1 package org.lcsim.spacegeom;
2 /**
3 * A line segment in two dimensions.
4 *
5 *
6 *@author Norman A. Graf
7 *@version $Id: TwoSegment.java,v 1.1.1.1 2010/12/01 00:15:57 jeremy Exp $
8 *
9 */
10
11 public class TwoSegment
12 {
13 private TwoSpacePoint _a;
14 private TwoSpacePoint _b;
15 private double _length;
16
17 /**
18 * Constructor
19 *
20 * @param a beginning TwoSpacePoint
21 * @param b ending TwoSpacePoint
22 */
23 public TwoSegment(TwoSpacePoint a, TwoSpacePoint b)
24 {
25 _a = a;
26 _b = b;
27 _length = TwoSpacePoint.distance(_a,_b);
28 }
29
30
31 /**
32 * Fetch the starting point
33 *
34 * @return beginning TwoSpacePoint
35 */
36 public TwoSpacePoint startPoint()
37 {
38 return _a;
39 }
40
41
42 /**
43 * Fetch the ending point
44 *
45 * @return ending TwoSpacePoint
46 */
47 public TwoSpacePoint endPoint()
48 {
49 return _b;
50 }
51
52
53 /**
54 * Return the length of the segment
55 *
56 * @return length of the segment
57 */
58 public double length()
59 {
60 return _length;
61 }
62
63
64 /**
65 * calculate the minimum distance between this segment and a point p
66 *
67 * @param p The TwoSpacePoint
68 * @return the distance between the point and this segment
69 */
70 public double minimumDistance(TwoSpacePoint p)
71 {
72 double u = ( ( ( p.x() - _a.x() ) * ( _b.x() - _a.x() ) ) +
73 ( ( p.y() - _a.y() ) * ( _b.y() - _a.y() ) ) )/(_length*_length);
74 double xint = ( _a.x() + u * (_b.x() - _a.x()) ) - p.x();
75 double yint = ( _a.y() + u * (_b.y() - _a.y()) ) - p.y();
76 double dist = Math.sqrt( xint*xint + yint*yint );
77 return dist;
78 }
79
80
81
82 /**
83 * return intersection point of two line segments
84 *
85 * @param a The first line segment
86 * @param b The second line segment
87 * @return The intersection point of the two segments.
88 * null if lines are parallel or intersect outside of segment boundaries.
89 */
90 public static TwoSpacePoint intersection(TwoSegment a, TwoSegment b)
91 {
92 // set some things up
93 double x1 = a.startPoint().x();
94 double y1 = a.startPoint().y();
95 double x2 = a.endPoint().x();
96 double y2 = a.endPoint().y();
97 double x3 = b.startPoint().x();
98 double y3 = b.startPoint().y();
99 double x4 = b.endPoint().x();
100 double y4 = b.endPoint().y();
101
102 double denom = (y4-y3)*(x2-x1)-(x4-x3)*(y2-y1);
103
104 if(denom==0) return null;// lines are parallel
105
106 double numa = (x4-x3)*(y1-y3)-(y4-y3)*(x1-x3);
107
108 if(numa==0) return null; // lines are coincident, no unique intersection
109
110 double ua = numa/denom;
111
112 if(ua<0 || ua >1.) return null; // intersect outside of segment a
113
114 double numb = (x2-x1)*(y1-y3)-(y2-y1)*(x1-x3);
115 double ub = numb/denom;
116 if(ub<0 || ub >1.) return null; // intersect outside of segment b
117
118 // should now have a valid intersection point...
119 double xend = x3+ub*(x4-x3);
120 double yend = y3+ub*(y4-y3);
121
122 return new CartesianTwoPoint(xend, yend);
123
124 }
125
126
127 /** String representation of this object
128 *
129 *
130 * @return String representation of this object
131 */
132 public String toString()
133 {
134 return "TwoSegment from "+_a+" \n to "+_b +" \n length: "+_length;
135 }
136
137 }