On Tuesday 12 February 2008, Paul Ramsey wrote: > OK, it was a bit harder than you interpreted (takes more than a SQL > cut'n'paste) and not as straightforward as I thought it might be (the > simplify function that was already there was a native implementation, > not a GEOS call, so cutting and pasting it would do no good). > > Anyhow, I have applied my incredible text editing skills and now there > is a ST_SimplifyPreserveTopology(geometry, float) in the -svn drop. > > Paul
Nice work Paul- Am I correct in understanding that the ST_SimplifyPreserveTopology() function would give topologically correct polygons after simplification? In other words, it would preserve adjacency after simplification? I have noticed that running simplify() on a set of adjacent polygons sometimes has the not-so-nice result of causing previously overlapping (dang simple features!) edges to separate. Dylan > > On Feb 8, 2008, at 10:21 PM, Brian Hamlin wrote: > > It recently came up in the IRC channel FreeNode #postgis > > that there is a GEOS TopologyPreserveSimplify, in addition > > to Simplify2D. > > > > I did a bit of checking just now.. on my machine, I see > > nm /usr/local/pgsql/lib/liblwgeom.1.3.so > > > > _LWGEOM_simplify2d > > _pg_finfo_LWGEOM_simplify2d > > _simplify2d_lwgeom > > > > and some others, but no TopologyPreserve anything > > > > nm /usr/lcoal/lib/libgeos_c.1.4.1.dylib > > > > _GEOSSimplify > > _GEOSTopologyPreserveSimplify > > > > and lastly, in lwpostgis.sql > > CREATE OR REPLACE FUNCTION simplify(geometry, float8) > > RETURNS geometry > > AS '$libdir/liblwgeom', 'LWGEOM_simplify2d' > > LANGUAGE 'C' IMMUTABLE STRICT; > > > > -- > > since the last SQL line matches the exported symbol name from > > liblwgeom > > exactly, minus the front _, looks like there is the exact entry point. > > > > Clearly no parallel entry for TopologyPreseveSimplify > > > > this makes me think that there is a little more to adding the call to > > lwpostgis.sql than copying and pasting the declaration. > > > > insights welcome > > -Brian > > > > _______________________________________________ > > postgis-users mailing list > > [email protected] > > http://postgis.refractions.net/mailman/listinfo/postgis-users > > _______________________________________________ > postgis-users mailing list > [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-users -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
