com.vividsolutions.jts.algorithm.Centroid Maven / Gradle / Ivy
The newest version!
/*
* 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.algorithm;
import com.vividsolutions.jts.geom.*;
/**
* Computes the centroid of a {@link Geometry} of any dimension.
* If the geometry is nominally of higher dimension,
* but has lower effective dimension
* (i.e. contains only components
* having zero length or area),
* the centroid will be computed as for the equivalent lower-dimension geometry.
* If the input geometry is empty, a
* null Coordinate is returned.
*
* Algorithm
*
* - Dimension 2 - the centroid is computed
* as the weighted sum of the centroids
* of a decomposition of the area into (possibly overlapping) triangles.
* Holes and multipolygons are handled correctly.
* See
http://www.faqs.org/faqs/graphics/algorithms-faq/
* for further details of the basic approach.
*
* - Dimension 1 - Computes the average of the midpoints
* of all line segments weighted by the segment length.
* Zero-length lines are treated as points.
*
*
- Dimension 0 - Compute the average coordinate for all points.
* Repeated points are all included in the average.
*
*
* @version 1.7
*/
public class Centroid
{
/**
* Computes the centroid point of a geometry.
*
* @param geom the geometry to use
* @return the centroid point, or null if the geometry is empty
*/
public static Coordinate getCentroid(Geometry geom)
{
Centroid cent = new Centroid(geom);
return cent.getCentroid();
}
private Coordinate areaBasePt = null;// the point all triangles are based at
private Coordinate triangleCent3 = new Coordinate();// temporary variable to hold centroid of triangle
private double areasum2 = 0; /* Partial area sum */
private Coordinate cg3 = new Coordinate(); // partial centroid sum
// data for linear centroid computation, if needed
private Coordinate lineCentSum = new Coordinate();
private double totalLength = 0.0;
private int ptCount = 0;
private Coordinate ptCentSum = new Coordinate();
/**
* Creates a new instance for computing the centroid of a geometry
*/
public Centroid(Geometry geom)
{
areaBasePt = null;
add(geom);
}
/**
* Adds a Geometry to the centroid total.
*
* @param geom the geometry to add
*/
private void add(Geometry geom)
{
if (geom.isEmpty())
return;
if (geom instanceof Point) {
addPoint(geom.getCoordinate());
}
else if (geom instanceof LineString) {
addLineSegments(geom.getCoordinates());
}
else if (geom instanceof Polygon) {
Polygon poly = (Polygon) geom;
add(poly);
}
else if (geom instanceof GeometryCollection) {
GeometryCollection gc = (GeometryCollection) geom;
for (int i = 0; i < gc.getNumGeometries(); i++) {
add(gc.getGeometryN(i));
}
}
}
/**
* Gets the computed centroid.
*
* @return the computed centroid, or null if the input is empty
*/
public Coordinate getCentroid()
{
/**
* The centroid is computed from the highest dimension components present in the input.
* I.e. areas dominate lineal geometry, which dominates points.
* Degenerate geometry are computed using their effective dimension
* (e.g. areas may degenerate to lines or points)
*/
Coordinate cent = new Coordinate();
if (Math.abs(areasum2) > 0.0) {
/**
* Input contains areal geometry
*/
cent.x = cg3.x / 3 / areasum2;
cent.y = cg3.y / 3 / areasum2;
}
else if (totalLength > 0.0) {
/**
* Input contains lineal geometry
*/
cent.x = lineCentSum.x / totalLength;
cent.y = lineCentSum.y / totalLength;
}
else if (ptCount > 0){
/**
* Input contains puntal geometry only
*/
cent.x = ptCentSum.x / ptCount;
cent.y = ptCentSum.y / ptCount;
}
else {
return null;
}
return cent;
}
private void setBasePoint(Coordinate basePt)
{
if (this.areaBasePt == null)
this.areaBasePt = basePt;
}
private void add(Polygon poly)
{
addShell(poly.getExteriorRing().getCoordinates());
for (int i = 0; i < poly.getNumInteriorRing(); i++) {
addHole(poly.getInteriorRingN(i).getCoordinates());
}
}
private void addShell(Coordinate[] pts)
{
if (pts.length > 0)
setBasePoint(pts[0]);
boolean isPositiveArea = ! CGAlgorithms.isCCW(pts);
for (int i = 0; i < pts.length - 1; i++) {
addTriangle(areaBasePt, pts[i], pts[i+1], isPositiveArea);
}
addLineSegments(pts);
}
private void addHole(Coordinate[] pts)
{
boolean isPositiveArea = CGAlgorithms.isCCW(pts);
for (int i = 0; i < pts.length - 1; i++) {
addTriangle(areaBasePt, pts[i], pts[i+1], isPositiveArea);
}
addLineSegments(pts);
}
private void addTriangle(Coordinate p0, Coordinate p1, Coordinate p2, boolean isPositiveArea)
{
double sign = (isPositiveArea) ? 1.0 : -1.0;
centroid3( p0, p1, p2, triangleCent3 );
double area2 = area2( p0, p1, p2 );
cg3.x += sign * area2 * triangleCent3.x;
cg3.y += sign * area2 * triangleCent3.y;
areasum2 += sign * area2;
}
/**
* Computes three times the centroid of the triangle p1-p2-p3.
* The factor of 3 is
* left in to permit division to be avoided until later.
*/
private static void centroid3( Coordinate p1, Coordinate p2, Coordinate p3, Coordinate c )
{
c.x = p1.x + p2.x + p3.x;
c.y = p1.y + p2.y + p3.y;
return;
}
/**
* Returns twice the signed area of the triangle p1-p2-p3.
* The area is positive if the triangle is oriented CCW, and negative if CW.
*/
private static double area2( Coordinate p1, Coordinate p2, Coordinate p3 )
{
return
(p2.x - p1.x) * (p3.y - p1.y) -
(p3.x - p1.x) * (p2.y - p1.y);
}
/**
* Adds the line segments defined by an array of coordinates
* to the linear centroid accumulators.
*
* @param pts an array of {@link Coordinate}s
*/
private void addLineSegments(Coordinate[] pts)
{
double lineLen = 0.0;
for (int i = 0; i < pts.length - 1; i++) {
double segmentLen = pts[i].distance(pts[i + 1]);
if (segmentLen == 0.0)
continue;
lineLen += segmentLen;
double midx = (pts[i].x + pts[i + 1].x) / 2;
lineCentSum.x += segmentLen * midx;
double midy = (pts[i].y + pts[i + 1].y) / 2;
lineCentSum.y += segmentLen * midy;
}
totalLength += lineLen;
if (lineLen == 0.0 && pts.length > 0)
addPoint(pts[0]);
}
/**
* Adds a point to the point centroid accumulator.
* @param pt a {@link Coordinate}
*/
private void addPoint(Coordinate pt)
{
ptCount += 1;
ptCentSum.x += pt.x;
ptCentSum.y += pt.y;
}
}