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Fix nested shell handling
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dr-jts committed Sep 25, 2024
1 parent 610dcea commit 20dcf03
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Showing 3 changed files with 132 additions and 11 deletions.
Original file line number Diff line number Diff line change
Expand Up @@ -58,7 +58,8 @@
* via {@link #setHolesAllowed(boolean)}.
* <p>
* The hull can be specified as being "tight", via {@link #setTight(boolean)}.
* This causes the result to follow the outer boundaries of the input polygons.
* This causes the result to follow the outer boundaries of the input polygons
* which "face away" from other input polygons.
* <p>
* Instead of the complete hull, the "fill area" between the input polygons
* can be computed using {@link #getFill()}.
Expand All @@ -68,6 +69,9 @@
* If needed, a set of possibly-overlapping Polygons
* can be converted to a valid MultiPolygon
* by using {@link Geometry#union()};
* <p>
* If the input contains holes (possibly containing nested polygon elements,
* these will be preserved in the output.
*
* @author Martin Davis
*
Expand Down Expand Up @@ -297,7 +301,7 @@ private Geometry createEmptyHull() {
}

private void buildHullTris() {
polygonRings = extractShellRings(inputPolygons);
polygonRings = OuterShellsExtracter.extractShells(inputPolygons);
Polygon frame = createFrame(inputPolygons.getEnvelopeInternal(), polygonRings, geomFactory);
ConstrainedDelaunayTriangulator cdt = new ConstrainedDelaunayTriangulator(frame);
List<Tri> tris = cdt.getTriangles();
Expand Down Expand Up @@ -589,12 +593,4 @@ private static Polygon createFrame(Envelope polygonsEnv, LinearRing[] polygonRin
return frame;
}

private static LinearRing[] extractShellRings(Geometry polygons) {
LinearRing[] rings = new LinearRing[polygons.getNumGeometries()];
for (int i = 0; i < polygons.getNumGeometries(); i++) {
Polygon consPoly = (Polygon) polygons.getGeometryN(i);
rings[i] = (LinearRing) consPoly.getExteriorRing().copy();
}
return rings;
}
}
Original file line number Diff line number Diff line change
@@ -0,0 +1,111 @@
/*
* Copyright (c) 2024 Martin Davis.
*
* All rights reserved. This program and the accompanying materials
* are made available under the terms of the Eclipse Public License 2.0
* and Eclipse Distribution License v. 1.0 which accompanies this distribution.
* The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
* and the Eclipse Distribution License is available at
*
* http://www.eclipse.org/org/documents/edl-v10.php.
*/
package org.locationtech.jts.algorithm.hull;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

import org.locationtech.jts.algorithm.PointLocation;
import org.locationtech.jts.geom.Coordinate;
import org.locationtech.jts.geom.Geometry;
import org.locationtech.jts.geom.GeometryFactory;
import org.locationtech.jts.geom.LinearRing;
import org.locationtech.jts.geom.Polygon;

/**
* Extracts the rings of outer shells from a polygonal geometry.
* Outer shells are the shells of polygon elements which
* are not nested inside holes of other polygons.
*
* @author mdavis
*
*/
class OuterShellsExtracter {

public static LinearRing[] extractShells(Geometry polygons) {
OuterShellsExtracter extracter = new OuterShellsExtracter(polygons);
return extracter.extractShells();
}

private Geometry polygons;

public OuterShellsExtracter(Geometry polygons) {
this.polygons = polygons;
}

private LinearRing[] extractShells() {
LinearRing[] shells = extractShellRings(polygons);
/**
* sort shells in order of increasing envelope area
* to ensure that shells are added before any of their inner shells
*/
Arrays.sort(shells, new EnvelopeAreaComparator());
List<LinearRing> outerShells = new ArrayList<LinearRing>();
for (int i = shells.length - 1; i >= 0; i--) {
LinearRing shell = shells[i];
if (outerShells.size() == 0
|| isOuter(shell, outerShells)) {
outerShells.add(shell);
}
}
return GeometryFactory.toLinearRingArray(outerShells);
}

private boolean isOuter(LinearRing shell, List<LinearRing> outerShells) {
for (LinearRing outShell : outerShells) {
if (covers(outShell, shell)) {
return false;
}
}
return true;
}

private boolean covers(LinearRing shellA, LinearRing shellB) {
//-- if shellB envelope is not covered then shell is not covered
if (! shellA.getEnvelopeInternal().covers(shellB.getEnvelopeInternal()))
return false;
//-- if a shellB point lies inside shellA, shell is covered (since shells do not overlap)
if (isPointInRing(shellB, shellA))
return true;
return false;
}

private boolean isPointInRing(LinearRing shell, LinearRing shellRing) {
//TODO: optimize this with cached index
Coordinate pt = shell.getCoordinate();
return PointLocation.isInRing(pt, shellRing.getCoordinates());
}

private static LinearRing[] extractShellRings(Geometry polygons) {
LinearRing[] rings = new LinearRing[polygons.getNumGeometries()];
for (int i = 0; i < polygons.getNumGeometries(); i++) {
Polygon consPoly = (Polygon) polygons.getGeometryN(i);
rings[i] = (LinearRing) consPoly.getExteriorRing().copy();
}
return rings;
}

private static class EnvelopeAreaComparator implements Comparator<Geometry> {

@Override
public int compare(Geometry o1, Geometry o2) {
return Double.compare(envArea(o1), envArea(o2));
}

private static double envArea(Geometry g) {
return g.getEnvelopeInternal().getArea();
}

}
}
Original file line number Diff line number Diff line change
Expand Up @@ -80,7 +80,7 @@ public void testPoly3Concave3() {
"POLYGON ((9 10, 10 9, 10 3, 7 0, 4 0, 0 2, 0 7, 4 10, 9 10))" );
}

public void testPoly3WithHole() {
public void testPoly3WithHullHole() {
String wkt = "MULTIPOLYGON (((1 9, 5 9, 5 7, 3 7, 3 5, 1 5, 1 9)), ((1 4, 3 4, 3 2, 5 2, 5 0, 1 0, 1 4)), ((6 9, 8 9, 9 5, 8 0, 6 0, 6 2, 8 5, 6 7, 6 9)))";
checkHullWithHoles( wkt, .9, wkt);
checkHullWithHoles( wkt, 1,
Expand All @@ -93,6 +93,20 @@ public void testPoly3WithHole() {
"POLYGON ((6 9, 8 9, 9 5, 8 0, 6 0, 5 0, 1 0, 1 4, 1 5, 1 9, 5 9, 6 9))");
}

public void testPolygonHole() {
checkHullByLenRatio(
"MULTIPOLYGON (((1 1, 10 3, 19 1, 16 8, 19 7, 19 19, 10 20, 8 17, 1 19, 1 1), (3 4, 5 10, 3 16, 9 14, 14 15, 15 9, 13 5, 3 4)))",
0.9,
"POLYGON ((10 20, 19 19, 19 7, 19 1, 10 3, 1 1, 1 19, 10 20), (13 5, 15 9, 14 15, 9 14, 3 16, 5 10, 3 4, 13 5))" );
}

public void testPolygonNestedPoly() {
checkHullByLenRatio(
"MULTIPOLYGON (((1 1, 10 3, 19 1, 16 8, 19 7, 19 19, 10 20, 8 17, 1 19, 1 1), (3 4, 5 10, 3 16, 9 14, 14 15, 15 9, 13 5, 3 4)), ((6 10, 7 13, 10 12, 12 13, 13 11, 11 9, 13 8, 9 6, 6 6, 7 8, 6 10)))",
0.9,
"MULTIPOLYGON (((10 20, 19 19, 19 7, 19 1, 10 3, 1 1, 1 19, 10 20), (13 5, 15 9, 14 15, 9 14, 3 16, 5 10, 3 4, 13 5)), ((7 13, 10 12, 12 13, 13 11, 11 9, 13 8, 9 6, 6 6, 7 8, 6 10, 7 13)))" );
}

private void checkHull(String wkt, double maxLen, String wktExpected) {
Geometry geom = read(wkt);
Geometry actual = ConcaveHullOfPolygons.concaveHullByLength(geom, maxLen);
Expand Down

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