MFC之画圆弧 任意三点绘制圆弧求弧长

1. 先求出线的方向角（用向量）

// 得到两点的角度 
// 0 - 360 以 point1 为圆心绕一周
double getAngleFromTwoPoint(CPoint point1, CPoint point2)
{
	double angle = 0;
	CPoint O = point1; //以第一点为圆心旋转
	CPoint A = point2;

	double r = getTheRealLengthOfTheLine(O, A, FALSE); //这只是求两点间距离很简单的我就不贴了
	CPoint B = CPoint(O.x + r, O.y); //以圆心O为起点 O点右侧距离r个像素长度做B点（O->B）
	CPoint OA = CPoint(A.x - O.x, A.y - O.y); //向量OA O->A
	CPoint OB = CPoint(B.x - O.x, B.y - O.y); //向量OB O->B

	double sin = (OA.x * OB.y - OB.x * OA.y) / (r * r); // |OA| = |OB| = r
	angle = ASIN(sin); //弧度 ASIN(c) = (asin(c) * (180.0 / PI))

	if (O.x < A.x && O.y > A.y)  //象限1
		angle = angle;
	else if (O.x > A.x && O.y > A.y) //象限2
		angle = 180 - angle;
	else if (O.x > A.x && O.y < A.y) //象限3
		angle = 180 - angle;
	else if (O.x <