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

[Martijn Meijers]

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...


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
>>
>> ---
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