计算两线段交点

function segmentsIntr(a, b, c, d){  
  
/** 1 解线性方程组, 求线段交点. **/  
// 如果分母为0 则平行或共线, 不相交  
    var denominator = (b.y - a.y)*(d.x - c.x) - (a.x - b.x)*(c.y - d.y);  
    if (denominator==0) {  
        return false;  
    }  
   
// 线段所在直线的交点坐标 (x , y)      
    var x = ( (b.x - a.x) * (d.x - c.x) * (c.y - a.y)   
                + (b.y - a.y) * (d.x - c.x) * a.x   
                - (d.y - c.y) * (b.x - a.x) * c.x ) / denominator ;  
    var y = -( (b.y - a.y) * (d.y - c.y) * (c.x - a.x)   
                + (b.x - a.x) * (d.y - c.y) * a.y   
                - (d.x - c.x) * (b.y - a.y) * c.y ) / denominator;  
  
/** 2 判断交点是否在两条线段上 **/  
    if (  
        // 交点在线段1上  
        (x - a.x) * (x - b.x) <= 0 && (y - a.y) * (y - b.y) <= 0  
        // 且交点也在线段2上  
         && (x - c.x) * (x - d.x) <= 0 && (y - c.y) * (y - d.y) <= 0  
        ){  
  
        // 返回交点p  
        return {  
                x :  x,  
                y :  y  
            }  
    }  
    //否则不相交  
    return false  
  
}