edge-enhance a portable graymap
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portable graymap as input. Enhances the edges, and writes a
portable graymap as output.
enhancing technique is taken from Philip R. Thompson’s
"xim" program, which in turn took it from section
6 of "Digital Halftones by Dot Diffusion", D. E.
Knuth, ACM Transaction on Graphics Vol. 6, No. 4, October
1987, which in turn got it from two 1976 papers by J. F.
Jarvis et. al.
-N flag should be a digit from 1 to 9. 1 is the
lowest level of enhancement, 9 is the highest, The default
1989 by Jef Poskanzer.