[geos-devel] Bug in GEOSEquals predicate ?

[Martin Davis]

Yep, that's what you'd have to do. 

Probably a better way to think of this is that you need to decide what 
dimension of geometry you are interested in out of the intersection 
operation.  Then you retain only those components of the result.  In 
this case, since you are checking whether the intersection is equal to 
one of the input polygons, you should retain only the 2D components of 
the intersection result.

I realize that it would be nice to have it a bit simpler to get the 
desired dimension components from an intersection result.  One idea is 
to ensure that a result GEOMETRYCOLLECTION contains at most 3 (possible 
multi) geometry components, one each for the 0-D, 1-D, and 2-D components. 

In general if you are testing spatial relationships you should try and 
use just predicates.  This will be faster and more accurate.  What are 
the other cases which you think will not be as simple?

Pascal Leroux wrote:
> Does it mean that, when I get a geometry collection, the very first 
> thing I have to do is to split it into simple/multiple but homogeneous 
> geometries to go on ?
>
> In the script I've posted, I could have use "contains" or "within" 
> with the two polygons instead of "equals" with the intersection, but 
> in others cases, I'm afraid it is not so easy/simple !
>
> Can you give me an advice ?
>
> Pascal
>
> 2009/3/16 Sean Gillies <sgillies at frii.com <mailto:sgillies at frii.com>>
>
>     The GEOS C API returns a integer value of 2 to indicate an error.
>     It's good to learn that geometry collection predicates aren't
>     supported.
>
>
>     On Mar 16, 2009, at 9:46 AM, Martin Davis wrote:
>
>         GEOS does not support evaluating predicates on
>         GEOMETRYCOLLECTIONS (due to the potential complexity and lack
>         of obvious semantics of heterogeneous collections).
>
>         In JTS trying to do this throws an exception.  I'm not sure
>         how GEOS reacts - or possibly shapely is not handling this
>         gracefully?
>
>         Pascal Leroux wrote:
>
>             Hi all
>
>             while searching for intersections between polygons
>             (buildings), I have found shapes that make "equals"
>             predicate crash. With simpler shapes, it occurs too :
>
>             >>> from shapely.geometry import Polygon
>             >>> p1 =
>             Polygon(((0,0),(0,10),(10,10),(10,0),(8,0),(8,8),(2,8),(2,0)))
>             >>> p2 = Polygon(((2,0),(2,8),(8,8),(7,4),(8,0)))
>             >>> inter = p1.intersection(p2)
>             >>> inter.is_valid and p1.is_valid and p2.is_valid
>             True
>             >>> inter.wkt
>             'GEOMETRYCOLLECTION (POINT (8.0000000000000000
>             0.0000000000000000), LINESTRING (8.0000000000000000
>             8.0000000000000000, 2.0000000000000000
>             8.0000000000000000), LINESTRING (2.0000000000000000
>             8.0000000000000000, 2.0000000000000000 0.0000000000000000))'
>             >>> inter.equals(p1)
>             False
>             >>> inter.equals(p2)
>             Traceback (most recent call last):
>              File "<stdin>", line 1, in <module>
>              File
>             "/usr/lib/python2.5/site-packages/Shapely-1.0.11-py2.5.egg/shapely/predicates.py",
>             line 34, in __call__
>               return bool(self.fn(self.context._geom, other._geom))
>              File
>             "/usr/lib/python2.5/site-packages/Shapely-1.0.11-py2.5.egg/shapely/predicates.py",
>             line 21, in errcheck
>               raise PredicateError, "Failed to evaluate %s" %
>             repr(self.fn)
>             shapely.geos.PredicateError: Failed to evaluate <_FuncPtr
>             object at 0x96a3bf4>
>
>             I have got a Bus Error on MacOSX (libgeos version 3.0.0)
>             and a Segmentation Fault on Ubuntu (libgeos version 3.0.3)
>             with an equivalent code written in C (so I think Shapely
>             is not involved).
>
>             Any idea ? Did I miss something ?
>
>             Pascal
>             ------------------------------------------------------------------------
>
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>
>         -- 
>         Martin Davis
>         Senior Technical Architect
>         Refractions Research, Inc.
>         (250) 383-3022
>
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022