轮廓的最小外接矩形、最小外接圆、三角形、椭圆等

朱雀 2022-03-22 14:22 419阅读 0赞

[OpenCV官网][OpenCV]

#include <opencv2/opencv.hpp>
    #include <iostream>
    
    
    using namespace cv;
    using namespace std;
    
    
    Mat src_gray;
    int thresh = 100;
    RNG rng(12345);
    
    void thresh_callback(int, void*);
    
    int main(void)
    { 
    	
        Mat src = imread("../res/balloon.png");
        if(src.empty())
        { 
    	cout << "can't load image" << endl;
    	return -1;
        }
    
    
        cvtColor(src, src_gray, cv::COLOR_BGR2GRAY);
        blur(src_gray, src_gray, Size(3,3));
    
        const char* source_window = "Source";
        namedWindow(source_window);
        imshow(source_window, src);
    
        const int max_thresh =255;
        createTrackbar("canny thresh:", source_window, &thresh, max_thresh, thresh_callback);
        thresh_callback(0,0);
    
        waitKey();
    
    	return 0;
    };
    
    
    
    void thresh_callback(int, void*)
    { 
        Mat canny_output;
        Canny(src_gray, canny_output, thresh, thresh*2);
    
    
        vector<vector<Point>> contours;
        findContours(canny_output, contours, RETR_TREE, CHAIN_APPROX_SIMPLE);  //从canny输出得到轮廓图
    
        vector<vector<Point>> contours_poly(contours.size());   //近似轮廓的多边形
        vector<Rect> boundRect (contours.size());     //矩形
        vector<Point2f> centers(contours.size());   // 圆的中心
        vector<float> radius(contours.size());      //圆的半径
    
    
        for(size_t i=0; i<contours.size();i++)  //对于每个轮廓都进行以下操作
        { 
    	cv::approxPolyDP(contours[i], contours_poly[i], 3, true); //将轮廓近似到多边形，最大距离不大于3
    	boundRect[i] = cv::boundingRect(contours_poly[i]);     //得到 能包围多边形的最小矩形
    	cv::minEnclosingCircle(contours_poly[i], centers[i], radius[i]); //得到能包围多边形的最小圆
        }
    
    
    //开始画图， 多边形、圆、矩形
        Mat drawing = Mat::zeros(canny_output.size(), CV_8UC3);
    
        for(size_t i=0; i<contours.size(); i++)
        { 
    	Scalar color = Scalar(rng.uniform(0,256), rng.uniform(0, 256), rng.uniform(0,256));
    	drawContours(drawing, contours_poly, (int)i, color); 
    	cv::rectangle(drawing, boundRect[i].tl(), boundRect[i].br(), color,2);
    	cv::circle(drawing, centers[i], (int)radius[i], color, 2);
        }	
    
        imshow("Contours", drawing);
    
    }

结果：

![2019012818494854.png][]

--------------------

## OpenCV API ##

1.  得到轮廓近似的多边形, 轮廓也是有一系列点组成，但是近似到多边行，**点会变少**，也会存在指定近似误差的问题（多边形和轮廓之间最大距离）

[void cv::approxPolyDP][void cv_approxPolyDP]

( 	    InputArray  	curve,    //输入的轮廓，存在std::vector<Point> 或者Mat
    		OutputArray  	approxCurve, // 输出的近似， 和输入类型一样
    		double  	epsilon, // 指定近似的误差， 近似的多边行和轮廓之间最大距离
    		bool  	closed   // true: 近似多边形是closed，第一个点和最后一个点连起来的， false: not closed
    )

1.  计算能 **包含一组点** 的最小外接矩形（up-right boundidng：正的，没有旋转的矩形）  
    [Rect cv::boundingRect ( InputArray array ) ][Rect cv_boundingRect _ InputArray array _]  
    输入灰度图 或者2D 一组点, 存在std::vector 或者 Mat.

1.  找到 **包含一组点** 的最小圆  
    [void cv::minEnclosingCircle][void cv_minEnclosingCircle]

( 	    InputArray  	points,  //输入点， 存在std::vector<> or Mat
    		Point2f &  	center,   //圆心
    		float &  	radius   //半径
    )

### 延伸 ###

1.  找到一组点的最小外接三角形

double cv::minEnclosingTriangle

(
    InputArray  	points,   //Input vector of 2D points with depth CV_32S or CV_32F, stored in std::vector<> or Mat
    OutputArray  	//triangle Output vector of three 2D points defining the vertices of the triangle. The depth of the OutputArray must be CV_32F.
    )

![在这里插入图片描述][20190128192234299.png]

1.  找到一组点的最小外接矩形（可能是旋转的）

[RotatedRect cv::minAreaRect ( InputArray points )  
][RotatedRect cv_minAreaRect _ InputArray points _]

值得注意的是， 当一组点的一部分接近边界的时候，所得到的矩形的顶点有可能是负数

1.  找到一组点的外接椭圆

[RotatedRect cv::fitEllipse ( InputArray points ) ][RotatedRect cv_fitEllipse _ InputArray points _]  
值得注意的是， 当一组点的一部分接近边界的时候，所得到的矩形的顶点有可能是负数，椭圆就是矩形内接椭圆

void thresh_callback(int, void* )
    { 
        Mat canny_output;
        Canny( src_gray, canny_output, thresh, thresh*2 );
        vector<vector<Point> > contours;
        findContours( canny_output, contours, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );
        vector<RotatedRect> minRect( contours.size() );
        vector<RotatedRect> minEllipse( contours.size() );
        for( size_t i = 0; i < contours.size(); i++ )
        { 
            minRect[i] = minAreaRect( contours[i] );
            if( contours[i].size() > 5 )
            { 
                minEllipse[i] = fitEllipse( contours[i] );
            }
        }
        Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
        for( size_t i = 0; i< contours.size(); i++ )
        { 
            Scalar color = Scalar( rng.uniform(0, 256), rng.uniform(0,256), rng.uniform(0,256) );
            // contour
            drawContours( drawing, contours, (int)i, color );
            // ellipse
            ellipse( drawing, minEllipse[i], color, 2 );
            // rotated rectangle
            Point2f rect_points[4];
            minRect[i].points( rect_points );
            for ( int j = 0; j < 4; j++ )
            { 
                line( drawing, rect_points[j], rect_points[(j+1)%4], color );
            }
        }
        imshow( "Contours", drawing );
    }

![在这里插入图片描述][20190128194229231.png]

[OpenCV]: https://docs.opencv.org/master/da/d0c/tutorial_bounding_rects_circles.html
[2019012818494854.png]: /images/20220322/26f514993cd443e4b5e4fa8c5755f083.png
[void cv_approxPolyDP]: https://docs.opencv.org/master/d3/dc0/group__imgproc__shape.html#ga0012a5fdaea70b8a9970165d98722b4c
[Rect cv_boundingRect _ InputArray array _]: https://docs.opencv.org/master/d3/dc0/group__imgproc__shape.html#ga103fcbda2f540f3ef1c042d6a9b35ac7
[void cv_minEnclosingCircle]: https://docs.opencv.org/master/d3/dc0/group__imgproc__shape.html#ga8ce13c24081bbc7151e9326f412190f1
[20190128192234299.png]: /images/20220322/31b238c9993848f7a65c94e64ec0ff5a.png
[RotatedRect cv_minAreaRect _ InputArray points _]: https://docs.opencv.org/master/d3/dc0/group__imgproc__shape.html#ga3d476a3417130ae5154aea421ca7ead9
[RotatedRect cv_fitEllipse _ InputArray points _]: https://docs.opencv.org/master/d3/dc0/group__imgproc__shape.html#gaf259efaad93098103d6c27b9e4900ffa
[20190128194229231.png]: /images/20220322/1f94b9686646486397aa373a6ae939cc.png