1. 先求出線的方向角（用向量）
// 得到兩點的角度
// 0 - 360 以 point1 為圓心繞一周
double getanglefromtwopoint(cpoint point1, cpoint point2)
{ double angle = 0;
cpoint o = point1; //以第一點為圓心旋轉
cpoint a = point2;
double r = getthereallengthoftheline(o, a, false); //這只是求兩點間距離很簡單的我就不貼了
cpoint b = cpoint(o.x + r, o.y); //以圓心o為起點 o點右側距離r個畫素長度做b點（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 <
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