Zerocrossings in an image

[Søren Hauberg]

Hi
Does anybody here know a simple way to find the zerocrossings in an 
image in Octave?
I usually do something like this (where f is the image (or 2d function))

zerocrossings = bwmorph(f>=0, "remove");

but then I depend on bwmorph which only is octave-forge (which I don't 
have at school).

Any help would be great,
\Søren



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