用OpenCV的话，也要会一些绘图的操作。主要是画线、圆、矩形、椭圆。
绘图的话，首先要了解两种类型：Point和Scalar。
Point就是点的类，我们用它来表示图像当中的点，比如Point pt; pt.x = 10; pt.y = 8;或者Point pt = Point(10, 8);
Scalar实质上就是4维的向量，也就是C++当中的含有4个元素的vector。一般我们只用到三维，比如Scalar(0, 0, 255);
一、线
void line(Mat& img, Point pt1, Point pt2, const Scalar& color, int thickness=1, int lineType=8, int shift=0)
pt1表示线段的起始点。
pt2表示线段的终止点。
color表示线段的颜色。
lineType有三种类型：8连接的线、4连接的线，经过高斯平滑的线。分别是8, 4, CV_AA。
shift暂时不清楚有什么用，默认为0就好。
二、圆
void circle(Mat& img, Point center, int radius, const Scalar& color, int thickness=1, int lineType=8, int shift=0)
center表示圆心坐标。
radius表示半径。
thickness为圆的曲线的厚度，如果thickness为负数，则圆被填满。
lineType的意思和line()函数当中lineType的意思一样。
三、矩形
void rectangle(Mat& img, Point pt1, Point pt2, const Scalar& color, int thickness=1, lineType=8, int shift=0)
pt1和pt2是对角线上的两个点
void rectangle(Mat& img, Rect rec, const Scalar& color, int thickness=1, lineType=8, int shift=0)
Rect是矩形类型，一般以x, y, width, height初始化，比如 Rect rect(10, 10, 200, 100)，表示从点(10,10)开始，宽200高100的矩形
四、椭圆
void ellipse(Mat& img, Point center, Size axes, double angle, double startAngle, double endAngle, const Scalar& color, int thickness=1, int lineType=8, int shift=0)
要知道axes的意思，首先要知道Size。Size是尺寸类型，一般以width, height初始化，Size size(50, 20)表示宽度为50，高度为20。axes是axis的复数形式，Size的第一个参数表示长半轴的长度，第二个参数表示短半轴的长度。
angle是椭圆相对图像的角度。
startAngle和endAngle是椭圆的圆弧的角度，0到360，说明是整个椭圆。0到180，只有半个椭圆。OpenCV采取逆时针绘制。
void ellipse(Mat& img, RotatedRect& box, const Scalar& color, int thickness=1, int lineType=8, int shift=0)
box给出一个旋转的矩形，然后ellipse方法给出在矩形的内接椭圆。RotatedRect(const Point2f& center, const Size2f& size, float angle)
用box给椭圆初始化和用axes+angle的差别在于：box中RotatedRect的Size是矩形的宽度和高度，而axes是椭圆的半轴的长度。
附OpenCV官方指南当中绘图操作的实例代码：
1 /**
2 * @file Drawing_1.cpp
3 * @brief Simple sample code
4 */
5
6 #include <opencv2/core/core.hpp>
7 #include <opencv2/highgui/highgui.hpp>
8
9 #define w 400
10
11 using namespace cv;
12
13 /// Function headers
14 void MyEllipse( Mat img, double angle );
15 void MyFilledCircle( Mat img, Point center );
16 void MyPolygon( Mat img );
17 void MyLine( Mat img, Point start, Point end );
18
19 /**
20 * @function main
21 * @brief Main function
22 */
23 int main( void ){
24
25 /// Windows names
26 char atom_window[] = "Drawing 1: Atom";
27 char rook_window[] = "Drawing 2: Rook";
28
29 /// Create black empty images
30 Mat atom_image = Mat::zeros( w, w, CV_8UC3 );
31 Mat rook_image = Mat::zeros( w, w, CV_8UC3 );
32
33 /// 1. Draw a simple atom:
34 /// -----------------------
35
36 /// 1.a. Creating ellipses
37 MyEllipse( atom_image, 90 );
38 MyEllipse( atom_image, 0 );
39 MyEllipse( atom_image, 45 );
40 MyEllipse( atom_image, -45 );
41
42 /// 1.b. Creating circles
43 MyFilledCircle( atom_image, Point( w/2, w/2) );
44
45 /// 2. Draw a rook
46 /// ------------------
47
48 /// 2.a. Create a convex polygon
49 MyPolygon( rook_image );
50 /*
51 /// 2.b. Creating rectangles
52 rectangle( rook_image,
53 Point( w, w),
54 Point( 0, 7*w/8 ),
55
56 Scalar( 0, 255, 255 ),
57 -1,
58 8 );
59 */
60 Rect rec(0, 7*w/8, w, 1*w/8);
61 rectangle(rook_image, rec, Scalar(0, 255, 255), -1, 8);
62 /// 2.c. Create a few lines
63 MyLine( rook_image, Point( 0, 15*w/16 ), Point( w, 15*w/16 ) );
64 MyLine( rook_image, Point( w/4, 7*w/8 ), Point( w/4, w ) );
65 MyLine( rook_image, Point( w/2, 7*w/8 ), Point( w/2, w ) );
66 MyLine( rook_image, Point( 3*w/4, 7*w/8 ), Point( 3*w/4, w ) );
67
68 /// 3. Display your stuff!
69 imshow( atom_window, atom_image );
70 moveWindow( atom_window, 0, 200 );
71 imshow( rook_window, rook_image );
72 moveWindow( rook_window, w, 200 );
73
74 waitKey( 0 );
75 return(0);
76 }
77
78 /// Function Declaration
79
80 /**
81 * @function MyEllipse
82 * @brief Draw a fixed-size ellipse with different angles
83 */
84 void MyEllipse( Mat img, double angle )
85 {
86 int thickness = 2;
87 int lineType = 8;
88
89 ellipse( img,
90 //Point( w/2, w/2 ),
91 //Size( w/4, w/16 ),
92 //angle,
93 //0,
94 //360,
95 RotatedRect(Point( w/2, w/2 ),Size( w/2, w/8 ),angle),
96 Scalar( 255, 0, 0 ),
97 thickness,
98 lineType );
99 /*
100 ellipse( img,
101 Point( w/2, w/2 ),
102 Size( w/4, w/16 ),
103 angle,
104 0,
105 360,
106 Scalar( 255, 0, 0 ),
107 thickness,
108 lineType );
109 */
110 }
111
112 /**
113 * @function MyFilledCircle
114 * @brief Draw a fixed-size filled circle
115 */
116 void MyFilledCircle( Mat img, Point center )
117 {
118 int thickness = -1;
119 int lineType = CV_AA;
120
121 circle( img,
122 center,
123 w/32,
124 Scalar( 0, 0, 255 ),
125 thickness,
126 lineType,
127 0);
128 }
129
130 /**
131 * @function MyPolygon
132 * @function Draw a simple concave polygon (rook)
133 */
134 void MyPolygon( Mat img )
135 {
136 int lineType = 8;
137
138 /** Create some points */
139 Point rook_points[1][20];
140 rook_points[0][0] = Point( w/4, 7*w/8 );
141 rook_points[0][1] = Point( 3*w/4, 7*w/8 );
142 rook_points[0][2] = Point( 3*w/4, 13*w/16 );
143 rook_points[0][3] = Point( 11*w/16, 13*w/16 );
144 rook_points[0][4] = Point( 19*w/32, 3*w/8 );
145 rook_points[0][5] = Point( 3*w/4, 3*w/8 );
146 rook_points[0][6] = Point( 3*w/4, w/8 );
147 rook_points[0][7] = Point( 26*w/40, w/8 );
148 rook_points[0][8] = Point( 26*w/40, w/4 );
149 rook_points[0][9] = Point( 22*w/40, w/4 );
150 rook_points[0][10] = Point( 22*w/40, w/8 );
151 rook_points[0][11] = Point( 18*w/40, w/8 );
152 rook_points[0][12] = Point( 18*w/40, w/4 );
153 rook_points[0][13] = Point( 14*w/40, w/4 );
154 rook_points[0][14] = Point( 14*w/40, w/8 );
155 rook_points[0][15] = Point( w/4, w/8 );
156 rook_points[0][16] = Point( w/4, 3*w/8 );
157 rook_points[0][17] = Point( 13*w/32, 3*w/8 );
158 rook_points[0][18] = Point( 5*w/16, 13*w/16 );
159 rook_points[0][19] = Point( w/4, 13*w/16 );
160
161 const Point* ppt[1] = { rook_points[0] };
162 int npt[] = { 20 };
163
164 fillPoly( img,
165 ppt,
166 npt,
167 1,
168 Scalar( 255, 255, 255 ),
169 lineType );
170 }
171
172 /**
173 * @function MyLine
174 * @brief Draw a simple line
175 */
176 void MyLine( Mat img, Point start, Point end )
177 {
178 int thickness = 2;
179 int lineType = 8;
180 line( img,
181 start,
182 end,
183 Scalar( 0, 0, 0 ),
184 thickness,
185 lineType,
186 0);
187 }