canny
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Fundamentals Canny: Non-Maximum Suppression and Hysteresis, Taken Apart
Lesson 4 of the Edge Detection unit. Canny is two independent mechanisms, not one tuned number. NMS collapses a five-pixel ridge to one pixel by comparing along the gradient; hysteresis keeps a weak chain because it touches a strong seed. Worked on nine numbers: a single threshold gives either 1 pixel or 9, and hysteresis gives the 6 that belong to a real contour. Measured on a photo, that is 204 contours averaging 18.68 px against 393 averaging 9.69 px at the same pixel budget.
Fundamentals Edge Detection: Finding Where One Thing Stops and Another Begins
The unit overview: what counts as an edge, the two derivatives that find one, why a single threshold never works, and how contours become the points a matcher can use. Measured on one photo: smoothing across rows recovers 80% of the clean edge set at noise σ=20 against 63% without it, and hysteresis returns contours twice as long as a single threshold at the same pixel budget.