[Gvsig_english] QUESTION gvSIG 1.9 (BN 1253) -- Corrupted Polygon / Create new polygon using nodes

Antonio Falciano afalciano at yahoo.it
Fri Jul 2 11:19:24 CEST 2010


Il 02/07/2010 9.45, Jorge Gaspar Sanz Salinas ha scritto:
> El 01/07/10 13:16, Antonio Falciano escribió:
>> Il 01/07/2010 12.30, Jorge Gaspar Sanz Salinas ha scritto:
>>> El 01/07/10 12:08, Antonio Falciano escribió:
>>>> Il 01/07/2010 10.56, Simon Cropper (Botanicus Australia Pty Ltd) ha scritto:
>>>>> Hi,
>>>>>
>>>>> I have inherited a polygon -- nodes in the polygon do not run
>>>>> sequentially from clockwise or anti-clockwise. This is throwing an error.
>>>>>
>>>>> I was able to extract the nodes from the polygon but have had problems
>>>>> finding the routine to create a new polygon using these points (there is
>>>>> no holes in the polygon).
>>>>>
>>>>> Can anyone please point me to a suitable routine?
>>>>
>>>> Hi Simon,
>>>> you can try with the SEXTANTE library: in detail, applying "Minimum
>>>> enclosing shapes" (Convex hull) on vertices and then subtract the more
>>>> external triangles (selected manually) computed with "Delauney
>>>> triangulation". This is a quite huge task, also known as "Concave hull".
>>>> I don't know if this problem has been solved in GFOSS world...
>>>> http://code.flickr.com/blog/2008/10/30/the-shape-of-alpha/
>>>> Cheers,
>>>> Antonio
>>>>
>>>
>>> Schuyler Erle developed for flickr a free software to create alpha
>>> shapes[1], but the trac/svn[2] is closed now...
>>>
>>> More info about the flickr project at [3]
>>>
>>> It would be great to have a SEXTANTE geoprocess for that, indeed.
>>>
>>> Best
>>>
>>> [1] http://biogeometry.duke.edu/software/alphashapes/index.html
>>> [2] http://code.flickr.com/trac/browser/trunk/clustr
>>> [3] http://code.flickr.com/blog/2008/10/30/the-shape-of-alpha/
>>
>> Hi Jorge,
>> I'm agree with you about the need of a such tool. Its algorithm is quite
>> complex, however there are a lot of interesting resources on the web [4]
>> or in the literature [5]. Furthermore, there are many possibilities to
>> obtain good approximations. For instance, see [6]. Maybe, with a good
>> SEXTANTE model or script it's possible to obtain something of useful.
>>
>> Cheers,
>> Antonio
>>
>> [4]
>> http://stackoverflow.com/questions/83593/is-there-an-efficient-algorithm-to-generate-a-2d-concave-hull
>> [5]
>> http://www.wipo.int/pctdb/en/wo.jsp?WO=2008107859&IA=IB2008050849&DISPLAY=DESC
>> [6] http://grass.osgeo.org/wiki/Create_concave_hull
>>
>
> Hi, thanks for the links Antonio.
>
> Schuyler finally pointed me to the current location of Clustr code at
> [1]. He told me that well, it's focused on Flickr needs but maybe can be
> interesting to someone else.
>
> BTW, two days ago I have nice developers workshop about SEXTANTE so just
> to do a matter of exercise I adapted (I mean, copy&pasted) some code
> posted at JTS mailing list [2] and created an algorithm.
>
> It's just a dirty hack and you have to do some "try and error" until you
> get something interesting but well, the results are not soy bad after
> all :-) [3].
>
> Cheers
>
> [1] http://github.com/straup/Clustr
> [2]
> http://jts-devel.219725.n2.nabble.com/polygonizer-and-line-strings-tp2572524p2591002.html
> [3] http://yfrog.com/7fe71p
>

Hi Jorge,
your results are very interesting, congrats! The alpha shapes algorithm
[1] cited on JTS mailing list is relatively easy to understand and it
seem very effective in order to solve the problem. I'm looking forward
to test your new SEXTANTE geoalgorithm! ;)

Cheers,
Antonio

[1] http://www.isprs.org/proceedings/XXXVII/congress/3b_pdf/37.pdf

-- 
Antonio Falciano
http://www.linkedin.com/in/antoniofalciano


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