OpenCV怎么画箭头(How to draw an arrow)

1: #include <opencv\highgui.h>

2: #include <iostream>

3: #pragma comment(lib, "opencv_core231d.lib")

4: #pragma comment(lib, "opencv_highgui231d.lib")

5: #pragma comment(lib, "opencv_imgproc231d.lib")

6: using namespace std;

7: using namespace cv;

8: 

9: void drawArrow(cv::Mat& img, cv::Point pStart, cv::Point pEnd, int len, int alpha,

10:     cv::Scalar& color, int thickness = 1, int lineType = 8);

11: 

12: int main()

13: {

14:     Mat M(400, 400, CV_8UC3, Scalar(0, 0, 0));

15:     Point pStart(380, 100), pEnd(100, 250);

16:     Scalar lineColor(0, 255, 255);

17:     drawArrow(M, pStart, pEnd, 10, 45, lineColor);

18:     pStart = Point(100, 100);

19:     pEnd = Point(320, 190);

20:     lineColor = Scalar(0, 0, 255);

21:     drawArrow(M, pStart, pEnd, 25, 30, lineColor, 2, CV_AA);

22:     pStart = Point(200, 420);

23:     pEnd = Point(370, 170);

24:     lineColor = Scalar(255, 0, 255);

25:     drawArrow(M, pStart, pEnd, 17, 15, lineColor, 1, 4);

26:     imshow("draw arrow", M);

27:     waitKey();

28:     return 0;

29: }

30: 

31: void drawArrow(cv::Mat& img, cv::Point pStart, cv::Point pEnd, int len, int alpha,

32:     cv::Scalar& color, int thickness, int lineType)

33: {

34:     const double PI = 3.1415926;

35:     Point arrow;

36:     //计算 θ 角（最简单的一种情况在下面图示中已经展示，关键在于 atan2 函数，详情见下面）

37:     double angle = atan2((double)(pStart.y - pEnd.y), (double)(pStart.x - pEnd.x));

38:     line(img, pStart, pEnd, color, thickness, lineType);

39:     //计算箭角边的另一端的端点位置（上面的还是下面的要看箭头的指向，也就是pStart和pEnd的位置）

40:     arrow.x = pEnd.x + len * cos(angle + PI * alpha / 180);

41:     arrow.y = pEnd.y + len * sin(angle + PI * alpha / 180);

42:     line(img, pEnd, arrow, color, thickness, lineType);

43:     arrow.x = pEnd.x + len * cos(angle - PI * alpha / 180);

44:     arrow.y = pEnd.y + len * sin(angle - PI * alpha / 180);

45:     line(img, pEnd, arrow, color, thickness, lineType);

46: }

运行结果

 原理 如图

箭头的指向有如图四种情况，由三角函数知识可以为各个情况分别写出计算公式，在编程中有一个函数 atan2 正好为这四种情况提供了很好的支持（atan2的更多知识链接：http://en.wikipedia.org/wiki/Atan2）； 另外注意的是windows窗口的坐标是如图垂直翻转的坐标系，也就是 x 从左到右变大， y 从上到下变大。那么根据我们习惯用的如图所示的坐标系计算出来的 y 方向坐标会正好相反。