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. |