Finding edges with Canny

Edges are sudden changes in brightness

The Canny edge detector finds them. It takes two thresholds:

• pixels changing faster than the high threshold are definitely edges
• pixels changing slower than the low threshold are definitely not
• anything between is kept only if it connects to a definite edge

Call cv2.Canny(img, low, high) and store the result in edges.

A common starting point is a high threshold around twice the low one. Too many thresholds too low and you detect noise; too high and you miss real edges. The tests check that you land in a sensible range.