Finds the convexity defects of a contour.
D = imconvexityDefects(contours, hullinds)
Contours in list
Indices of contours which representing convex hulls in list of each contours.
The output vector of convexity defects.(start_index, end_index, farthest_pt_index, fixpt_depth)
The functions finds the convexity defects of a contour.