OpenCV 雖是開源的計算機視覺庫，但裡面也有一些基礎的繪圖函式，本文將介紹幾種常用繪圖函式：直線、圓、橢圓、長方形、多邊形等。

1  資料結構

1.1  二維向量

     cv::Point 代表的是二維點 (int 型)，可用來表示影象座標 (x, y)  

// one way
Point pt;
pt.x = 10;
pt.y = 8;

// another way
Point pt = Point(10, 8);

     OpenCV 中，二維點型別可分為 Point2i, Point2l, Point2f, Point2d 四種，各自定義如下：

// 4 type of Point
typedef Point_<int>    cv::Point2i
typedef Point_<int64>  cv::Point2l
typedef Point_<float>  cv::Point2f
typedef Point_<double> cv::Point2d

// cv::Point
typedef Point2i cv::Point

 1.2  四維向量

     cv::Scalar 代表的是四維向量，常用來傳遞畫素值，尤其是 BGR 通道的畫素值 (最後一個元素不用，則不定義)

     $\texttt{Scalar} (blue \_ component, green \_ component, red \_ component)$

2  繪圖函式

2.1  line()

    OpenCV 中，繪製直線段較簡單，就是過兩點畫一條直線，函式為 line()

void cv::line (     
    InputOutputArray  img,
    Point      pt1,           // first point
    Point      pt2,           // second point
    const Scalar &   color,
    int      thickness = 1,
    int      lineType = LINE_8,
    int      shift = 0
)     

  2.2 circle() 和 ellipse()

     知道圓心和半徑，就可以繪製圓了，函式為 circle()

void cv::circle (     
       InputOutputArray   img,
       Point      center,              // center of the circle
       int        radius,              // radius of the circle
       const Scalar &     color,
       int      thickness = 1,
       int      lineType = LINE_8,
       int      shift = 0 
)     

    橢圓稍微複雜，橢圓中心，長、短軸半徑，以及橢圓弧的旋轉角度，則可得到一段橢圓弧 ellipse()

void cv::ellipse (     
    InputOutputArray   img,
    Point       center,         // center of the ellipse
    Size        axes,           // half size of the main axes
    double      angle,          // ellipse rotation angle in degrees
    double      startAngle,
    double      endAngle,
    const Scalar &   color,
    int      thickness = 1,
    int      lineType = LINE_8,
    int      shift = 0 
)     

2.3  rectangle()

   長方形的繪製，主要是靠其對角線上的兩個點 pt1 和 pt2，函式為 rectangle()

void cv::rectangle (
    InputOutputArray  img,
    Point      pt1,          // vertex of the rectangle
    Point      pt2,          // vertex of the rectangle opposite to pt1
    const Scalar &    color,
    int      thickness = 1,
    int      lineType = LINE_8,
    int      shift = 0 
)     

 2.4 fillpoly()

void cv::fillPoly (     
    InputOutputArray  img,
    const Point **   pts,   //
    const int *      npts,  //
    int      ncontours,
    const Scalar &  color,
    int      lineType = LINE_8,
    int      shift = 0,
    Point    offset = Point() 
)     

3 程式碼示例

3.1  直線和長方形

#include <opencv2/core.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/highgui.hpp>

using namespace cv;

#define w 300

int main()
{
    // creat a white background image
    Mat img;
    img.create(w,w,CV_8UC3);
    img.setTo(Scalar(255,255,255));

    // draw lines
    line(img, Point(w/4,w/4), Point(3*w/4, w/4), Scalar(255, 0, 0));
    line(img, Point(w/6,w/2), Point(5*w/6, w/2), Scalar(0, 255, 0));
    line(img, Point(w/10,3*w/4), Point(9*w/10, 3*w/4), Scalar(0, 0, 255));
    
    // draw rectangle
    rectangle(img,Point(w/12,w/12),Point(11*w/12,9*w/10),Scalar(200,200,100));

    // show lines in the image
    imshow("line and rectangle", img);

    waitKey(0);
}

 3.2  圓和橢圓

// draw circle and ellipse
circle(img, Point(w/2,w/2), 50, Scalar(255, 0, 0));
ellipse(img, Point(w/2,w/2), Size(100,50), 0, 0, 360, Scalar(0, 255, 0));

3.3  多邊形

Point rook_points[1][20];
rook_points[0][0]  = Point(    w/4,   7*w/8 );
rook_points[0][1]  = Point(  3*w/4,   7*w/8 );
rook_points[0][2]  = Point(  3*w/4,  13*w/16 );
rook_points[0][3]  = Point( 11*w/16, 13*w/16 );
rook_points[0][4]  = Point( 19*w/32,  3*w/8 );
rook_points[0][5]  = Point(  3*w/4,   3*w/8 );
rook_points[0][6]  = Point(  3*w/4,     w/8 );
rook_points[0][7]  = Point( 26*w/40,    w/8 );
rook_points[0][8]  = Point( 26*w/40,    w/4 );
rook_points[0][9]  = Point( 22*w/40,    w/4 );
rook_points[0][10] = Point( 22*w/40,    w/8 );
rook_points[0][11] = Point( 18*w/40,    w/8 );
rook_points[0][12] = Point( 18*w/40,    w/4 );
rook_points[0][13] = Point( 14*w/40,    w/4 );
rook_points[0][14] = Point( 14*w/40,    w/8 );
rook_points[0][15] = Point(    w/4,     w/8 );
rook_points[0][16] = Point(    w/4,   3*w/8 );
rook_points[0][17] = Point( 13*w/32,  3*w/8 );
rook_points[0][18] = Point(  5*w/16, 13*w/16 );
rook_points[0][19] = Point(    w/4,  13*w/16 );

const Point* ppt[1] = { rook_points[0]};
int npt[] = { 20 };

// draw polygon
fillPoly(img, ppt, npt, 1, Scalar(0, 255, 255 ));

3.4  顯示效果

 參考資料:

    OpenCV Tutorials / imgproc module / Basic Drawing

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