I think so. I was planning to give it a try in the mapnik2 version.
> -----Mensaje original----- > De: Dane Springmeyer [mailto:[email protected]] > Enviado el: lunes, 07 de junio de 2010 3:28 > Para: Clos Crespo, Manel > CC: [email protected] > Asunto: Re: [Mapnik-users] [SOLVED] TextSymbolizer label > placement for polygon > > Should we add point_on_surface support to Mapnik's Symbolizers? > > Dane > > > On May 31, 2010, at 4:15 AM, <[email protected]> > <[email protected]> wrote: > > > This took a while to automate because of GDAL 1.5, so here it is: > > > > If you think this fits in the TextSymbolizer documentation > (http://trac.mapnik.org/wiki/TextSymbolizer), just tell me. > Corrections welcomed. > > > > PostGIS only > > ============ > > > > TextSymbolizer with polygon geometries: > > > > Default position for labels in polygons is polygon's > centroid. If you want your label to be placed on the surface > of the polygon you can use the ST_PointOnSurface function. > > > > 1. Create a geometry field "point_on_surface" in the same > table/layer > > 2. Run "update table_with_polygons set > point_on_surface=ST_PointOnSurface(the_geom)" on your database > > 3. Use the generated points to draw the labels > > > > Would you like to automate this, use a trigger to call a > function like: > > > > <code> > > CREATE OR REPLACE FUNCTION coloma_point_on_surface() > RETURNS "trigger" AS > > $BODY$ > > BEGIN > > NEW.point_on_surface:=ST_PointOnSurface(NEW.the_geom); > > RETURN NEW; > > END; > > $BODY$ > > LANGUAGE 'plpgsql' VOLATILE; > > ALTER FUNCTION coloma_point_on_surface() OWNER TO postgres; > > > > CREATE TRIGGER table_with_polygons_point_on_surface > > BEFORE INSERT OR UPDATE ON table_with_polygons FOR EACH ROW > > EXECUTE PROCEDURE coloma_point_on_surface(); > > </code> > > > > > > Workaround for GDAL < 1.6.0: > > > > * GDAL < 1.6.0 won't let you select which column to use > for the geometry, so perhaps selecting > > the "point_on_surface" geometry field as the main geometry. > > > > 1. Create a Character Varying field "point_on_surface" > > 2. Modify the above code to look like this: > > > > <code> > > -- Use ST_GeomFromText('point_on_surface', SRS) to get the > geometry back > > -- where SRS is the same as the layer SRS, i.e. 23031 > > CREATE OR REPLACE FUNCTION coloma_point_on_surface() > RETURNS "trigger" AS > > $BODY$ > > BEGIN > > -- Store geometry as text so GDAL uses "the_geom" as the > feature geometry > > NEW.point_on_surface:=ST_AsText(ST_PointOnSurface(NEW.the_geom)); > > RETURN NEW; > > END; > > $BODY$ > > LANGUAGE 'plpgsql' VOLATILE; > > ALTER FUNCTION coloma_point_on_surface() OWNER TO postgres; > > > > CREATE TRIGGER table_with_polygons_point_on_surface > > BEFORE INSERT OR UPDATE ON table_with_polygons FOR EACH ROW > > EXECUTE PROCEDURE coloma_point_on_surface(); > > </code> > > > > > > > >> -----Mensaje original----- > >> De: [email protected] > >> [mailto:[email protected]] En nombre de > >> [email protected] > >> Enviado el: lunes, 31 de mayo de 2010 10:17 > >> Para: [email protected] > >> CC: [email protected] > >> Asunto: Re: [Mapnik-users] TextSymbolizer label placement > for polygon > >> > >> Hi Numenor, > >> > >> ST_PointOnSurface helped a lot. It first killed postgres > >> because I tried using it directly in the SQL for ogcserver. I > >> tried adding a new field "point_on_surface" and updating it. > >> It works flawlessly! > >> > >> Still, this is not as optimal as a good label placement > >> algorithm, but will work meanwhile... > >> > >> And by the way, the polygon inside polygon ended being an > >> internal polygon sharing limits with the bigger one, but NOT > >> overlapping. So this is "solved" too. > >> > >> Thank you very much for your help! > >> > >> > >>> -----Mensaje original----- > >>> De: numenor [mailto:[email protected]] > >>> Enviado el: viernes, 28 de mayo de 2010 14:55 > >>> Para: Clos Crespo, Manel > >>> CC: [email protected] > >>> Asunto: Re: [Mapnik-users] TextSymbolizer label placement > >> for polygon > >>> > >>> Hi Manel, > >>> > >>> On Fri, 28 May 2010 14:21:08 +0200, > <[email protected]> wrote: > >>>> I've two problems with label placement: > >>>> > >>>> 1) When the centroid is outside of the polygon the label > >>> will appear to > >>> be > >>>> labeling just another polygon (confusing). > >>> > >>> Maybe the SQL extension function 'ST_PointOnSurface' can > >> help you (at > >>> least as a workaround), if you use PostGIS as data source. > >> For a given > >>> surface, this function returns a point which is guaranteed to > >>> lie on the > >>> surface. It does not guarantee any more, but with PostGIS, > >> for me for > >>> convex shapes the labels appear similarly placed as when > >>> using ST_Centroid > >>> (or mapnik's algorithm), more or less in the middle. > >>> > >>>> 2) When a polygon is inside a bigger one, and they both > >>> have a similar > >>>> centroid, both labels will end in the smaller polygon (really > >>> confusing), > >>>> and the bigger one will appear to have no label at all > >>> (more confusion). > >>> > >>> Is this problem solvable at all, in general? What if the > >>> outer polygon is > >>> completely covered by smaller polygons? > >>> > >>> If this can happen, maybe a better solution would be to label > >>> the border > >>> of the polygon, as is often done for borders between > administrative > >>> entities. > >>> > >>> If you know which smaller polygons lie inside the bigger one, > >>> you could > >>> subtract their area from the bigger one, and then use > >>> ST_PointOnSurface on > >>> the result to get the point for placement of the label. > >> This might of > >>> course be computationally expensive ... > >>> > >>> Hope this helps, > >>> -- > >>> Holger Schöner - [email protected] > >>> > >> _______________________________________________ > >> Mapnik-users mailing list > >> [email protected] > >> https://lists.berlios.de/mailman/listinfo/mapnik-users > >> > > _______________________________________________ > > Mapnik-users mailing list > > [email protected] > > https://lists.berlios.de/mailman/listinfo/mapnik-users > > _______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
