Name
CG_Simplify — Reduces the complexity of a geometry while preserving essential features and Z/M values.
Synopsis
geometry CG_Simplify(geometry geom, double precision threshold, boolean preserveTopology = false);
Description
Simplifies a geometry using SFCGAL's simplification algorithm, which reduces the number of points or vertices while preserving the essential features of the geometry. This function preserves Z and M values during simplification.
The algorithm is based on constrained triangulation and uses the CGAL Polyline Simplification 2 library with additional handling to preserve Z and M coordinates. When topology is preserved and geometries intersect, Z and M values are interpolated at intersection points.
This function works well with 3D terrain-like geometries (2.5D) but is not designed for vertical surfaces like walls.
Availability: 3.6.0 - requires SFCGAL >= 2.1.0
This method needs SFCGAL backend.
This function supports 3d and will not drop the z-index.
This function supports M coordinates.
Parameters
-
geom -
Input geometry
-
threshold -
Maximum distance threshold (in geometry unit) for simplification. The higher this value, the more simplified the resulting geometry will be.
-
preserveTopology -
If set to true, the function ensures that the topology of the geometry is preserved. When geometries intersect in this mode, Z and M values at intersection points are interpolated. The default value is false.
Return Value
Returns a simplified geometry with preserved Z and M values.
Examples
This example simplifies a polygon with a threshold of 0.5.
SELECT CG_Simplify(
'POLYGON((0 0,0 1,0.1 1,0.2 1,0.3 1,0.4 1,0.5 1,1 1,1 0,0 0))',
0.5
);
POLYGON((0 0,0 1,1 1,1 0,0 0))
Preserving topology can keep additional support points in a mixed ZM collection.
SELECT CG_Simplify(
'GEOMETRYCOLLECTION ZM(
LINESTRING ZM(-1 -1 3 4,0 0 10 100,1 1 20 200,0 2 15 150,0 5 30 300,2 19 25 250,-4 20 15 150),
POLYGON ZM((0 0 10 100,1 1 20 200,0 2 15 150,0 5 30 300,2 19 25 250,-4 20 15 150,0 0 10 100))
)',
2,
false
);
GEOMETRYCOLLECTION ZM (LINESTRING ZM (-1 -1 3 4,2 19 25 250,-4 20 15 150),POLYGON ZM ((0 0 10 100,2 19 25 250,-4 20 15 150,0 0 10 100)))
The topology-preserving variant keeps the shared anchor point at the collection intersection.
SELECT CG_Simplify(
'GEOMETRYCOLLECTION ZM(
LINESTRING ZM(-1 -1 3 4,0 0 10 100,1 1 20 200,0 2 15 150,0 5 30 300,2 19 25 250,-4 20 15 150),
POLYGON ZM((0 0 10 100,1 1 20 200,0 2 15 150,0 5 30 300,2 19 25 250,-4 20 15 150,0 0 10 100))
)',
2,
true
);
GEOMETRYCOLLECTION ZM (LINESTRING ZM (-1 -1 3 4,0 0 10 100,2 19 25 250,-4 20 15 150),POLYGON ZM ((0 0 10 100,2 19 25 250,-4 20 15 150,0 0 10 100)))
Compare simplification with and without topology preservation, using a threshold of 50.
WITH data AS (
SELECT ST_GeomFromText('GEOMETRYCOLLECTION(
POLYGON((88.46 158.85,90.77 171.54,147.31 173.85,146.15 145,173.85 119.62,146.15 103.46,112.69 118.46,91.92 93.08,65.38 101.15,34.23 121.92,41.15 142.69,49.23 143.85,88.46 158.85)),
POLYGON((112.69 118.46,146.15 103.46,190 60.77,185.38 43.46,126.54 26.15,83.85 28.46,67.69 64.23,43.46 58.46,10 83.85,34.23 121.92,65.38 101.15,91.92 93.08,112.69 118.46)))
') AS geom
)
SELECT geom AS input_geometry,
CG_Simplify(geom, 50, true) AS topology_preserved,
CG_Simplify(geom, 50, false) AS topology_not_preserved
FROM data;
GEOMETRYCOLLECTION(POLYGON((88.46 158.85,90.77 171.54,147.31 173.85,146.15 145,173.85 119.62,146.15 103.46,112.69 118.46,91.92 93.08,65.38 101.15,34.23 121.92,41.15 142.69,49.23 143.85,88.46 158.85)), POLYGON((112.69 118.46,146.15 103.46,190 60.77,185.38 43.46,126.54 26.15,83.85 28.46,67.69 64.23,43.46 58.46,10 83.85,34.23 121.92,65.38 101.15,91.92 93.08,112.69 118.46))) | GEOMETRYCOLLECTION(POLYGON((88.46 158.85,147.31 173.85,146.15 103.46,112.69 118.46,34.23 121.92,88.46 158.85)), POLYGON((112.69 118.46,146.15 103.46,185.38 43.46,10 83.85,34.23 121.92,112.69 118.46))) | GEOMETRYCOLLECTION(POLYGON((88.46 158.85,173.85 119.62,34.23 121.92,88.46 158.85)), POLYGON((112.69 118.46,190 60.77,10 83.85,112.69 118.46)))