[mapguide-users] OGR / GEOS selection problem
Kenneth, GEOGRAF A/S
ks at geograf.dk
Tue Jun 12 10:36:21 EDT 2007
Yes, I have tried doing the selection with a triangle, and it also
works, but there is another bug in GEOS, that makes it EXTREMELY slow
for any geometries with +2000 points.
I have posted some info on trac about it:
http://trac.osgeo.org/mapguide/ticket/172
So, the original problem is "fixed", but it opens another problem, which
may have more to do with GEOS than with the provider.
I think the related speed problem makes the provider more useless with
the fix than without.
I can build a new provider, but I don't have the setup for Oracle and
PostGIS.
Regards, Kenneth, GEOGRAF A/S
Traian Stanev skrev:
>
>
>
> Hi.
>
>
>
> I checked in a temporary fix to this problem. I simply warped one of
> the coordinates of the input polygon by 10^-10. This is sufficient to
> make OGR call GEOS to compute the exact intersection. Selection should
> work correctly now in the AJAX viewer.
>
>
>
> I have not posted a compiled DLL yet, so Kenneth feel free to do so, (-:
>
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> Traian
>
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>
>
>
> *From:* mapguide-users-bounces at lists.osgeo.org
> [mailto:mapguide-users-bounces at lists.osgeo.org] *On Behalf Of *Traian
> Stanev
> *Sent:* Thursday, June 07, 2007 3:46 PM
> *To:* MapGuide Users Mail List; MapGuide Users Mail List
> *Subject:* RE: [mapguide-users] OGR / GEOS selection problem
>
>
>
> 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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