计算几何学的相关算法 可以根据这个函数确定两条线段在交点处的转向,比如确定p0 p1和p1 p2在p1处是左转还是右转，只要求(p2-p0)*(p1-p0)，若0则右转，=0则共线-geometry calculation of correlation algorithm can determine the function of the two segments to the intersection, such as identifying p0 p1 p1 and p2 p1 is the left or the right, only requirements (p2 - p0) * (p1 - p0), and if <0 left,> 0 right, a total = 0 line