/*
* GeoTools - The Open Source Java GIS Toolkit
* http://geotools.org
*
* (C) 2004-2008, Open Source Geospatial Foundation (OSGeo)
*
* 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;
* version 2.1 of the License.
*
* 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.
*
* Created on 31-dic-2004
*/
package org.geotools.geometry.jts.coordinatesequence;
import com.vividsolutions.jts.algorithm.RobustDeterminant;
import com.vividsolutions.jts.geom.Coordinate;
import com.vividsolutions.jts.geom.CoordinateSequence;
/**
* Utility functions for coordinate sequences (extends the same named JTS class)
* @author Andrea Aime - OpenGeo
*
* @source $URL: http://svn.osgeo.org/geotools/trunk/modules/library/main/src/main/java/org/geotools/geometry/jts/coordinatesequence/CoordinateSequences.java $
*/
public class CoordinateSequences extends com.vividsolutions.jts.geom.CoordinateSequences {
/**
* Computes whether a ring defined by an array of {@link Coordinate}s is
* oriented counter-clockwise.
* <ul>
* <li>The list of points is assumed to have the first and last points equal.
* <li>This will handle coordinate lists which contain repeated points.
* </ul>
* This algorithm is <b>only</b> guaranteed to work with valid rings.
* If the ring is invalid (e.g. self-crosses or touches),
* the computed result may not be correct.
*
* @param ring an array of Coordinates forming a ring
* @return true if the ring is oriented counter-clockwise.
*/
public static boolean isCCW(CoordinateSequence ring) {
// # of points without closing endpoint
int nPts = ring.size() - 1;
// find highest point
double hiy = ring.getOrdinate(0, 1);
int hiIndex = 0;
for (int i = 1; i <= nPts; i++) {
if (ring.getOrdinate(i, 1) > hiy) {
hiy = ring.getOrdinate(i, 1);
hiIndex = i;
}
}
// find distinct point before highest point
int iPrev = hiIndex;
do {
iPrev = iPrev - 1;
if (iPrev < 0) iPrev = nPts;
} while (equals2D(ring, iPrev, hiIndex) && iPrev != hiIndex);
// find distinct point after highest point
int iNext = hiIndex;
do {
iNext = (iNext + 1) % nPts;
} while (equals2D(ring, iNext, hiIndex) && iNext != hiIndex);
/**
* This check catches cases where the ring contains an A-B-A configuration of points.
* This can happen if the ring does not contain 3 distinct points
* (including the case where the input array has fewer than 4 elements),
* or it contains coincident line segments.
*/
if (equals2D(ring, iPrev, hiIndex) || equals2D(ring, iNext, hiIndex)|| equals2D(ring, iPrev, iNext))
return false;
int disc = computeOrientation(ring, iPrev, hiIndex, iNext);
/**
* If disc is exactly 0, lines are collinear. There are two possible cases:
* (1) the lines lie along the x axis in opposite directions
* (2) the lines lie on top of one another
*
* (1) is handled by checking if next is left of prev ==> CCW
* (2) will never happen if the ring is valid, so don't check for it
* (Might want to assert this)
*/
boolean isCCW = false;
if (disc == 0) {
// poly is CCW if prev x is right of next x
isCCW = (ring.getOrdinate(iPrev, 0) > ring.getOrdinate(iNext, 0));
} else {
// if area is positive, points are ordered CCW
isCCW = (disc > 0);
}
return isCCW;
}
private static boolean equals2D(CoordinateSequence cs, int i, int j) {
return cs.getOrdinate(i, 0) == cs.getOrdinate(j, 0) &&
cs.getOrdinate(i, 1) == cs.getOrdinate(j, 1);
}
public static int computeOrientation(CoordinateSequence cs, int p1, int p2, int q) {
// travelling along p1->p2, turn counter clockwise to get to q return 1,
// travelling along p1->p2, turn clockwise to get to q return -1,
// p1, p2 and q are colinear return 0.
double p1x = cs.getOrdinate(p1, 0);
double p1y = cs.getOrdinate(p1, 1);
double p2x = cs.getOrdinate(p2, 0);
double p2y = cs.getOrdinate(p2, 1);
double qx = cs.getOrdinate(q, 0);
double qy = cs.getOrdinate(q, 1);
double dx1 = p2x - p1x;
double dy1 = p2y - p1y;
double dx2 = qx - p2x;
double dy2 = qy - p2y;
return RobustDeterminant.signOfDet2x2(dx1, dy1, dx2, dy2);
}
}