[geos-devel] [GEOS] #241: GEOSBuffer produces an invalid geometry

[Martin Davis]

This seems fine in JTS 1.11.  There's been several bug 
fixes/improvements to the buffer code since it was last ported to GEOS, 
I think - and any of them could be causing the problem.

Ah, the joys of two code bases.... gcj, anyone?

GEOS wrote:
> #241: GEOSBuffer produces an invalid geometry
> ------------------------+---------------------------------------------------
>  Reporter:  pleroux     |       Owner:  geos-devel at lists.osgeo.org
>      Type:  defect      |      Status:  new                       
>  Priority:  major       |   Milestone:  3.1.1                     
> Component:  Default     |     Version:  3.0.0                     
>  Severity:  Unassigned  |    Keywords:                            
> ------------------------+---------------------------------------------------
>  Here's a a valid polygon for which the GEOSBuffer function produces an
>  invalid geometry.
>
>  At the beginning, I didn't test the returned geometry but only wrote it in
>  WKT format (and it "worked"). I tested different values for dist (1 (m)
>  and 2) and, with 1, it is obvious than the result is not valid (a big hole
>  of the poygon disappears and two small polygons are created). With 2, it
>  "seems" good and the obvious error obtained with 1 don't occur any more.
>
>  Even if they are not valid, I think they're interesting to investigate
>
>
>
>  {{{
>  >>> from shapely.wkb import loads
>  >>> polygon =
>  loads(open('polygon.hexwkb').readline().rstrip().decode('hex'))
>  >>> polygon.is_valid
>  True
>  >>> buffer_1 = polygon.buffer(1.)
>  >>> buffer_1.is_valid
>  False
>  >>> buffer_2 = polygon.buffer(2.)
>  >>> buffer_2.is_valid
>  False
>  >>> print >> open('buffer_1.hexwkb','w'), buffer_1.wkb.encode('hex')
>  >>> print >> open('buffer_2.hexwkb','w'), buffer_2.wkb.encode('hex')
>  }}}
>
>  On MacOSX, libgeos 3.0.0, with an equivalent code in C, the problem occurs
>  too.
>
>   
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022