[Octave-bug-tracker] [bug #46547] image package: canny detector in edge() should use LoG as first step

[Hartmut]

Follow-up Comment #1, bug #46547 (project octave):

And here is my personal comment on it:

If you run this script for example with "thresholds = []" then you will get at
least a visibly reasonable result. (The result will still have un-closed
contours, which would most probable be fixed with the supplied patch of
comment #1 in issue #46099).

The root cause of this (Matlab incompatible) behavior is that in the first
calculation step of the Canny edges
* Octave calculates the gradient with a Prewitt filter, whereas
* Matlab calculates the gradient with a LoG filter (if we believe their
documention). Matlab also states that they changed this some time ago. Before
Matlab R2011a they also used a Prewitt filter for the first step.

This different calculation step for the initial edge pixel candidates
explains, why currently Octave and Matlab interpret the "threshold" input
differently. And I think this is the reason why this anonymous user gets
results he doesn't like.

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