[Octave-bug-tracker] [bug #46099] image package: edge(I, 'canny') gives very bad quality results

[Carnë Draug]

Follow-up Comment #8, bug #46099 (project octave):

> @Carne: Your simplification looks reasonable to me. (But what would be
benefit? execution time?) 

The benefit would be on reduced maintenance, to replace 30 lines of untested
C++ (the part of nonmax_supress that performs hysteresis) with 2 lines of m
code with well tested functions.  And by then removing the nonmax_supress code
that did it, there's no longer two pieces of code around with almost equal
purposes.

Maybe using imreconstruct is not a very clear simplication and bwselect would
be a better fit (but bwselect also does not have tests). Also I get different
results from nonmax_supress's hysteresis than I get from imreconstruct and
bwselect so I'm questioning if nonmax_supress hysteresis is correct.

There seems to be a very small increase time by making use of imreconstruct
though, but I'd rather spend the time on speeding up imreconstruct for logical
input (a simple algorithm, already described in the source as TODO
http://hg.code.sf.net/p/octave/image/file/90a52e024691/src/imreconstruct.cc#l285
) than figuring out nonmax_supress.

> I am on the way to make the Canny method of edge.m much more Matlab
compatible, and to add some tests to it. I should be able to supply a patch in
a couple of days. (It will necessarily also cover bug #46547). So if you are
patient enough, you could test your simplicition step then.

That is great. Thank you for working on this. By the way, although
nonmax_supress is not clearly marked as private (it does not have the double
underscores on the name), it is meant as such (according to its help text) so
you can just remove it or change it as much as you want.

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