[postgis-users] [postgis-devel] Problem in st_StriaghtSkeleton - Hangs!

Thu Jan 18 06:48:38 PST 2018

This provided some output for me:

select st_straightskeleton(ST_Rotate(ST_SnapToGrid(ST_Rotate('POLYGON((256
760,518 760,518 630,674 630,674
239,673 239,127 239,127 240,126 240,126 513,127 513,127 514,126 514,126
630,255 630,256 630,256 760),(128 629,128 423,270 423,270 422,271
422,271 240,672 240,672 629,128 629),(258 759,258 631,516 631,516
759,258 759),(128 421,128 240,269 240,269 421,128 421))'::geometry, 0.001),
0.001), -0.001));

чт, 18 янв. 2018 г. в 17:44, Stephen Woodbridge <woodbri at swoodbridge.com>:

> On 1/18/2018 3:54 AM, Martijn Meijers wrote:
> > Hi,
> >
> > I think the problem with this input is that there is *a lot* of parallel
> > lines, meaning that in the algorithm of skeleton construction many
> > things happen at exactly the same time (if you see it as a wavefront
> > propagation). I also tried with my Python implementation [1], but this
> > also crashes (-:
> >
> > One option I'd try is to slightly perturb/add some randomness to your
> > input coordinates (to take away the lines being exactly parallel) and
> > see if that goes through...
>
> This data is generated from a scan of a building floor plan by passing
> it through st_dumpaspolygons(rast, 1, true) so it will always have lots
> of parallel lines. This is just one of ten similar drawings but its the
> only one that fails. I'll be running a larger set today and may have
> some additional failures.
>
> I tried Regina's suggestion of using ST_ApproximateMedialAxis and
> ST_SubDivide but that also seems to hang in my work flow. I'll take
> another look through my queries to make sure I incorporated it correctly.
>
> How would you go about perturbing the input coordinates?
>
> Thanks,
>    -Steve
>
> > Martijn
> >
> >
> > [1] https://bitbucket.org/bmmeijers/grassfire/
> >
> > On 18-01-18 02:21, Stephen Woodbridge wrote:
> >> Hi Regina,
> >>
> >> Thanks, this is a great idea. I rework my queries to work in
> >> ST_ApproximateMedialAxis and ST_SubDivide and see how that does.
> >>
> >> -Steve
> >>
> >> On 1/17/2018 8:13 PM, Regina Obe wrote:
> >>> Steve,
> >>>
> >>> One more thought.  Can you try this and see if it works.  I think
> >>> ST_StraightSkeleton and ST_ApproximateMedialAxis are having issues,
> >>> because in this case your holes are so big that it almost cuts into
> >>> another.
> >>>
> >>> So to work around the issue, you can subdivide the polygon and then
> >>> do the analyze like this:
> >>>
> >>> SELECT  ST_ApproximateMedialAxis(ST_SubDivide('POLYGON((256 760,518
> >>> 760,518 630,674 630,674
> >>> 239,673 239,127 239,127 240,126 240,126 513,127 513,127 514,126 514,126
> >>> 630,255 630,256 630,256 760),(128 629,128 423,270 423,270 422,271
> >>> 422,271 240,672 240,672 629,128 629),(258 759,258 631,516 631,516
> >>> 759,258 759),(128 421,128 240,269 240,269 421,128 421))'::geometry,
> >>> 12));
> >>>
> >>> You might need to tweak with the subdivide param.
> >>>
> >>> http://postgis.net/docs/manual-dev/ST_Subdivide.html
> >>>
> >>> The subdivide cuts right into the holes turning them not into holes
> >>> so you end up with simple polygons that approximate medial axis can
> >>> handle.
> >>>
> >>> Once you are done you can run ST_Union on your set of Linestrings.
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> -----Original Message-----
> >>> From: postgis-devel [mailto:postgis-devel-bounces at lists.osgeo.org] On
> >>> Behalf Of Stephen Woodbridge
> >>> Sent: Wednesday, January 17, 2018 2:47 PM
> >>> To: PostGIS Development Discussion <postgis-devel at lists.osgeo.org>
> >>> Subject: [postgis-devel] Problem in st_StriaghtSkeleton - Hangs!
> >>>
> >>> Hi all,
> >>>
> >>> I have run into a problem using st_StraightSkelton() where it appears
> >>> to hang at 100 cpu. Maybe an endless loop?
> >>>
> >>> select st_straightskeleton('POLYGON((256 760,518 760,518 630,674
> 630,674
> >>> 239,673 239,127 239,127 240,126 240,126 513,127 513,127 514,126 514,126
> >>> 630,255 630,256 630,256 760),(128 629,128 423,270 423,270 422,271
> >>> 422,271 240,672 240,672 629,128 629),(258 759,258 631,516 631,516
> >>> 759,258 759),(128 421,128 240,269 240,269 421,128 421))'::geometry);
> >>>
> >>> POSTGIS="2.3.3 r15473" GEOS="3.5.1-CAPI-1.9.1 r4246" SFCGAL="1.2.2"
> >>> PROJ="Rel. 4.9.2, 08 September 2015" GDAL="GDAL 1.11.3, released
> >>> 2015/09/16" LIBXML="2.9.3" LIBJSON="0.11.99" RASTER
> >>>
> >>> Can anyone reproduce this?
> >>> Any idea how to work around it?
> >>>
> >>> Is there a non-CGAL version of skeleton?
> >>>
> >>> Thanks,
> >>>     -Steve
> >>>
> >>> ---
> >>> This email has been checked for viruses by Avast antivirus software.
> >>> https://www.avast.com/antivirus
> >>>
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