CANNY EDGE OPERATOR
Only consider pixel points with first order edge-strength above thresh_1.
Look for ‘zero-crossings’ of second derivative in direction of image gradient.
Nonmaximum Supression: Only consider those zero-crossings that constitute a maximum of the image intensity gradient.
Edge Tracking: Pick thresh_2<thresh_1 and travel perpendicular to gradient selecting pixels with image intensity gradient >thresh_2.