com.vividsolutions.jts.algorithm.MinimumDiameter Maven / Gradle / Ivy
/*
* 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 minimum diameter of a {@link Geometry}.
* The minimum diameter is defined to be the
* width of the smallest band that
* contains the geometry,
* where a band is a strip of the plane defined
* by two parallel lines.
* This can be thought of as the smallest hole that the geometry can be
* moved through, with a single rotation.
*
* The first step in the algorithm is computing the convex hull of the Geometry.
* If the input Geometry is known to be convex, a hint can be supplied to
* avoid this computation.
*
* This class can also be used to compute a line segment representing
* the minimum diameter, the supporting line segment of the minimum diameter,
* and a minimum rectangle enclosing the input geometry.
* This rectangle will
* have width equal to the minimum diameter, and have one side
* parallel to the supporting segment.
*
* @see ConvexHull
*
* @version 1.7
*/
public class MinimumDiameter
{
/**
* Gets the minimum rectangle enclosing a geometry.
*
* @param geom the geometry
* @return the minimum rectangle enclosing the geometry
*/
public static Geometry getMinimumRectangle(Geometry geom) {
return (new MinimumDiameter(geom)).getMinimumRectangle();
}
/**
* Gets the length of the minimum diameter enclosing a geometry
* @param geom the geometry
* @return the length of the minimum diameter of the geometry
*/
public static Geometry getMinimumDiameter(Geometry geom) {
return (new MinimumDiameter(geom)).getDiameter();
}
private final Geometry inputGeom;
private final boolean isConvex;
private Coordinate[] convexHullPts = null;
private LineSegment minBaseSeg = new LineSegment();
private Coordinate minWidthPt = null;
private int minPtIndex;
private double minWidth = 0.0;
/**
* Compute a minimum diameter for a given {@link Geometry}.
*
* @param geom a Geometry
*/
public MinimumDiameter(Geometry inputGeom)
{
this(inputGeom, false);
}
/**
* Compute a minimum diameter for a giver {@link Geometry},
* with a hint if
* the Geometry is convex
* (e.g. a convex Polygon or LinearRing,
* or a two-point LineString, or a Point).
*
* @param geom a Geometry which is convex
* @param isConvex true if the input geometry is convex
*/
public MinimumDiameter(Geometry inputGeom, boolean isConvex)
{
this.inputGeom = inputGeom;
this.isConvex = isConvex;
}
/**
* Gets the length of the minimum diameter of the input Geometry
*
* @return the length of the minimum diameter
*/
public double getLength()
{
computeMinimumDiameter();
return minWidth;
}
/**
* Gets the {@link Coordinate} forming one end of the minimum diameter
*
* @return a coordinate forming one end of the minimum diameter
*/
public Coordinate getWidthCoordinate()
{
computeMinimumDiameter();
return minWidthPt;
}
/**
* Gets the segment forming the base of the minimum diameter
*
* @return the segment forming the base of the minimum diameter
*/
public LineString getSupportingSegment()
{
computeMinimumDiameter();
return inputGeom.getFactory().createLineString(new Coordinate[] { minBaseSeg.p0, minBaseSeg.p1 } );
}
/**
* Gets a {@link LineString} which is a minimum diameter
*
* @return a {@link LineString} which is a minimum diameter
*/
public LineString getDiameter()
{
computeMinimumDiameter();
// return empty linestring if no minimum width calculated
if (minWidthPt == null)
return inputGeom.getFactory().createLineString((Coordinate[])null);
Coordinate basePt = minBaseSeg.project(minWidthPt);
return inputGeom.getFactory().createLineString(new Coordinate[] { basePt, minWidthPt } );
}
private void computeMinimumDiameter()
{
// check if computation is cached
if (minWidthPt != null)
return;
if (isConvex)
computeWidthConvex(inputGeom);
else {
Geometry convexGeom = (new ConvexHull(inputGeom)).getConvexHull();
computeWidthConvex(convexGeom);
}
}
private void computeWidthConvex(Geometry convexGeom)
{
//System.out.println("Input = " + geom);
if (convexGeom instanceof Polygon)
convexHullPts = ((Polygon) convexGeom).getExteriorRing().getCoordinates();
else
convexHullPts = convexGeom.getCoordinates();
// special cases for lines or points or degenerate rings
if (convexHullPts.length == 0) {
minWidth = 0.0;
minWidthPt = null;
minBaseSeg = null;
}
else if (convexHullPts.length == 1) {
minWidth = 0.0;
minWidthPt = convexHullPts[0];
minBaseSeg.p0 = convexHullPts[0];
minBaseSeg.p1 = convexHullPts[0];
}
else if (convexHullPts.length == 2 || convexHullPts.length == 3) {
minWidth = 0.0;
minWidthPt = convexHullPts[0];
minBaseSeg.p0 = convexHullPts[0];
minBaseSeg.p1 = convexHullPts[1];
}
else
computeConvexRingMinDiameter(convexHullPts);
}
/**
* Compute the width information for a ring of {@link Coordinate}s.
* Leaves the width information in the instance variables.
*
* @param pts
*/
private void computeConvexRingMinDiameter(Coordinate[] pts)
{
// for each segment in the ring
minWidth = Double.MAX_VALUE;
int currMaxIndex = 1;
LineSegment seg = new LineSegment();
// compute the max distance for all segments in the ring, and pick the minimum
for (int i = 0; i < pts.length - 1; i++) {
seg.p0 = pts[i];
seg.p1 = pts[i + 1];
currMaxIndex = findMaxPerpDistance(pts, seg, currMaxIndex);
}
}
private int findMaxPerpDistance(Coordinate[] pts, LineSegment seg, int startIndex)
{
double maxPerpDistance = seg.distancePerpendicular(pts[startIndex]);
double nextPerpDistance = maxPerpDistance;
int maxIndex = startIndex;
int nextIndex = maxIndex;
while (nextPerpDistance >= maxPerpDistance) {
maxPerpDistance = nextPerpDistance;
maxIndex = nextIndex;
nextIndex = nextIndex(pts, maxIndex);
nextPerpDistance = seg.distancePerpendicular(pts[nextIndex]);
}
// found maximum width for this segment - update global min dist if appropriate
if (maxPerpDistance < minWidth) {
minPtIndex = maxIndex;
minWidth = maxPerpDistance;
minWidthPt = pts[minPtIndex];
minBaseSeg = new LineSegment(seg);
// System.out.println(minBaseSeg);
// System.out.println(minWidth);
}
return maxIndex;
}
private static int nextIndex(Coordinate[] pts, int index)
{
index++;
if (index >= pts.length) index = 0;
return index;
}
/**
* Gets the minimum rectangular {@link Polygon} which encloses the input geometry.
* The rectangle has width equal to the minimum diameter,
* and a longer length.
* If the convex hull of the input is degenerate (a line or point)
* a {@link LineString} or {@link Point} is returned.
*
* The minimum rectangle can be used as an extremely generalized representation
* for the given geometry.
*
* @return the minimum rectangle enclosing the input (or a line or point if degenerate)
*/
public Geometry getMinimumRectangle()
{
computeMinimumDiameter();
// check if minimum rectangle is degenerate (a point or line segment)
if (minWidth == 0.0) {
if (minBaseSeg.p0.equals2D(minBaseSeg.p1)) {
return inputGeom.getFactory().createPoint(minBaseSeg.p0);
}
return minBaseSeg.toGeometry(inputGeom.getFactory());
}
// deltas for the base segment of the minimum diameter
double dx = minBaseSeg.p1.x - minBaseSeg.p0.x;
double dy = minBaseSeg.p1.y - minBaseSeg.p0.y;
/*
double c0 = computeC(dx, dy, minBaseSeg.p0);
double c1 = computeC(dx, dy, minBaseSeg.p1);
*/
double minPara = Double.MAX_VALUE;
double maxPara = -Double.MAX_VALUE;
double minPerp = Double.MAX_VALUE;
double maxPerp = -Double.MAX_VALUE;
// compute maxima and minima of lines parallel and perpendicular to base segment
for (int i = 0; i < convexHullPts.length; i++) {
double paraC = computeC(dx, dy, convexHullPts[i]);
if (paraC > maxPara) maxPara = paraC;
if (paraC < minPara) minPara = paraC;
double perpC = computeC(-dy, dx, convexHullPts[i]);
if (perpC > maxPerp) maxPerp = perpC;
if (perpC < minPerp) minPerp = perpC;
}
// compute lines along edges of minimum rectangle
LineSegment maxPerpLine = computeSegmentForLine(-dx, -dy, maxPerp);
LineSegment minPerpLine = computeSegmentForLine(-dx, -dy, minPerp);
LineSegment maxParaLine = computeSegmentForLine(-dy, dx, maxPara);
LineSegment minParaLine = computeSegmentForLine(-dy, dx, minPara);
// compute vertices of rectangle (where the para/perp max & min lines intersect)
Coordinate p0 = maxParaLine.lineIntersection(maxPerpLine);
Coordinate p1 = minParaLine.lineIntersection(maxPerpLine);
Coordinate p2 = minParaLine.lineIntersection(minPerpLine);
Coordinate p3 = maxParaLine.lineIntersection(minPerpLine);
LinearRing shell = inputGeom.getFactory().createLinearRing(
new Coordinate[] { p0, p1, p2, p3, p0 });
return inputGeom.getFactory().createPolygon(shell, null);
}
private static double computeC(double a, double b, Coordinate p)
{
return a * p.y - b * p.x;
}
private static LineSegment computeSegmentForLine(double a, double b, double c)
{
Coordinate p0;
Coordinate p1;
/*
* Line eqn is ax + by = c
* Slope is a/b.
* If slope is steep, use y values as the inputs
*/
if (Math.abs(b) > Math.abs(a)) {
p0 = new Coordinate(0.0, c/b);
p1 = new Coordinate(1.0, c/b - a/b);
}
else {
p0 = new Coordinate(c/a, 0.0);
p1 = new Coordinate(c/a - b/a, 1.0);
}
return new LineSegment(p0, p1);
}
}