Class OffsetPointGenerator

1	/*
2	* Copyright (c) 2016 Vivid Solutions.
3	*
4	* All rights reserved. This program and the accompanying materials
5	* are made available under the terms of the Eclipse Public License 2.0
6	* and Eclipse Distribution License v. 1.0 which accompanies this distribution.
7	* The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
8	* and the Eclipse Distribution License is available at
9	*
10	* http://www.eclipse.org/org/documents/edl-v10.php.
11	*/
12
13	package org.locationtech.jts.operation.overlay.validate;
14
15	import java.util.ArrayList;
16	import java.util.Iterator;
17	import java.util.List;
18
19	import org.locationtech.jts.geom.Coordinate;
20	import org.locationtech.jts.geom.Geometry;
21	import org.locationtech.jts.geom.LineString;
22	import org.locationtech.jts.geom.util.LinearComponentExtracter;
23
24	/**
25	* Generates points offset by a given distance
26	* from both sides of the midpoint of
27	* all segments in a {@link Geometry}.
28	* Can be used to generate probe points for
29	* determining whether a polygonal overlay result
30	* is incorrect.
31	* The input geometry may have any orientation for its rings,
32	* but {@link #setSidesToGenerate(boolean, boolean)} is
33	* only meaningful if the orientation is known.
34	*
35	* @author Martin Davis
36	* @version 1.7
37	*/
38	public class OffsetPointGenerator
39	{
40	private Geometry g;
41	private boolean doLeft = true;
42	private boolean doRight = true;
43
44	public OffsetPointGenerator(Geometry g)
45	{
46	this.g = g;
47	}
48
49	/**
50	* Set the sides on which to generate offset points.
51	*
52	* @param doLeft
53	* @param doRight
54	*/
55	public void setSidesToGenerate(boolean doLeft, boolean doRight)
56	{
57	this.doLeft = doLeft;
58	this.doRight = doRight;
59	}
60
61	/**
62	* Gets the computed offset points.
63	*
64	* @return List<Coordinate>
65	*/
66	public List getPoints(double offsetDistance)
67	{
68	List offsetPts = new ArrayList();
69	List lines = LinearComponentExtracter.getLines(g);
70	for (Iterator i = lines.iterator(); i.hasNext(); ) {
71	LineString line = (LineString) i.next();
72	extractPoints(line, offsetDistance, offsetPts);
73	}
74	//System.out.println(toMultiPoint(offsetPts));
75	return offsetPts;
76	}
77
78	private void extractPoints(LineString line, double offsetDistance, List offsetPts)
79	{
80	Coordinate[] pts = line.getCoordinates();
81	for (int i = 0; i < pts.length - 1; i++) {
82	computeOffsetPoints(pts[i], pts[i + 1], offsetDistance, offsetPts);
83	}
84	}
85
86	/**
87	* Generates the two points which are offset from the
88	* midpoint of the segment <tt>(p0, p1)</tt> by the
89	* <tt>offsetDistance</tt>.
90	*
91	* @param p0 the first point of the segment to offset from
92	* @param p1 the second point of the segment to offset from
93	*/
94	private void computeOffsetPoints(Coordinate p0, Coordinate p1, double offsetDistance, List offsetPts)
95	{
96	double dx = p1.x - p0.x;
97	double dy = p1.y - p0.y;
98	double len = Math.sqrt(dx * dx + dy * dy);
99	// u is the vector that is the length of the offset, in the direction of the segment
100	double ux = offsetDistance * dx / len;
101	double uy = offsetDistance * dy / len;
102
103	double midX = (p1.x + p0.x) / 2;
104	double midY = (p1.y + p0.y) / 2;
105
106	if (doLeft) {
107	Coordinate offsetLeft = new Coordinate(midX - uy, midY + ux);
108	offsetPts.add(offsetLeft);
109	}
110
111	if (doRight) {
112	Coordinate offsetRight = new Coordinate(midX + uy, midY - ux);
113	offsetPts.add(offsetRight);
114	}
115	}
116
117	}
118