pgr_alphaShape

pgr_alphaShape — Polygon part of an alpha shape.

Availability

  • Version 3.0.0
    • Breaking change on signature
  • Version 2.1.0
    • Added alpha argument with default 0 (use optimal value)
    • Support to return multiple outer/inner ring
  • Renamed in v2.0.0

Description

Returns the polygon part of an alpha shape.

Characteristics

  • Input is a geometry and returns a geometry
  • Uses PostGis ST_DelaunyTriangles
  • Instead of using CGAL’s definition of alpha it use the spoon_radius
    • \(spoon\_radius = \sqrt alpha\)
  • A Triangle area is considered part of the alpha shape when \(circumcenter\ radius < spoon\_radius\)
  • When the total number of points is less than 3, returns an EMPTY geometry

Signatures

Summary

pgr_alphaShape(geometry,   [spoon_radius])
RETURNS geometry

Example: passing a geometry collection with spoon radius \(1.5\) using the return variable geom

SELECT ST_Area(pgr_alphaShape((SELECT ST_Collect(the_geom) FROM edge_table_vertices_pgr), 1.5));
 st_area
---------
    9.75
(1 row)

Parameters

Parameter Type Default Description
geometry geometry   Geometry with at least \(3\) points
spoon_radius FLOAT   The radius of the spoon

Return Value

Kind of geometry Description
GEOMETRY COLLECTION A Geometry collection of Polygons

See Also

Indices and tables