`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
- 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
- \(spoon\_radius = \sqrt alpha\)
A Triangle area is considered part of the alpha shape when \(circumcenter\ radius < spoon\_radius\)
The alpha parameter is the spoon radius
When the total number of points is less than 3, returns an EMPTY geometry

Summary

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)

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

Kind of geometry	Description
GEOMETRY COLLECTION	A Geometry collection of Polygons