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Re: checking for connectedness of a given set of pixels [message #73430 is a reply to message #73311]

Mon, 08 November 2010 11:22

James[2]
Messages: 44
Registered: November 2009

Member

LABEL_REGION's speed relies on a fast implementation of the union and
find operations on disjoint set data structures (http://
en.wikipedia.org/wiki/Disjoint-set_data_structure). I'm guessing that
the IDL programmers have implemented this in C for LABEL_REGION,
probably with a higher speed than you'd be able to get with anything
written in IDL. If they did it "right" (with path compression) then
it's an optimally fast algorithm for this problem (see Wikipedia
article).

One problem is that LABEL_REGION runs over a whole array even if it is
mostly empty (sparse). This is why LABEL_REGION might be slow for
your task - it could be doing a lot of unnecessary extra work. Since
your data is already in 2D coordinates, I'd take the max/min values
and use a tightly-fitted array. Supposing your coordinates are in a 2-
by-N array POINTS:

function ISCONNECTED(points, _extra=ex)
mins = min(points, dimension=2, max=maxs)
shift = mins # replicate(1, n_elements(points)/2)
arr = bytarr(maxs-mins+1)
arr[points - shift] = 1
return, max(label_region(arr, _extra=ex)) le 1
end

the _extra keywords allow you to pass ALL_NEIGHBORS along to choose
between 4- and 8-connectivity. Of course, this function adds a lot of
extra baggage, so it's only going to be faster if your regions are
significantly smaller than the whole array.

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[Message index]

		Re: checking for connectedness of a given set of pixels By: David Fanning on Wed, 03 November 2010 06:35
		Re: checking for connectedness of a given set of pixels By: guillermo.castilla.ca on Wed, 03 November 2010 06:32
		Re: checking for connectedness of a given set of pixels By: David Fanning on Tue, 02 November 2010 15:14
		Re: checking for connectedness of a given set of pixels By: guillermo.castilla.ca on Tue, 02 November 2010 15:06
		Re: checking for connectedness of a given set of pixels By: David Fanning on Tue, 02 November 2010 13:37
		Re: checking for connectedness of a given set of pixels By: James[2] on Mon, 08 November 2010 11:22
		Re: checking for connectedness of a given set of pixels By: guillermo.castilla.ca on Sun, 14 November 2010 13:56
		Re: checking for connectedness of a given set of pixels By: guillermo.castilla.ca on Mon, 03 January 2011 13:50

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