逆時針畫圓弧，原理：將360度分割成36份，分別標出每10度角度時的座標點，然後將每個點連線起來。 

#include <iostream>    
#include <opencv2\core\core.hpp>
#include <opencv2\opencv.hpp>    
#include <opencv2\highgui\highgui.hpp>    
#include <opencv2\contrib\contrib.hpp>    

#include <fstream> 
#include <windows.h>

using namespace cv;
using namespace std;

// 影象、圓心、開始點、結束點、線寬
void DrawArc(Mat *src, Point ArcCenter, Point StartPoint, Point EndPoint, int Fill)
{
	if (Fill <= 0) return;

	vector<Point> Dots;
	double Angle1 = atan2((StartPoint.y - ArcCenter.y), (StartPoint.x - ArcCenter.x));
	double Angle2 = atan2((EndPoint.y - ArcCenter.y), (EndPoint.x - ArcCenter.x));
	double Angle = Angle1 - Angle2;
	Angle = Angle * 180.0 / CV_PI;

	if (Angle < 0) Angle = 360 + Angle;
	if (Angle == 0) Angle = 360;
	int  brim = floor(Angle / 10); // 向下取整

	Dots.push_back(StartPoint);
	for (int i = 0; i < brim; i++)
	{
		double dSinRot = sin(-(10 * (i + 1)) * CV_PI / 180);
		double dCosRot = cos(-(10 * (i + 1)) * CV_PI / 180);
		int x = ArcCenter.x + dCosRot * (StartPoint.x - ArcCenter.x) - dSinRot * (StartPoint.y - ArcCenter.y);
		int y = ArcCenter.y + dSinRot * (StartPoint.x - ArcCenter.x) + dCosRot * (StartPoint.y - ArcCenter.y);
		Dots.push_back(Point(x, y));
	}
	Dots.push_back(EndPoint);
	RNG &rng = theRNG();
	Scalar color = Scalar(rng.uniform(100, 255), rng.uniform(100, 255), rng.uniform(100, 255));
	for (int i = 0; i < Dots.size() - 1; i++) {
		line(*src, Dots[i], Dots[i + 1], color, Fill);
	}
	Dots.clear();
}

int main()
{
	Mat	Img = Mat::zeros(800, 800, CV_8UC3);
	int64 tim = getTickCount();

	// 座標零點 400,400
	Point ZeroPoint = Point(400, 400);
	// 起始座標 150，-100
	Point StartPoint = Point(ZeroPoint.x +150, ZeroPoint.y - (-100));
	// 結束座標 -150，-100
	Point EndPoint = Point(ZeroPoint.x - 150, ZeroPoint.y - (-100));

	// 圓心相對起始點的座標 -150,200
	int I = StartPoint.x - 150;
	int J = StartPoint.y - (+200);

	Point Arc = Point(I, J);

	// 顯示圓心座標
	circle(Img, Arc, 5, Scalar(0, 0, 255), -1);
	// 顯示起始點座標
	circle(Img, StartPoint, 5, Scalar(255, 0, 0), -1);
	// 顯示結束點座標
	circle(Img, EndPoint, 5, Scalar(0, 255, 0), -1);

	// 影象、圓心、開始點、結束點、線寬
	DrawArc(&Img, Arc, StartPoint, EndPoint, 2);
	imshow("正多邊形", Img);

	tim = getTickCount() - tim;
	printf("處理耗時: %fms\n\n", tim * 1000 / getTickFrequency());
	waitKey(0);
	return 0;
}
 

效果如下：