判断两条线段是否相交

//排斥实验bool IsRectCross(const double a1[2],const double a2[2],                const double b1[2],const double b2[2]){    bool ret =  (min(a1[0],a2[0]) <= max(b1[0],b2[0])) &&            (min(b1[0],b2[0]) <= max(a1[0],a2[0])) &&            (min(a1[1],a2[1]) <= max(b1[1],b2[1]))  &&            (min(b1[1],b2[1]) <= max(a1[1],a2[1]));    return ret;}//跨立判断bool IsLineSegmentCross(const double a1[2],const double a2[2],                const double b1[2],const double b2[2]){    double b1a1[2] = {a1[0]-b1[0],a1[1]-b1[1]};    double b1b2[2] = {b2[0]-b1[0],b2[1]-b1[1]};    double b1a2[2] = {a2[0]-b1[0],a2[1]-b1[1]};    double seg1 = (b1a1[0]*b1b2[1]-b1a1[1]*b1b2[0]) * (b1a2[0]*b1b2[1]-b1a2[1]*b1b2[0]);    double a1b1[2] = {b1[0]-a1[0],b1[1]-a1[1]};    double a1a2[2] = {a2[0]-a1[0],a2[1]-a1[1]};    double a1b2[2] = {b2[0]-a1[0],b2[1]-a1[1]};    double seg2 = (a1b1[0]*a1a2[1]-a1b1[1]*a1a2[0]) * (a1b2[0]*a1a2[1]-a1b2[1]*a1a2[0]);    if(seg1<0 && seg2<0){        return true;    }else{        return false;    }}//XOY面两条线是否相交bool Intersection (double a1[2], double a2[2],                  double b1[2], double b2[2],                  double& u, double& v){    u = v = 0.0;    double denominator = (a2[1]-a1[1])*(b2[0]-b1[0])-(b2[1]-b1[1])*(a2[0]-a1[0]);    //平行    if(denominator < 0.0001 && denominator > -0.0001){        return 0;    }    if(IsRectCross(a1,a2,b1,b2)){        if(IsLineSegmentCross(a1,a2,b1,b2)){            double elementX = (a2[1]-a1[1])*(b2[0]-b1[0])*a1[0]-(b2[1]-b1[1])*(a2[0]-a1[0])*b1[0];            double elementY = (b2[1]-b1[1])*(a2[0]-a1[0])*a1[1]-(a2[1]-a1[1])*(b2[0]-b1[0])*b1[1];            u = elementX/denominator;            v = elementY/denominator;            return true;        }    }    return false;}

详细说明请看https://segmentfault.com/a/1190000004070478