DouglasPeuckerLineSimplifier.java

/*
* The JTS Topology Suite is a collection of Java classes that
* implement the fundamental operations required to validate a given
* geo-spatial data set to a known topological specification.
*
* Copyright (C) 2001 Vivid Solutions
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*
* For more information, contact:
*
*     Vivid Solutions
*     Suite #1A
*     2328 Government Street
*     Victoria BC  V8T 5G5
*     Canada
*
*     (250)385-6040
*     www.vividsolutions.com
*/

package com.vividsolutions.jts.simplify;

import java.util.*;
import com.vividsolutions.jts.geom.*;

/**
 * Simplifies a linestring (sequence of points) using
 * the standard Douglas-Peucker algorithm.
 *
 * @version 1.7
 */
class DouglasPeuckerLineSimplifier
{
  public static Coordinate[] simplify(Coordinate[] pts, double distanceTolerance)
  {
    DouglasPeuckerLineSimplifier simp = new DouglasPeuckerLineSimplifier(pts);
    simp.setDistanceTolerance(distanceTolerance);
    return simp.simplify();
  }

  private Coordinate[] pts;
  private boolean[] usePt;
  private double distanceTolerance;

  public DouglasPeuckerLineSimplifier(Coordinate[] pts)
  {
    this.pts = pts;
  }
  /**
   * Sets the distance tolerance for the simplification.
   * All vertices in the simplified linestring will be within this
   * distance of the original linestring.
   *
   * @param distanceTolerance the approximation tolerance to use
   */
  public void setDistanceTolerance(double distanceTolerance) {
    this.distanceTolerance = distanceTolerance;
  }

  public Coordinate[] simplify()
  {
    usePt = new boolean[pts.length];
    for (int i = 0; i < pts.length; i++) {
      usePt[i] = true;
    }
    simplifySection(0, pts.length - 1);
    CoordinateList coordList = new CoordinateList();
    for (int i = 0; i < pts.length; i++) {
      if (usePt[i])
        coordList.add(new Coordinate(pts[i]));
    }
    return coordList.toCoordinateArray();
  }

  private LineSegment seg = new LineSegment();

  private void simplifySection(int i, int j)
  {
    if((i+1) == j) {
      return;
    }
    seg.p0 = pts[i];
    seg.p1 = pts[j];
    double maxDistance = -1.0;
    int maxIndex = i;
    for (int k = i + 1; k < j; k++) {
      double distance = seg.distance(pts[k]);
      if (distance > maxDistance) {
        maxDistance = distance;
        maxIndex = k;
      }
    }
    if (maxDistance <= distanceTolerance) {
      for(int k = i + 1; k < j; k++) {
        usePt[k] = false;
      }
    }
    else {
      simplifySection(i, maxIndex);
      simplifySection(maxIndex, j);
    }
  }

}