Class Area

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	package org.locationtech.jts.algorithm;
13
14	import org.locationtech.jts.geom.Coordinate;
15	import org.locationtech.jts.geom.CoordinateSequence;
16
17	/**
18	* Functions for computing area.
19	*
20	* @author Martin Davis
21	*
22	*/
23	public class Area {
24
25	/**
26	* Computes the area for a ring.
27	*
28	* @param ring the coordinates forming the ring
29	* @return the area of the ring
30	*/
31	public static double ofRing(Coordinate[] ring)
32	{
33	return Math.abs(ofRingSigned(ring));
34	}
35
36	/**
37	* Computes the area for a ring.
38	*
39	* @param ring the coordinates forming the ring
40	* @return the area of the ring
41	*/
42	public static double ofRing(CoordinateSequence ring)
43	{
44	return Math.abs(ofRingSigned(ring));
45	}
46
47	/**
48	* Computes the signed area for a ring. The signed area is positive if the
49	* ring is oriented CW, negative if the ring is oriented CCW, and zero if the
50	* ring is degenerate or flat.
51	*
52	* @param ring
53	* the coordinates forming the ring
54	* @return the signed area of the ring
55	*/
56	public static double ofRingSigned(Coordinate[] ring)
57	{
58	if (ring.length < 3)
59	return 0.0;
60	double sum = 0.0;
61	/**
62	* Based on the Shoelace formula.
63	* http://en.wikipedia.org/wiki/Shoelace_formula
64	*/
65	double x0 = ring[0].x;
66	for (int i = 1; i < ring.length - 1; i++) {
67	double x = ring[i].x - x0;
68	double y1 = ring[i + 1].y;
69	double y2 = ring[i - 1].y;
70	sum += x * (y2 - y1);
71	}
72	return sum / 2.0;
73	}
74
75	/**
76	* Computes the signed area for a ring. The signed area is:
77	* <ul>
78	* <li>positive if the ring is oriented CW
79	* <li>negative if the ring is oriented CCW
80	* <li>zero if the ring is degenerate or flat
81	* </ul>
82	*
83	* @param ring
84	* the coordinates forming the ring
85	* @return the signed area of the ring
86	*/
87	public static double ofRingSigned(CoordinateSequence ring)
88	{
89	int n = ring.size();
90	if (n < 3)
91	return 0.0;
92	/**
93	* Based on the Shoelace formula.
94	* http://en.wikipedia.org/wiki/Shoelace_formula
95	*/
96	Coordinate p0 = new Coordinate();
97	Coordinate p1 = new Coordinate();
98	Coordinate p2 = new Coordinate();
99	ring.getCoordinate(0, p1);
100	ring.getCoordinate(1, p2);
101	double x0 = p1.x;
102	p2.x -= x0;
103	double sum = 0.0;
104	for (int i = 1; i < n - 1; i++) {
105	p0.y = p1.y;
106	p1.x = p2.x;
107	p1.y = p2.y;
108	ring.getCoordinate(i + 1, p2);
109	p2.x -= x0;
110	sum += p1.x * (p0.y - p2.y);
111	}
112	return sum / 2.0;
113	}
114
115	}
116