e93247899f1b6e3ba071e721b1a60f30b1f1a32b
1 /**
2 * \file src/geom.cpp
3 * \brief Various geometrical calculations.
4 */
6 #ifdef HAVE_CONFIG_H
7 # include <config.h>
8 #endif
9 #include "geom.h"
10 #include "point.h"
12 namespace Geom {
13 /**
14 * Finds the intersection of the two (infinite) lines
15 * defined by the points p such that dot(n0, p) == d0 and dot(n1, p) == d1.
16 *
17 * If the two lines intersect, then \a result becomes their point of
18 * intersection; otherwise, \a result remains unchanged.
19 *
20 * This function finds the intersection of the two lines (infinite)
21 * defined by n0.X = d0 and x1.X = d1. The algorithm is as follows:
22 * To compute the intersection point use kramer's rule:
23 * \verbatim
24 * convert lines to form
25 * ax + by = c
26 * dx + ey = f
27 *
28 * (
29 * e.g. a = (x2 - x1), b = (y2 - y1), c = (x2 - x1)*x1 + (y2 - y1)*y1
30 * )
31 *
32 * In our case we use:
33 * a = n0.x d = n1.x
34 * b = n0.y e = n1.y
35 * c = d0 f = d1
36 *
37 * so:
38 *
39 * adx + bdy = cd
40 * adx + aey = af
41 *
42 * bdy - aey = cd - af
43 * (bd - ae)y = cd - af
44 *
45 * y = (cd - af)/(bd - ae)
46 *
47 * repeat for x and you get:
48 *
49 * x = (fb - ce)/(bd - ae) \endverbatim
50 *
51 * If the denominator (bd-ae) is 0 then the lines are parallel, if the
52 * numerators are then 0 then the lines coincide.
53 *
54 * \todo Why not use existing but outcommented code below
55 * (HAVE_NEW_INTERSECTOR_CODE)?
56 */
57 IntersectorKind
58 line_intersection(Geom::Point const &n0, double const d0,
59 Geom::Point const &n1, double const d1,
60 Geom::Point &result)
61 {
62 double denominator = dot(Geom::rot90(n0), n1);
63 double X = n1[Geom::Y] * d0 -
64 n0[Geom::Y] * d1;
65 /* X = (-d1, d0) dot (n0[Y], n1[Y]) */
67 if (denominator == 0) {
68 if ( X == 0 ) {
69 return coincident;
70 } else {
71 return parallel;
72 }
73 }
75 double Y = n0[Geom::X] * d1 -
76 n1[Geom::X] * d0;
78 result = Geom::Point(X, Y) / denominator;
80 return intersects;
81 }
86 /* ccw exists as a building block */
87 int
88 intersector_ccw(const Geom::Point& p0, const Geom::Point& p1,
89 const Geom::Point& p2)
90 /* Determine which way a set of three points winds. */
91 {
92 Geom::Point d1 = p1 - p0;
93 Geom::Point d2 = p2 - p0;
94 /* compare slopes but avoid division operation */
95 double c = dot(Geom::rot90(d1), d2);
96 if(c > 0)
97 return +1; // ccw - do these match def'n in header?
98 if(c < 0)
99 return -1; // cw
101 /* Colinear [or NaN]. Decide the order. */
102 if ( ( d1[0] * d2[0] < 0 ) ||
103 ( d1[1] * d2[1] < 0 ) ) {
104 return -1; // p2 < p0 < p1
105 } else if ( dot(d1,d1) < dot(d2,d2) ) {
106 return +1; // p0 <= p1 < p2
107 } else {
108 return 0; // p0 <= p2 <= p1
109 }
110 }
112 /** Determine whether two line segments intersect. This doesn't find
113 the point of intersection, use the line_intersect function above,
114 or the segment_intersection interface below.
116 \pre neither segment is zero-length; i.e. p00 != p01 and p10 != p11.
117 */
118 static bool
119 segment_intersectp(Geom::Point const &p00, Geom::Point const &p01,
120 Geom::Point const &p10, Geom::Point const &p11)
121 {
122 if(p00 == p01) return false;
123 if(p10 == p11) return false;
125 /* true iff ( (the p1 segment straddles the p0 infinite line)
126 * and (the p0 segment straddles the p1 infinite line) ). */
127 return ((intersector_ccw(p00,p01, p10)
128 *intersector_ccw(p00, p01, p11)) <=0 )
129 &&
130 ((intersector_ccw(p10,p11, p00)
131 *intersector_ccw(p10, p11, p01)) <=0 );
132 }
135 /** Determine whether \& where two line segments intersect.
137 If the two segments don't intersect, then \a result remains unchanged.
139 \pre neither segment is zero-length; i.e. p00 != p01 and p10 != p11.
140 **/
141 IntersectorKind
142 segment_intersect(Geom::Point const &p00, Geom::Point const &p01,
143 Geom::Point const &p10, Geom::Point const &p11,
144 Geom::Point &result)
145 {
146 if(segment_intersectp(p00, p01, p10, p11)) {
147 Geom::Point n0 = (p01 - p00).ccw();
148 double d0 = dot(n0,p00);
150 Geom::Point n1 = (p11 - p10).ccw();
151 double d1 = dot(n1,p10);
152 return line_intersection(n0, d0, n1, d1, result);
153 } else {
154 return no_intersection;
155 }
156 }
158 /** Determine whether \& where two line segments intersect.
160 If the two segments don't intersect, then \a result remains unchanged.
162 \pre neither segment is zero-length; i.e. p00 != p01 and p10 != p11.
163 **/
164 IntersectorKind
165 line_twopoint_intersect(Geom::Point const &p00, Geom::Point const &p01,
166 Geom::Point const &p10, Geom::Point const &p11,
167 Geom::Point &result)
168 {
169 Geom::Point n0 = (p01 - p00).ccw();
170 double d0 = dot(n0,p00);
172 Geom::Point n1 = (p11 - p10).ccw();
173 double d1 = dot(n1,p10);
174 return line_intersection(n0, d0, n1, d1, result);
175 }
177 /**
178 * polyCentroid: Calculates the centroid (xCentroid, yCentroid) and area of a polygon, given its
179 * vertices (x[0], y[0]) ... (x[n-1], y[n-1]). It is assumed that the contour is closed, i.e., that
180 * the vertex following (x[n-1], y[n-1]) is (x[0], y[0]). The algebraic sign of the area is
181 * positive for counterclockwise ordering of vertices in x-y plane; otherwise negative.
183 * Returned values:
184 0 for normal execution;
185 1 if the polygon is degenerate (number of vertices < 3);
186 2 if area = 0 (and the centroid is undefined).
188 * for now we require the path to be a polyline and assume it is closed.
189 **/
191 int centroid(std::vector<Geom::Point> p, Geom::Point& centroid, double &area) {
192 const unsigned n = p.size();
193 if (n < 3)
194 return 1;
195 Geom::Point centroid_tmp(0,0);
196 double atmp = 0;
197 for (unsigned i = n-1, j = 0; j < n; i = j, j++) {
198 const double ai = -cross(p[j], p[i]);
199 atmp += ai;
200 centroid_tmp += (p[j] + p[i])*ai; // first moment.
201 }
202 area = atmp / 2;
203 if (atmp != 0) {
204 centroid = centroid_tmp / (3 * atmp);
205 return 0;
206 }
207 return 2;
208 }
210 }
212 /*
213 Local Variables:
214 mode:c++
215 c-file-style:"stroustrup"
216 c-file-offsets:((innamespace . 0)(inline-open . 0)(case-label . +))
217 indent-tabs-mode:nil
218 fill-column:99
219 End:
220 */
221 // vim: filetype=cpp:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:encoding=utf-8:textwidth=99 :