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

[Jorge Gaspar Sanz Salinas]

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

-- 
Jorge Gaspar Sanz Salinas
gvSIG Team at Prodevelop
Technical Collaborations Manager
http://www.gvsig.org
http://www.gvsig.com