Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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centroid

Calculates the centroid of a geometry.

Description

The free function centroid calculates the geometric center (or: center of mass) of a geometry. It uses the default strategy, based on the coordinate system of the geometry.

Synopsis

template<typename Geometry, typename Point>
void centroid(Geometry const & geometry, Point & c)

Parameters

Type

Concept

Name

Description

Geometry const &

Any type fulfilling a Geometry Concept

geometry

A model of the specified concept

Point &

Any type fulfilling a Point Concept

c

The calculated centroid will be assigned to this point reference

Header

Either

#include <boost/geometry/geometry.hpp>

Or

#include <boost/geometry/algorithms/centroid.hpp>

Behavior

Case

Behavior

Point

Returns the point itself as the centroid

Multi Point

Calculates centroid (based on average)

linear (e.g. linestring)

Calculates centroid (based on weighted length)

areal (e.g. polygon)

Calculates centroid

Empty (e.g. polygon without points)

Throws a centroid_exception

Cartesian

Implemented

Spherical

Calculates the centroid as if based on Cartesian coordinates

Supported geometries

2D

3D

Point

ok

ok

Segment

ok

ok

Box

ok

ok

Linestring

ok

nyi

Ring

ok

nyi

Polygon

ok

nyi

MultiPoint

ok

ok

MultiLinestring

ok

nyi

MultiPolygon

ok

nyi

Complexity

Linear

Example

Shows calculation of a centroid of a polygon

#include <iostream>
#include <list>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>
#include <boost/geometry/geometries/polygon.hpp>

#include <boost/geometry/domains/gis/io/wkt/wkt.hpp>


int main()
{
    typedef boost::geometry::model::d2::point_xy<double> point_type;
    typedef boost::geometry::model::polygon<point_type> polygon_type;

    polygon_type poly;
    boost::geometry::read_wkt(
        "POLYGON((2 1.3,2.4 1.7,2.8 1.8,3.4 1.2,3.7 1.6,3.4 2,4.1 3,5.3 2.6,5.4 1.2,4.9 0.8,2.9 0.7,2 1.3)"
            "(4.0 2.0, 4.2 1.4, 4.8 1.9, 4.4 2.2, 4.0 2.0))", poly);

    point_type p;
    boost::geometry::centroid(poly, p);

    std::cout << "centroid: " << boost::geometry::dsv(p) << std::endl;
    
    return 0;
}

Output:

centroid: (4.04663, 1.6349)
centroid

Note that the centroid might be located in a hole or outside a polygon, easily.


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