求两个向量夹角

/******************************************

  输入：

  3点坐标值

 返回：

  (first-cen)和(second-cen)两个向量的角度；

******************************************/ 

 

float CImageDoc::CalcAngle(CPoint cen,CPoint first,CPoint second)
{
 float dx1,dx2,dy1,dy2;
 float angle;

 dx1 = first.x - cen.x;
 dx2 = second.x - cen.x;
 dy1 = first.y - cen.y;
 dy2 = second.y - cen.y;

 float c=(float)sqrt((dx1*dx1+dy1*dy1)*(dx2*dx2+dy2*dy2));
 if(c==0) return -1;
 angle = (float)asin((dx1*dy2-dy1*dx2)/c);
 angle=angle*180/PI;
 return angle;
}