Question: It is quite easy to defeat hysteresis in edge detectors that implement it - essentially, one sets the lower and higher thresholds to have the same value. Use this trick to compare the behaviour of an edge detector with and without hysteresis. There are a variety of issues to look at:
• What are you trying to do with the edge detector output? it is sometimes very helpful to have linked chains of edge points - does hysteresis help significantly here?
• Noise suppression: we often wish to force edge detectors to ignore some edge points and mark others. One diagnostic that an edge is useful is high contrast (it is by no means reliable). How reliably can you use hysteresis to suppress low contrast edges without breaking high contrast edges?