Supported versions: latest (3.8) 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 main dev
Unsupported versions:2.6 2.5 2.4 2.3 2.2 2.1 2.0

pgr_alphaShape

pgr_alphaShape — Polygon part of an alpha shape.

Availability

• Version 3.0.0

  • Breaking change on signature

  • Old signature no longer supported

  • Boost 1.54 & Boost 1.55 are supported

  • Boost 1.56+ is preferable

    • Boost Geometry is stable on Boost 1.56

• Version 2.1.0

  • Added alpha argument with default 0 (use optimal value)

  • Support to return multiple outer/inner ring

• Version 2.0.0

  • New official function.

  • Renamed from version 1.x

Support

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

  • $spoon\mathrm{\_}radius=\sqrt{a}lpha$

• A Triangle area is considered part of the alpha shape when $circumcenterradius<spoon\mathrm{\_}radius$

• The alpha parameter is the spoon radius

• When the total number of points is less than 3, returns an EMPTY geometry

Boost Graph Inside

Signatures

pgr_alphaShape(geometry, [alpha])

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(geom)
      FROM vertices), 1.5));
 st_area
---------
    9.75
(1 row)

Parameters

Parameter	Type	Default	Description
geometry	geometry		Geometry with at least $3$ points
alpha	FLOAT	0	The radius of the spoon.

Return Value

Kind of geometry	Description
GEOMETRY COLLECTION	A Geometry collection of Polygons

See Also

• pgr_drivingDistance

• Sample Data

• ST_ConcaveHull

Indices and tables

• Index

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