env/opencv-4.8.0/samples/cpp/tutorial_code/ShapeDescriptors/generalContours_demo2.cpp

/**
 * @function generalContours_demo2.cpp
 * @brief Demo code to obtain ellipses and rotated rectangles that contain detected contours
 * @author OpenCV team
 */

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include <iostream>

using namespace cv;
using namespace std;

Mat src_gray;
int thresh = 100;
RNG rng(12345);

/// Function header
void thresh_callback(int, void* );

/**
 * @function main
 */
int main( int argc, char** argv )
{
    /// Load source image and convert it to gray
    CommandLineParser parser( argc, argv, "{@input | stuff.jpg | input image}" );
    Mat src = imread( samples::findFile( parser.get<String>( "@input" ) ) );
    if( src.empty() )
    {
        cout << "Could not open or find the image!\n" << endl;
        cout << "Usage: " << argv[0] << " <Input image>" << endl;
        return -1;
    }

    /// Convert image to gray and blur it
    cvtColor( src, src_gray, COLOR_BGR2GRAY );
    blur( src_gray, src_gray, Size(3,3) );

    /// Create Window
    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;
}

/**
 * @function thresh_callback
 */
void thresh_callback(int, void* )
{
    /// Detect edges using Canny
    Mat canny_output;
    Canny( src_gray, canny_output, thresh, thresh*2 );
    /// Find contours
    vector<vector<Point> > contours;
    findContours( canny_output, contours, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );

    /// Find the rotated rectangles and ellipses for each contour
    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] );
        }
    }

    /// Draw contours + rotated rects + ellipses
    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 );
        }
    }

    /// Show in a window
    imshow( "Contours", drawing );
}