with the gausing kernel applied.
The kernel weights always sum to one and the clamping attribute is set on this kernel so no change in the input range of values will be effected by the application of this kernel. Creates a kernel with the specified sharpness and radius . The sharpness typically ranges from -1 for maximum blur, to 0 for no effect, to positive values up to about 10 for various degrees of sharpening. The radius can be any non-negative value. A radius of zero will perform no filtering, while successively larger values will filter lower and lower frequencies.