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/* $Id: intersect.c,v 1.2 2005/02/24 00:57:33 ellson Exp $ $Revision: 1.2 $ */
/* vim:set shiftwidth=4 ts=8: */
/**********************************************************
* This software is part of the graphviz package *
* http://www.graphviz.org/ *
* *
* Copyright (c) 1994-2004 AT&T Corp. *
* and is licensed under the *
* Common Public License, Version 1.0 *
* by AT&T Corp. *
* *
* Information and Software Systems Research *
* AT&T Research, Florham Park NJ *
**********************************************************/
#include "neato.h"
#include "simple.h"
/* find the sign of the area of each of the triangles
formed by adding a vertex of m to l
also find the sign of their product */
static void sgnarea(struct vertex *l, struct vertex *m, int i[])
{
double a, b, c, d, e, f, g, h, t;
a = l->pos.x;
b = l->pos.y;
c = after(l)->pos.x - a;
d = after(l)->pos.y - b;
e = m->pos.x - a;
f = m->pos.y - b;
g = after(m)->pos.x - a;
h = after(m)->pos.y - b;
t = (c * f) - (d * e);
i[0] = ((t == 0) ? 0 : (t > 0 ? 1 : -1));
t = (c * h) - (d * g);
i[1] = ((t == 0) ? 0 : (t > 0 ? 1 : -1));
i[2] = i[0] * i[1];
}
/* determine if g lies between f and h */
static int between(double f, double g, double h)
{
if ((f == g) || (g == h))
return (0);
return ((f < g) ? (g < h ? 1 : -1) : (h < g ? 1 : -1));
}
/* determine if vertex i of line m is on line l */
static int online(struct vertex *l, struct vertex *m, int i)
{
struct position a, b, c;
a = l->pos;
b = after(l)->pos;
c = (i == 0) ? m->pos : after(m)->pos;
return ((a.x == b.x) ? ((a.x == c.x)
&& (-1 !=
between(a.y, c.y, b.y))) : between(a.x,
c.x,
b.x));
}
/* determine point of detected intersections */
static int intpoint(struct vertex *l, struct vertex *m, double *x, double *y, int cond)
{
struct position ls, le, ms, me, pt1, pt2;
double m1, m2, c1, c2;
if (cond <= 0)
return (0);
ls = l->pos;
le = after(l)->pos;
ms = m->pos;
me = after(m)->pos;
switch (cond) {
case 3: /* a simple intersection */
if (ls.x == le.x) {
*x = ls.x;
*y = me.y + SLOPE(ms, me) * (*x - me.x);
} else if (ms.x == me.x) {
*x = ms.x;
*y = le.y + SLOPE(ls, le) * (*x - le.x);
} else {
m1 = SLOPE(ms, me);
m2 = SLOPE(ls, le);
c1 = ms.y - (m1 * ms.x);
c2 = ls.y - (m2 * ls.x);
*x = (c2 - c1) / (m1 - m2);
*y = ((m1 * c2) - (c1 * m2)) / (m1 - m2);
}
break;
case 2: /* the two lines have a common segment */
if (online(l, m, 0) == -1) { /* ms between ls and le */
pt1 = ms;
pt2 =
(online(m, l, 1) ==
-1) ? ((online(m, l, 0) == -1) ? le : ls) : me;
} else if (online(l, m, 1) == -1) { /* me between ls and le */
pt1 = me;
pt2 =
(online(l, m, 0) ==
-1) ? ((online(m, l, 0) == -1) ? le : ls) : ms;
} else {
/* may be degenerate? */
if (online(m, l, 0) != -1)
return 0;
pt1 = ls;
pt2 = le;
}
*x = (pt1.x + pt2.x) / 2;
*y = (pt1.y + pt2.y) / 2;
break;
case 1: /* a vertex of line m is on line l */
if ((ls.x - le.x) * (ms.y - ls.y) == (ls.y - le.y) * (ms.x - ls.x)) {
*x = ms.x;
*y = ms.y;
} else {
*x = me.x;
*y = me.y;
}
} /* end switch */
return (1);
}
/*detect whether lines l and m intersect */
void find_intersection(struct vertex *l,
struct vertex *m,
struct intersection ilist[], struct data *input)
{
double x, y;
int i[3];
sgnarea(l, m, i);
if (i[2] > 0)
return;
if (i[2] < 0) {
sgnarea(m, l, i);
if (i[2] > 0)
return;
if (!intpoint
(l, m, &x, &y, (i[2] < 0) ? 3 : online(m, l, ABS(i[0]))))
return;
}
else if (!intpoint(l, m, &x, &y, (i[0] == i[1]) ?
2 * MAX(online(l, m, 0),
online(l, m, 1)) : online(l, m, ABS(i[0]))))
return;
if (input->ninters >= MAXINTS) {
agerr(AGERR, "using too many intersections\n");
exit(1);
}
ilist[input->ninters].firstv = l;
ilist[input->ninters].secondv = m;
ilist[input->ninters].firstp = l->poly;
ilist[input->ninters].secondp = m->poly;
ilist[input->ninters].x = x;
ilist[input->ninters].y = y;
input->ninters++;
}
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