Area.java
/*
* Copyright (c) 2016 Vivid Solutions.
*
* All rights reserved. This program and the accompanying materials
* are made available under the terms of the Eclipse Public License 2.0
* and Eclipse Distribution License v. 1.0 which accompanies this distribution.
* The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
* and the Eclipse Distribution License is available at
*
* http://www.eclipse.org/org/documents/edl-v10.php.
*/
package org.locationtech.jts.algorithm;
import org.locationtech.jts.geom.Coordinate;
import org.locationtech.jts.geom.CoordinateSequence;
/**
* Functions for computing area.
*
* @author Martin Davis
*
*/
public class Area {
/**
* Computes the area for a ring.
*
* @param ring the coordinates forming the ring
* @return the area of the ring
*/
public static double ofRing(Coordinate[] ring)
{
return Math.abs(ofRingSigned(ring));
}
/**
* Computes the area for a ring.
*
* @param ring the coordinates forming the ring
* @return the area of the ring
*/
public static double ofRing(CoordinateSequence ring)
{
return Math.abs(ofRingSigned(ring));
}
/**
* Computes the signed area for a ring. The signed area is positive if the
* ring is oriented CW, negative if the ring is oriented CCW, and zero if the
* ring is degenerate or flat.
*
* @param ring
* the coordinates forming the ring
* @return the signed area of the ring
*/
public static double ofRingSigned(Coordinate[] ring)
{
if (ring.length < 3)
return 0.0;
double sum = 0.0;
/*
* Based on the Shoelace formula.
* http://en.wikipedia.org/wiki/Shoelace_formula
*/
double x0 = ring[0].x;
for (int i = 1; i < ring.length - 1; i++) {
double x = ring[i].x - x0;
double y1 = ring[i + 1].y;
double y2 = ring[i - 1].y;
sum += x * (y2 - y1);
}
return sum / 2.0;
}
/**
* Computes the signed area for a ring. The signed area is:
* <ul>
* <li>positive if the ring is oriented CW
* <li>negative if the ring is oriented CCW
* <li>zero if the ring is degenerate or flat
* </ul>
*
* @param ring
* the coordinates forming the ring
* @return the signed area of the ring
*/
public static double ofRingSigned(CoordinateSequence ring)
{
int n = ring.size();
if (n < 3)
return 0.0;
/*
* Based on the Shoelace formula.
* http://en.wikipedia.org/wiki/Shoelace_formula
*/
Coordinate p0 = ring.createCoordinate();
Coordinate p1 = ring.createCoordinate();
Coordinate p2 = ring.createCoordinate();
ring.getCoordinate(0, p1);
ring.getCoordinate(1, p2);
double x0 = p1.x;
p2.x -= x0;
double sum = 0.0;
for (int i = 1; i < n - 1; i++) {
p0.y = p1.y;
p1.x = p2.x;
p1.y = p2.y;
ring.getCoordinate(i + 1, p2);
p2.x -= x0;
sum += p1.x * (p0.y - p2.y);
}
return sum / 2.0;
}
}