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
     */
    double x0 = ring.getX(0);
    double yPrev = ring.getY(0);
    double yCurr = ring.getY(1);
    double sum = 0.0;
    for (int i = 1; i < n - 1; i++) {
      double yNext = ring.getY(i + 1);
      sum += (ring.getX(i) - x0) * (yPrev - yNext);
      yPrev = yCurr;
      yCurr = yNext;
    }
    return sum / 2.0;
  }

}