On Mon, May 28, 2012 at 11:56:13AM +0200, Michiel J. van Heek wrote:
> Sandro,
>
> Based on your suggestions, I changed the script in three ways (see
> below for the result):
>
> 1. The number of iterations is now a parameter and running down the
> tolerance uses linear steps for each iteration (0.5, 0.45, 0.4,
> 0.35, etc.). The division by 2 solution (0.5, 0.25, 0.125, etc.) put
> a heavy weight on the lower tolerances, whereas it is often
> worthwile to also try higher tolerances (between initial tolerance
> and half of the initial tolerance).
>
> 2. If an iteration fails because of "not simple" the same iteration
> is run again, but with "PreserveTopology" now.
>
> 3. When all iterations fail, the function returns NULL now, and does
> not throw an exception. It is now possible to find "unwilling" edges
> by running:
>
> SELECT *
> FROM (
> SELECT
> edge_id,
> SimplifyEdgeGeom('countries_topology_4', edge_id, 0.5) AS tolerance
> FROM countries_topology_4.edge
> ) AS foo
> WHERE (tolerance IS NULL);
>
> Also, I removed the STABLE attribute, because the funtion *does*
> make changes to the database.
Good work ! Maybe make the "curve not simple" handling more tolerant of
eventual changes in the exception text (only look for "not simple"?).
How do your country boundaries look now ? :)
--strk;
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> CREATE OR REPLACE FUNCTION SimplifyEdgeGeom(topology_param text,
> edge_id_param integer, tolerance_param double precision,
> num_iterations_param integer DEFAULT 10) RETURNS double precision AS
> $BD$
> DECLARE
> i integer := 1;
> xi text := '';
> tolerance_var double precision := tolerance_param;
> sql_var text;
> BEGIN
> WHILE (i <= num_iterations_param) LOOP
> sql_var := 'SELECT topology.ST_ChangeEdgeGeom(' ||
> quote_literal(topology_param) || ', ' || edge_id_param
> || ', ST_Simplify' || xi || '(geom, ' || tolerance_var || ')) FROM '
> || quote_ident(topology_param) || '.edge WHERE edge_id = '
> || edge_id_param;
> BEGIN
> RAISE DEBUG 'Running %', sql_var;
> EXECUTE sql_var;
>
> RETURN tolerance_var;
> EXCEPTION
> WHEN OTHERS THEN
> RAISE WARNING 'Simplification of edge % failed in
> iteration %_%, with tolerance %: %',
> edge_id_param, i, xi, tolerance_var, SQLERRM;
>
> IF (SQLERRM = 'SQL/MM Spatial exception - curve not
> simple') AND (xi = '') THEN
> xi := 'PreserveTopology';
> ELSE
> i := i + 1;
> xi := '';
> tolerance_var := tolerance_var -
> (tolerance_param / num_iterations_param);
> END IF;
> END;
> -- END EXCEPTION
> END LOOP;
>
> RETURN NULL;
> END;
> $BD$ LANGUAGE 'plpgsql' STRICT;
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