This filter uses a subtractive smoothing method to sharpen an
image. First an average filter is applied to the image. The averaged
image retains all low spatial frequency information but has its high
frequency features, such as edges and lines, attenuated.
Consequently, the averaged image is subtracted from its original
image and the resultant difference image will have primarily the
edges and lines remaining. After the edges are determined in this
manner, the difference image is added back to the original image
to give an edge enhanced image. The resultant image will have
clearer high frequency detail; however, there is a tendency for
noise to be enhanced, as might be expected