Hi All,
Running 1.3.5 still :). Found some unexpected (to me), but
pleasant behavior with ST_DWithin. What we're doing is creating a function
that we can use to randomize a point within a minimum and maximum bounds.
Our original approach was to generate the geometry of the difference of two
buffers (a donut, if you will) and then place a random point within that.
Better to test with ST_DWithin directly then buffer, so we rewrote:
BEGIN
xmin = ST_XMin(geom) - dough;
deltax = ST_XMax(geom) - xmin + dough;
ymin = ST_YMin(geom) - dough;
deltay = ST_YMax(geom) - ymin + dough;
WHILE i < itermax LOOP
i = i + 1;
pointx = xmin + deltax * random();
pointy = ymin + deltay * random();
returnpoint = ST_SetSRID( ST_MakePoint(
pointx, pointy ), ST_SRID(geom) );
EXIT WHEN
ST_DWithin( returnpoint,
geom, dough )
AND NOT
ST_DWithin( returnpoint,
geom, nut );
END LOOP;
IF i > itermax THEN
RAISE NOTICE 'Reached maximum iterations
without placing point inside donut.';
END IF;
RETURN returnpoint;
END;
The puzzling part is that even if the parameters are passed out of order,
i.e. the inner ring is passed for the outer ring and vice versa, we get a
fine result. It's a nice unexpected behavior, but unexpected none-the-less.
Anyone have any thoughts on why this works? Full function below:
http://www.clemetparks.com/images/esig/cmp-ms-90x122.pngStephen Mather
Geographic Information Systems (GIS) Manager
(216) 635-3243
s...@clevelandmetroparks.com
<http://www.clemetparks.com/> clevelandmetroparks.com
CREATE OR REPLACE FUNCTION ST_Donut
( geom Geometry,
dough DOUBLE PRECISION,
nut DOUBLE PRECISION,
itermax INTEGER)
RETURNS Geometry
AS $$
DECLARE
xmin DOUBLE PRECISION;
ymin DOUBLE PRECISION;
deltax DOUBLE PRECISION;
deltay DOUBLE PRECISION;
pointx DOUBLE PRECISION;
pointy DOUBLE PRECISION;
returnpoint Geometry;
i INTEGER := 0;
nut1 DOUBLE PRECISION;
dough1 DOUBLE PRECISION;
BEGIN
xmin = ST_XMin(geom) - dough;
deltax = ST_XMax(geom) - xmin + dough;
ymin = ST_YMin(geom) - dough;
deltay = ST_YMax(geom) - ymin + dough;
WHILE i < itermax LOOP
i = i + 1;
pointx = xmin + deltax * random();
pointy = ymin + deltay * random();
returnpoint = ST_SetSRID( ST_MakePoint(
pointx, pointy ), ST_SRID(geom) );
EXIT WHEN
ST_DWithin( returnpoint,
geom, dough )
AND NOT
ST_DWithin( returnpoint,
geom, nut );
END LOOP;
IF i > itermax THEN
RAISE NOTICE 'Reached maximum iterations
without placing point inside donut.';
END IF;
RETURN returnpoint;
END;
$$ LANGUAGE plpgsql;
<<image001.png>>
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