OffsetPointGenerator.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.operation.overlay.validate;

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

/**
 * Generates points offset by a given distance 
 * from both sides of the midpoint of
 * all segments in a {@link Geometry}.
 * Can be used to generate probe points for
 * determining whether a polygonal overlay result
 * is incorrect.
 * The input geometry may have any orientation for its rings,
 * but {@link #setSidesToGenerate(boolean, boolean)} is
 * only meaningful if the orientation is known.
 *
 * @author Martin Davis
 * @version 1.7
 */
public class OffsetPointGenerator
{
  private Geometry g;
  private boolean doLeft = true; 
  private boolean doRight = true;
  
  public OffsetPointGenerator(Geometry g)
  {
    this.g = g;
  }

  /**
   * Set the sides on which to generate offset points.
   * 
   * @param doLeft
   * @param doRight
   */
  public void setSidesToGenerate(boolean doLeft, boolean doRight)
  {
    this.doLeft = doLeft;
    this.doRight = doRight;
  }
  
  /**
   * Gets the computed offset points.
   *
   * @return List<Coordinate>
   */
  public List getPoints(double offsetDistance)
  {
    List offsetPts = new ArrayList();
    List lines = LinearComponentExtracter.getLines(g);
    for (Iterator i = lines.iterator(); i.hasNext(); ) {
      LineString line = (LineString) i.next();
      extractPoints(line, offsetDistance, offsetPts);
    }
    //System.out.println(toMultiPoint(offsetPts));
    return offsetPts;
  }

  private void extractPoints(LineString line, double offsetDistance, List offsetPts)
  {
    Coordinate[] pts = line.getCoordinates();
    for (int i = 0; i < pts.length - 1; i++) {
    	computeOffsetPoints(pts[i], pts[i + 1], offsetDistance, offsetPts);
    }
  }

  /**
   * Generates the two points which are offset from the 
   * midpoint of the segment <tt>(p0, p1)</tt> by the
   * <tt>offsetDistance</tt>.
   * 
   * @param p0 the first point of the segment to offset from
   * @param p1 the second point of the segment to offset from
   */
  private void computeOffsetPoints(Coordinate p0, Coordinate p1, double offsetDistance, List offsetPts)
  {
    double dx = p1.x - p0.x;
    double dy = p1.y - p0.y;
    double len = Math.sqrt(dx * dx + dy * dy);
    // u is the vector that is the length of the offset, in the direction of the segment
    double ux = offsetDistance * dx / len;
    double uy = offsetDistance * dy / len;

    double midX = (p1.x + p0.x) / 2;
    double midY = (p1.y + p0.y) / 2;

    if (doLeft) {
      Coordinate offsetLeft = new Coordinate(midX - uy, midY + ux);
      offsetPts.add(offsetLeft);
    }
    
    if (doRight) {
      Coordinate offsetRight = new Coordinate(midX + uy, midY - ux);
      offsetPts.add(offsetRight);
    }
  }

}