c++ 根据三个点，测出角度

适用于opencv，居于c++，二维平面，角度测量

float angle(Point pt1, Point pt0, Point pt2)//角度计算
{
	double dx1 = (pt1.x - pt0.x);
	double dy1 = (pt1.y - pt0.y);
	double dx2 = (pt2.x - pt0.x);
	double dy2 = (pt2.y - pt0.y);
	double angle_line = (dx1*dx2 + dy1 * dy2) / sqrt((dx1*dx1 + dy1 * dy1)*(dx2*dx2 + dy2 * dy2) + 1e-10);
	double cosM;
	if (pt2.y > pt1.y)
	{
		cosM = 1;
	}
	else if (pt2.y <= pt1.y)
	{
		cosM = -1;
	}

	double a = acos(angle_line) * (cosM)*180 / 3.141592653;

	Point linefirst =  Point(pt1.x - pt0.x, pt1.y - pt0.y );
	Point linesecond =  Point(pt2.x - pt0.x, pt2.y - pt0.y );
	bool isClockwise = linefirst.x * linesecond.y - linesecond.x * linefirst.y < 0;
	if (isClockwise)
	{
		a = 360 - a;
	}
	return a;
}