[geos-devel] [Fwd: [mapguide-trac] #172: Bug in Selection polygon from PostGiS (PostgreSql)]

[Mateusz Loskot]

Hi,

I'm forwarding Ticket reported to the MapGuide Trac system:

http://trac.osgeo.org/mapguide/ticket/172#comment:6

It's also related to GEOS and may be interesting.

Shortly, one of datasource supported by MapGuide is OGR.
OGR uses GEOS, so MapGuide indirectly uses GEOS capabilities
for various geometric operations.

I believe it's worth to check the whole ticket, but below
I pasted most interesting comment:

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#172: Bug in Selection polygon from PostGiS (PostgreSql)
------------------------------------------+---------------------------------
 Reporter:  maciej.skorczewski at procad.pl  |         Owner:
     Type:  defect                        |        Status:  new
 Priority:  high                          |     Milestone:
Component:  General                       |       Version:  1.2.0
 Severity:  major                         |    Resolution:
 Keywords:                                |   External_id:  939561
------------------------------------------+---------------------------------
Comment (by ksgeograf):

 I have been examining the problem, using a custom build of the OGR
 provider.
 It is true that without GEOS support in OGR, the Intersect only does a
 BBOX check.

 However, merely enabling GEOS will not work, as the viewer sends a square
 that is orthogonal to the axes (like a BBOX). The GEOS library will
 interpret this as another BBOX, and only perform the BBOX check anyway.

 When I change the polgyon sent from the viewer to a triangle, it takes 22
 seconds to calculate the Intersects result with a polygon with 8000
 linesegments. I have a 2+ GHz machine.

 It seems that GEOS always builds a full intersection matrix, before
 returning the result. This is obviously not optimal for a simple hit test.
 Even with this knowledge, I still don't see how it can take 22 seconds to
 calculate 8000 * 3 intersections.

 Can someone explain this, or how to fix it?
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Cheers
-- 
Mateusz Loskot
http://mateusz.loskot.net