[postgis-devel] Re: Patch for PostGis Envelope Bug

[Charlie Savage]

Anyone have feedback on this patch?  It seems like the current behavior 
is incorrect, and I don't see any easy way to work around it without 
fixing it.

Charlie

Charlie Savage wrote:
>>> However, is this another bug then?
>>>
>>> select astext(extent(GeomFromText('POINT(-104 40)')))
>>>
>>> "POLYGON((-104 40,-104 40,-104 40,-104 40,-104 40))"
> 
> Martin Davis wrote:
> 
>> Looks like the same bug/design issue.  JTS would return the envelope 
>> as a POINT in this case.
> 
> This issue has been causing me problems, so I dug into.  Geos in fact
> does return a POINT when you ask for the envelope of a POINT.
> 
> It's PostGIS that is wrong.  The method LWGEOM_envelope (line 2493 in
> lwgeom_functions_basic.c) *always* returns a polygon.  So I changed it
> to return a point if the input geometry is a point.
> 
> I've attached a patch that fixes this issue.  The patch checks to see if
> the input geometry is a point or a multipoint with just one geometry.
> 
> There may be 2 issues with the patch:
> 
> 1.  The check ins't foolproof, one could subvert it by:
> 
> select asText(Envelope(GeomFromText('MULTIPOINT(10 10, 10 10)')));
> 
> Maybe checking the bounds (or area) of the envelope would be better?
> 
> 2.  I'm not sure I handled the SRID correctly for a mutlipoint.  I just
> took it from the multipoint itself.  Can the first point in a multipoint
> have a different SRID?
> 
> Anyway, it would be great if someone could review this patch and apply
> it if it seems ok.  I think its important because its easy to create
> invalid geometries that don't work with GeomUnion/Contains/Within/etc.
> We run into this problem a lot now that we've upgraded to GEOS 3 (GEOS
> 2.1 seemed to work ok, although maybe that was pure luck.).
> 
> Thanks,
> 
> Charlie
> 
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