[mapguide-users] OGR / GEOS selection problem

[Traian Stanev]

Hi Kenneth,
 
Thanks for tracking this down. I would try to fix this by bypassing Geos. FDO has utility code that can check for an intersection. We could take the BBOX result and pass that through a real intersection check on the provider side. This way the intersection check would be performed only on geometries that are close enough to the query polygon so the overhead should not be too bad.
 
 
Traian
 

	-----Original Message----- 
	From: mapguide-users-bounces at lists.osgeo.org on behalf of Kenneth, GEOGRAF A/S 
	Sent: Thu 6/7/2007 2:54 AM 
	To: MapGuide Users Mail List 
	Cc: 
	Subject: [mapguide-users] OGR / GEOS selection problem
	
	

	Maciej previously reported problems with the OGR provider using PostGIS.
	
	The problem is with the OGR provider.
	Se the Trac ticket for details:
	http://trac.osgeo.org/mapguide/ticket/172
	
	I have posted some follow up on the issue, but in hope someone knows a
	lot about GEOS, I'm also posting it here:
	
	
	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?
	
	
	
	--
	Regards, Kenneth, GEOGRAF A/S
	
	
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