OpenCV 之 基本绘图

OpenCV 虽是开源的计算机视觉库，但里面也有一些基础的绘图函数，本文将介绍几种常用绘图函数：直线、圆、椭圆、长方形、多边形等。

1  数据结构

1.1  二维向量

cv::Point 代表的是二维点 (int 型)，可用来表示图像坐标 (x, y)

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

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

OpenCV 中，二维点类型可分为 Point2i, Point2l, Point2f, Point2d 四种，各自定义如下：

// 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  四维向量

cv::Scalar 代表的是四维向量，常用来传递像素值，尤其是 BGR 通道的像素值 (最后一个元素不用，则不定义)

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

2  绘图函数

2.1  line()

OpenCV 中，绘制直线段较简单，就是过两点画一条直线，函数为 line()

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

2.2 circle() 和 ellipse()

知道圆心和半径，就可以绘制圆了，函数为 circle()

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

椭圆稍微复杂，椭圆中心，长、短轴半径，以及椭圆弧的旋转角度，则可得到一段椭圆弧 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 = ,
    int      lineType = LINE_8,
    int      shift =
)     

2.3  rectangle()

长方形的绘制，主要是靠其对角线上的两个点 pt1 和 pt2，函数为 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 = ,
    int      lineType = LINE_8,
    int      shift =
)     

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 = ,
    Point    offset = Point()
)     

3 代码示例

3.1  直线和长方形

#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(,,));

    // draw lines
    line(img, Point(w/,w/), Point(*w/, w/), Scalar(, , ));
    line(img, Point(w/,w/), Point(*w/, w/), Scalar(, , ));
    line(img, Point(w/,*w/), Point(*w/, *w/), Scalar(, , ));

    // draw rectangle
    rectangle(img,Point(w/,w/),Point(*w/,*w/),Scalar(,,));

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

    waitKey();
}

3.2  圆和椭圆

// draw circle and ellipse
circle(img, Point(w/,w/), , Scalar(, , ));
ellipse(img, Point(w/,w/), Size(,), , , , Scalar(, , ));

3.3  多边形

Point rook_points[][];
rook_points[][]  = Point(    w/,   *w/ );
rook_points[][]  = Point(  *w/,   *w/ );
rook_points[][]  = Point(  *w/,  *w/ );
rook_points[][]  = Point( *w/, *w/ );
rook_points[][]  = Point( *w/,  *w/ );
rook_points[][]  = Point(  *w/,   *w/ );
rook_points[][]  = Point(  *w/,     w/ );
rook_points[][]  = Point( *w/,    w/ );
rook_points[][]  = Point( *w/,    w/ );
rook_points[][]  = Point( *w/,    w/ );
rook_points[][] = Point( *w/,    w/ );
rook_points[][] = Point( *w/,    w/ );
rook_points[][] = Point( *w/,    w/ );
rook_points[][] = Point( *w/,    w/ );
rook_points[][] = Point( *w/,    w/ );
rook_points[][] = Point(    w/,     w/ );
rook_points[][] = Point(    w/,   *w/ );
rook_points[][] = Point( *w/,  *w/ );
rook_points[][] = Point(  *w/, *w/ );
rook_points[][] = Point(    w/,  *w/ );
const Point* ppt[] = { rook_points[]};
int npt[] = {  };
// draw polygon
fillPoly(img, ppt, npt, , Scalar(, ,  ));

3.4  显示效果

       

参考资料:

OpenCV Tutorials / imgproc module / Basic Drawing