[postgis-users] Re: Buffering a multipolygon gives Empty Geometry.

[Paul Ramsey]

Could you post your killer polygons to geos-devel? Hopefully some will 
be better with the new GEOS, and others will shake out bug cases we have 
not fixed yet.

rm_postgis at cheapcomplexdevices.com wrote:

>  
> Paul Ramsey <pramsey at refractions.net> wroe:
> 
>>OK, here is the odd bit about your examples: they are both buffer(0) 
>>cases.
> 
> 
> The buffer(0) example was the test-case that I could isolate
> in something small enough to post to the mailing list.  I can find
> where the "geomunion" crashes with positive polygons
> 
> 
> 
>>What are you trying to accomplish with this?
> 
> 
> A generic user-defined interface using mapserver-CGI that lets the end 
> user say "give me all events withing X miles of the Florida A&M campus".
> If the end user happens to say '0', it ran the same query as if he said
> 1 mile that looks something like:
> 
>    geomunion(buffer(the_geom,X))
> 
> 
>>That said, they should still work, so perhaps our buffer(0) semantics in 
>>GEOS far fubar.
> 
> 	
> With various geometries I can have geomunion crash on positive-sized
> buffers too; but the first one I saw had a zero, so I was thinking
> the empty geometry collection might have caused the geomunion to crash.
> 
> 
>>I found that example 1 worked with a positive buffer, but with 0 my test 
>>platform gave the same results as yours, an empty geometrycollection.
> 
> 
> I can post larger examples (a dozen large polygons) that crash the backend 
> but haven't been on the mailinglist long enough to know the etiquite of 
> posting large exmaples..  Perhaps I'll put together a SQL script and
> post a link to it on the internet that show the problem with non-zero
> buffer sizes if that would be of interest here.
> 
>    Ron
> 
> 
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      | Paul Ramsey
      | Refractions Research
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