LineDissolver.java
/*
* Copyright (c) 2016 Vivid Solutions.
*
* 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.dissolve;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.Stack;
import org.locationtech.jts.edgegraph.HalfEdge;
import org.locationtech.jts.edgegraph.MarkHalfEdge;
import org.locationtech.jts.geom.Coordinate;
import org.locationtech.jts.geom.CoordinateList;
import org.locationtech.jts.geom.CoordinateSequence;
import org.locationtech.jts.geom.Geometry;
import org.locationtech.jts.geom.GeometryComponentFilter;
import org.locationtech.jts.geom.GeometryFactory;
import org.locationtech.jts.geom.LineString;
/**
* Dissolves the linear components
* from a collection of {@link Geometry}s
* into a set of maximal-length {@link LineString}s
* in which every unique segment appears once only.
* The output linestrings run between node vertices
* of the input, which are vertices which have
* either degree 1, or degree 3 or greater.
* <p>
* Use cases for dissolving linear components
* include generalization
* (in particular, simplifying polygonal coverages),
* and visualization
* (in particular, avoiding symbology conflicts when
* depicting shared polygon boundaries).
* <p>
* This class does <b>not</b> node the input lines.
* If there are line segments crossing in the input,
* they will still cross in the output.
*
* @author Martin Davis
*
*/
public class LineDissolver
{
/**
* Dissolves the linear components in a geometry.
*
* @param g the geometry to dissolve
* @return the dissolved lines
*/
public static Geometry dissolve(Geometry g)
{
LineDissolver d = new LineDissolver();
d.add(g);
return d.getResult();
}
private Geometry result;
private GeometryFactory factory;
private final DissolveEdgeGraph graph;
private final List lines = new ArrayList();
public LineDissolver()
{
graph = new DissolveEdgeGraph();
}
/**
* Adds a {@link Geometry} to be dissolved.
* Any number of geometries may be added by calling this method multiple times.
* Any type of Geometry may be added. The constituent linework will be
* extracted to be dissolved.
*
* @param geometry geometry to be line-merged
*/
public void add(Geometry geometry) {
geometry.apply(new GeometryComponentFilter() {
public void filter(Geometry component) {
if (component instanceof LineString) {
add((LineString)component);
}
}
});
}
/**
* Adds a collection of Geometries to be processed. May be called multiple times.
* Any dimension of Geometry may be added; the constituent linework will be
* extracted.
*
* @param geometries the geometries to be line-merged
*/
public void add(Collection geometries)
{
for (Iterator i = geometries.iterator(); i.hasNext(); ) {
Geometry geometry = (Geometry) i.next();
add(geometry);
}
}
private void add(LineString lineString) {
if (factory == null) {
this.factory = lineString.getFactory();
}
CoordinateSequence seq = lineString.getCoordinateSequence();
boolean doneStart = false;
for (int i = 1; i < seq.size(); i++) {
DissolveHalfEdge e = (DissolveHalfEdge) graph.addEdge(seq.getCoordinate(i-1), seq.getCoordinate(i));
// skip zero-length edges
if (e == null) continue;
/**
* Record source initial segments, so that they can be reflected in output when needed
* (i.e. during formation of isolated rings)
*/
if (! doneStart) {
e.setStart();
doneStart = true;
}
}
}
/**
* Gets the dissolved result as a MultiLineString.
*
* @return the dissolved lines
*/
public Geometry getResult()
{
if (result == null)
computeResult();
return result;
}
private void computeResult() {
Collection edges = graph.getVertexEdges();
for (Iterator i = edges.iterator(); i.hasNext(); ) {
HalfEdge e = (HalfEdge) i.next();
if (MarkHalfEdge.isMarked(e)) continue;
process(e);
}
result = factory.buildGeometry(lines);
}
private final Stack nodeEdgeStack = new Stack();
private void process(HalfEdge e) {
HalfEdge eNode = e.prevNode();
// if edge is in a ring, just process this edge
if (eNode == null)
eNode = e;
stackEdges(eNode);
// extract lines from node edges in stack
buildLines();
}
/**
* For each edge in stack
* (which must originate at a node)
* extracts the line it initiates.
*/
private void buildLines() {
while (! nodeEdgeStack.empty()) {
HalfEdge e = (HalfEdge) nodeEdgeStack.pop();
if (MarkHalfEdge.isMarked(e))
continue;
buildLine(e);
}
}
private DissolveHalfEdge ringStartEdge;
/**
* Updates the tracked ringStartEdge
* if the given edge has a lower origin
* (using the standard {@link Coordinate} ordering).
*
* Identifying the lowest starting node meets two goals:
* <ul>
* <li>It ensures that isolated input rings are created using the original node and orientation
* <li>For isolated rings formed from multiple input linestrings,
* it provides a canonical node and orientation for the output
* (rather than essentially random, and thus hard to test).
* </ul>
*
* @param e
*/
private void updateRingStartEdge(DissolveHalfEdge e)
{
if (! e.isStart()) {
e = (DissolveHalfEdge) e.sym();
if (! e.isStart()) return;
}
// here e is known to be a start edge
if (ringStartEdge == null) {
ringStartEdge = e;
return;
}
if (e.orig().compareTo(ringStartEdge.orig()) < 0) {
ringStartEdge = e;
}
}
/**
* Builds a line starting from the given edge.
* The start edge origin is a node (valence = 1 or >= 3),
* unless it is part of a pure ring.
* A pure ring has no other incident lines.
* In this case the start edge may occur anywhere on the ring.
*
* The line is built up to the next node encountered,
* or until the start edge is re-encountered
* (which happens if the edges form a ring).
*
* @param eStart
*/
private void buildLine(HalfEdge eStart) {
CoordinateList line = new CoordinateList();
DissolveHalfEdge e = (DissolveHalfEdge) eStart;
ringStartEdge = null;
MarkHalfEdge.markBoth(e);
line.add(e.orig().copy(), false);
// scan along the path until a node is found (if one exists)
while (e.sym().degree() == 2) {
updateRingStartEdge(e);
DissolveHalfEdge eNext = (DissolveHalfEdge) e.next();
// check if edges form a ring - if so, we're done
if (eNext == eStart) {
buildRing(ringStartEdge);
return;
}
// add point to line, and move to next edge
line.add(eNext.orig().copy(), false);
e = eNext;
MarkHalfEdge.markBoth(e);
}
// add final node
line.add(e.dest().clone(), false);
// queue up the final node edges
stackEdges(e.sym());
// store the scanned line
addLine(line);
}
private void buildRing(HalfEdge eStartRing) {
CoordinateList line = new CoordinateList();
HalfEdge e = eStartRing;
line.add(e.orig().copy(), false);
// scan along the path until a node is found (if one exists)
while (e.sym().degree() == 2) {
HalfEdge eNext = e.next();
// check if edges form a ring - if so, we're done
if (eNext == eStartRing)
break;
// add point to line, and move to next edge
line.add(eNext.orig().copy(), false);
e = eNext;
}
// add final node
line.add(e.dest().copy(), false);
// store the scanned line
addLine(line);
}
private void addLine(CoordinateList line) {
lines.add(factory.createLineString(line.toCoordinateArray()));
}
/**
* Adds edges around this node to the stack.
*
* @param node
*/
private void stackEdges(HalfEdge node) {
HalfEdge e = node;
do {
if (! MarkHalfEdge.isMarked(e))
nodeEdgeStack.add(e);
e = e.oNext();
} while (e != node);
}
}