pgr_alphaShape
¶
pgr_alphaShape
— Polygon part of an alpha shape.
Availability
Version 3.8.0
Deprecated function.
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
Migration of pgr_alphaShape
¶
Starting from v3.8.0
Before Deprecation: The following was calculated:
An alphaShape was calculated
After Deprecation:
PostGIS has two ways of generating alphaShape.
If you have SFCGAL, which you can install using
CREATE EXTENSION postgis_sfcgal
Since PostGIS 3.5+ use CG_AlphaShape
For PostGIS 3.5+ use the old name
ST_AlphaShape
Other PostGIS options are * ST_ConvexHull * ST_ConcaveHull
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 radiusWhen the total number of points is less than 3, returns an EMPTY geometry
Signatures¶
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));
WARNING: pgr_alphashape(geometry,double precision) deprecated function on v3.8.0
st_area
---------
9.75
(1 row)
Parameters¶
Parameter |
Type |
Default |
Description |
---|---|---|---|
geometry |
|
Geometry with at least \(3\) points |
|
|
|
0 |
The radius of the spoon. |
Return Value¶
Kind of geometry |
Description |
---|---|
GEOMETRY COLLECTION |
A Geometry collection of Polygons |
See Also¶
Indices and tables