Edge Detection

From CGAFaq

A simple method is to put the bitmap through the filter:

-1    -1    -1
-1     8    -1
-1    -1    -1

This will highlight changes in contrast. Then any part of the picture where the absolute filtered value is higher than some threshold is an "edge".

A more appropriate edge detector for noisy images is described by Van Vliet et al. "A nonlinear Laplace operator as edge detector in noisy images", in Computer Vision, Graphics, and image processing 45, pp. 167–195, 1989.