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

Paul Ramsey pramsey at refractions.net
Tue Sep 18 22:03:59 PDT 2007

Applied to trunk. I don't see any way it can do harm.

On 18-Sep-07, at 8:57 PM, Charlie Savage wrote:

> 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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