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/**************************************************************
* find interesection between two lines defined by points on the lines
* line segment A is (ax1,ay1) to (ax2,ay2)
* line segment B is (bx1,by1) to (bx2,by2)
* returns
* -1 segment A and B do not intersect (parallel without overlap)
* 0 segment A and B do not intersect but extensions do intersect
* 1 intersection is a single point
* 2 intersection is a line segment (colinear with overlap)
* x,y intersection point
* ra - ratio that the intersection divides A
* rb - ratio that the intersection divides B
*
* B2
* /
* /
* r=p/(p+q) : A1---p-------*--q------A2
* /
* /
* B1
*
**************************************************************/
/**************************************************************
*
* A point P which lies on line defined by points A1=(x1,y1) and A2=(x2,y2)
* is given by the equation r * (x2,y2) + (1-r) * (x1,y1).
* if r is between 0 and 1, p lies between A1 and A2.
*
* Suppose points on line (A1, A2) has equation
* (x,y) = ra * (ax2,ay2) + (1-ra) * (ax1,ay1)
* or for x and y separately
* x = ra * ax2 - ra * ax1 + ax1
* y = ra * ay2 - ra * ay1 + ay1
* and the points on line (B1, B2) are represented by
* (x,y) = rb * (bx2,by2) + (1-rb) * (bx1,by1)
* or for x and y separately
* x = rb * bx2 - rb * bx1 + bx1
* y = rb * by2 - rb * by1 + by1
*
* when the lines intersect, the point (x,y) has to
* satisfy a system of 2 equations:
* ra * ax2 - ra * ax1 + ax1 = rb * bx2 - rb * bx1 + bx1
* ra * ay2 - ra * ay1 + ay1 = rb * by2 - rb * by1 + by1
*
* or
*
* (ax2 - ax1) * ra - (bx2 - bx1) * rb = bx1 - ax1
* (ay2 - ay1) * ra - (by2 - by1) * rb = by1 - ay1
*
* by Cramer's method, one can solve this by computing 3
* determinants of matrices:
*
* M = (ax2-ax1) (bx1-bx2)
* (ay2-ay1) (by1-by2)
*
* M1 = (bx1-ax1) (bx1-bx2)
* (by1-ay1) (by1-by2)
*
* M2 = (ax2-ax1) (bx1-ax1)
* (ay2-ay1) (by1-ay1)
*
* Which are exactly the determinants D, D2, D1 below:
*
* D ((ax2-ax1)*(by1-by2) - (ay2-ay1)*(bx1-bx2))
*
* D1 ((bx1-ax1)*(by1-by2) - (by1-ay1)*(bx1-bx2))
*
* D2 ((ax2-ax1)*(by1-ay1) - (ay2-ay1)*(bx1-ax1))
***********************************************************************/
#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))
#define SWAP(x,y) {int t; t=x; x=y; y=t;}
int G_intersect_line_segments(double ax1, double ay1, double ax2, double ay2,
double bx1, double by1, double bx2, double by2,
double *ra, double *rb, double *x, double *y)
{
double d;
d = D;
if (d) { /* lines are not parallel */
*ra = D1 / d;
*rb = D2 / d;
*x = ax1 + (*ra) * (ax2 - ax1);
*y = ay1 + (*ra) * (ay2 - ay1);
return (*ra >= 0.0 && *ra <= 1.0 && *rb >= 0.0 && *rb <= 1.0);
}
if (D1 || D2)
return -1; /* lines are parallel, not colinear */
if (ax1 > ax2) {
SWAP(ax1, ax2)
}
if (bx1 > bx2) {
SWAP(bx1, bx2)
}
if (ax1 > bx2)
return -1;
if (ax2 < bx1)
return -1;
/* there is overlap */
if (ax1 == bx2) {
*x = ax1;
*y = ay1;
return 1; /* at endpoints only */
}
if (ax2 == bx1) {
*x = ax2;
*y = ay2;
return 1; /* at endpoints only */
}
return 2; /* colinear with overlap on an interval, not just a single point */
}
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