(Adaptive unsharp masking) A powerful method of sharpening images in the presence of low levels of noise (such as film grain noise) is via the following algorithm [15] . The high-pass filter
which can be used for unsharp masking, can be written as ft times the sum of the following eight directional masks (H0):
The input image is filtered by each of these masks and the outputs that exceed a threshold are summed and mixed with the input image (see Fig. P7.5). Perform this algorithm on a scanned photograph and compare with nonadaptive unsharp masking.
