求两条直线夹角的算法

/*求角度  ；  向右方向为x正方向，向下方向为y正方向*/

double dX = pntTo.x - pntFrom.x;
double dY = pntTo.y - pntFrom.y;

double dAngle = 0;
//1
if (dX > 0 && dY > 0)
{
dAngle = PI/2 - atan(dY/dX);
}
//2
if (dX < 0 && dY > 0)
{
dAngle = 2*PI-(PI/2 + atan(dY/dX));
}
//3
if (dX < 0 && dY < 0)
{
dAngle = 2*PI-(PI/2 + atan(dY/dX));
}
//4
if (dX > 0 && dY < 0)
{
dAngle = PI/2 - atan(dY/dX);
}

//5
if (dX == 0 && dY > 0)