Gets the geometric center of a closed polygon. - Java 2D Graphics

Java examples for 2D Graphics:Polygon

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Description

Gets the geometric center of a closed polygon.

Demo Code


//package com.java2s;
import java.awt.geom.Point2D;

public class Main {
    /**// ww  w  .  j  a v  a2s  .c o m
     * Gets the centroid (geometric center) of a closed polygon.
     * The polygon must not be self intersecting, or this gives incorrect results.
     * Algorithm described here: http://local.wasp.uwa.edu.au/~pbourke/geometry/polyarea/
     *
     * @param p
     * @return
     */
    public static Point2D getCentroid(Point2D[] p) {
        double cx = 0;
        double cy = 0;
        for (int i = 0; i < p.length; i++) {
            int j = (i + 1) % p.length;
            double n = ((p[i].getX() * p[j].getY()) - (p[j].getX() * p[i]
                    .getY()));
            cx += (p[i].getX() + p[j].getX()) * n;
            cy += (p[i].getY() + p[j].getY()) * n;
        }
        double a = getArea(p);
        double f = 1 / (a * 6d);
        cx *= f;
        cy *= f;
        return new Point2D.Double(cx, cy);
    }

    /**
     * Gets the area of a closed polygon.
     * The polygon must not be self intersecting, or this gives incorrect results.
     * Algorithm described here: http://local.wasp.uwa.edu.au/~pbourke/geometry/polyarea/
     *
     * @param p points that define the polygon
     * @return
     */
    public static double getArea(Point2D[] p) {
        double a = 0;
        for (int i = 0; i < p.length; i++) {
            int j = (i + 1) % p.length;
            a += (p[i].getX() * p[j].getY());
            a -= (p[j].getX() * p[i].getY());
        }
        a *= 0.5;
        return a;
    }
}

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