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