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/*
****************************************************************************
*
* MODULE: Vector library
*
* AUTHOR(S): Original author CERL, probably Dave Gerdes.
* Update to GRASS 5.7 Radim Blazek.
*
* PURPOSE: Lower level functions for reading/writing/manipulating vectors.
*
* COPYRIGHT: (C) 2001 by the GRASS Development Team
*
* This program is free software under the GNU General Public
* License (>=v2). Read the file COPYING that comes with GRASS
* for details.
*
*****************************************************************************/
#include <stdio.h>
/***************************************************************
* test_for_intersection (ax1,ay1,ax2,ay2,bx1,by1,bx2,by2)
* double ax1,ax2,ay1,ay2;
* double bx1,bx2,by1,by2;
*
* returns
* 0 no intersection at all
* 1 the line segments intersect at only one point
* -1 the line segments intersect at many points, i.e., overlapping
* segments from the same line
*
* find_intersection (ax1,ay1,ax2,ay2,bx1,by1,bx2,by2,x,y)
* double ax1,ax2,ay1,ay2;
* double bx1,bx2,by1,by2;
* double *x,*y;
*
* returns
* 0 no intersection
* 1 x,y set to (unique) intersection
* -1 lines overlap, no unique intersection
*
* Based on the following:
*
* (ax2-ax1)r1 - (bx2-bx1)r2 = ax2 - ax1
* (ay2-ay1)r1 - (by2-by1)r2 = ay2 - ay1
*
* Solving for r1 and r2, if r1 and r2 are between 0 and 1,
* then line segments (ax1,ay1)(ax2,ay2) and (bx1,by1)(bx2,by2)
* intersect
****************************************************************/
#define D ((ax2-ax1)*(by1-by2) - (ay2-ay1)*(bx1-bx2))
#define D1 ((bx1-ax1)*(by1-by2) - (by1-ay1)*(bx1-bx2))
#define D2 ((ax2-ax1)*(by1-ay1) - (ay2-ay1)*(bx1-ax1))
int
dig_test_for_intersection(double ax1, double ay1,
double ax2, double ay2,
double bx1, double by1, double bx2, double by2)
{
register double d, d1, d2;
double t;
d = D;
d1 = D1;
d2 = D2;
if (d > 0)
return (d1 >= 0 && d2 >= 0 && d >= d1 && d >= d2);
if (d < 0)
return (d1 <= 0 && d2 <= 0 && d <= d1 && d <= d2);
/* lines are parallel */
if (d1 || d2)
return 0;
/* segments are colinear. check for overlap */
if (ax1 > ax2) {
t = ax1;
ax1 = ax2;
ax2 = t;
}
if (bx1 > bx2) {
t = bx1;
bx1 = bx2;
bx2 = t;
}
if (ax1 > bx2)
return 0;
if (ax2 < bx1)
return 0;
/* there is overlap */
if (ax1 == bx2 || ax2 == bx1)
return 1; /* endpoints only */
return -1; /* true overlap */
}
int
dig_find_intersection(double ax1, double ay1,
double ax2, double ay2,
double bx1, double by1,
double bx2, double by2, double *x, double *y)
{
register double d, r1, r2;
double t;
d = D;
if (d) {
r1 = D1 / d;
r2 = D2 / d;
if (r1 < 0 || r1 > 1 || r2 < 0 || r2 > 1) {
return 0;
}
*x = ax1 + r1 * (ax2 - ax1);
*y = ay1 + r1 * (ay2 - ay1);
return 1;
}
/* lines are parallel */
if (D1 || D2) {
return 0;
}
/* segments are colinear. check for overlap */
if (ax1 > ax2) {
t = ax1;
ax1 = ax2;
ax2 = t;
}
if (bx1 > bx2) {
t = bx1;
bx1 = bx2;
bx2 = t;
}
if (ax1 > bx2)
return 0;
if (ax2 < bx1)
return 0;
/* there is overlap */
if (ax1 == bx2) {
*x = ax1;
*y = ay1;
return 1; /* endpoints only */
}
if (ax2 == bx1) {
*x = ax2;
*y = ay2;
return 1; /* endpoints only */
}
return -1; /* overlap, no single intersection point */
}
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