These constants define the edge detection methods available.

Values:

IG_EDGE_DETECTION_MAXGRADIENT	Edge detection using the maxima of gradient, i.e., maxima of the first order derivative.
IG_EDGE_DETECTION_ZEROXC_DERIV2ND	Edge detection using the zero-crossings of second order derivative along the gradient.
IG_EDGE_DETECTION_DIFF_RECURSIVE	Edge detection using an optimal difference recursive filter.