CG_ApproximateMedialAxis

CG_ApproximateMedialAxis
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Name

CG_ApproximateMedialAxis — Compute the approximate medial axis of an areal geometry.

Synopsis

geometry CG_ApproximateMedialAxis(geometry geom);

geometry CG_ApproximateMedialAxis(geometry geom, boolean projected);

Description

Return an approximate medial axis for the areal input based on its straight skeleton. Uses an SFCGAL specific API when built against a capable version (1.2.0+). Otherwise the function is just a wrapper around CG_StraightSkeleton (slower case).

When projected is true, free endpoints of the medial axis are extended to reach the polygon boundary (projected medial axis). Requires SFCGAL 2.3.0+. When built against an older SFCGAL version, a notice is emitted and the non-projected result is returned instead.

Availability: 3.5.0

Availability: 3.7.0 - projected parameter. Requires SFCGAL >= 2.3.0 for projected result; falls back to non-projected with a notice on older versions.


	This function ignores the Z dimension. It always gives a 2D result even when used on a 3D geometry.

This method needs SFCGAL backend.

This function supports Polyhedral surfaces.

This function supports Triangles and Triangulated Irregular Network Surfaces (TIN).

Examples

A polygon and its approximate medial axis.

Code

WITH data AS (
  SELECT 'POLYGON((190 190,10 190,10 10,190 10,190 20,160 30,60 30,60 130,190 140,190 190))'::geometry AS polygon
)
SELECT polygon AS input_polygon,
       CG_ApproximateMedialAxis(polygon) AS medial_axis
FROM data;

Output

POLYGON((190 190,10 190,10 10,190 10,190 20,160 30,60 30,60 130,190 140,190 190)) | MULTILINESTRING((184.189 15.811,158.377 20),(50 20,158.377 20),(50 20,35 35),(35 153.151,35 35),(35 153.151,40.697 159.303),(164.04 164.04,40.697 159.303))

0103000000010000000A0000000000000000C067400000000000C0674000000000000024400000000000C06740000000000000244000000000000024400000000000C0674000000000000024400000000000C06740000000000000344000000000000064400000000000003E400000000000004E400000000000003E400000000000004E4000000000004060400000000000C0674000000000008061400000000000C067400000000000C06740 | 010500000006000000010200000002000000DEF0661B0906674027F290496E9F2F40BBE1CD3612CC6340000000000000344001020000000200000000000000000049400000000000003440BBE1CD3612CC634000000000000034400102000000020000000000000000004940000000000000344000000000008041400000000000804140010200000002000000000000000080414025E6642DD324634000000000008041400000000000804140010200000002000000000000000080414025E6642DD324634012E86DD339594440FB85248BB1E963400102000000020000002250E0B1468164402250E0B14681644012E86DD339594440FB85248BB1E96340

Figure

This example computes the projected medial axis with free endpoints extended to the polygon boundary.