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
Signatures¶
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)
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 |