env/opencv-4.8.0/samples/cpp/contours2.cpp

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

using namespace cv;
using namespace std;

static void help(char** argv)
{
    cout
        << "\nThis program illustrates the use of findContours and drawContours\n"
        << "The original image is put up along with the image of drawn contours\n"
        << "Usage:\n";
    cout
        << argv[0]
        << "\nA trackbar is put up which controls the contour level from -3 to 3\n"
        << endl;
}

const int w = 500;
int levels = 3;

vector<vector<Point> > contours;
vector<Vec4i> hierarchy;

static void on_trackbar(int, void*)
{
    Mat cnt_img = Mat::zeros(w, w, CV_8UC3);
    int _levels = levels - 3;
    drawContours( cnt_img, contours, _levels <= 0 ? 3 : -1, Scalar(128,255,255),
                  3, LINE_AA, hierarchy, std::abs(_levels) );

    imshow("contours", cnt_img);
}

int main( int argc, char** argv)
{
    cv::CommandLineParser parser(argc, argv, "{help h||}");
    if (parser.has("help"))
    {
        help(argv);
        return 0;
    }
    Mat img = Mat::zeros(w, w, CV_8UC1);
    //Draw 6 faces
    for( int i = 0; i < 6; i++ )
    {
        int dx = (i%2)*250 - 30;
        int dy = (i/2)*150;
        const Scalar white = Scalar(255);
        const Scalar black = Scalar(0);

        if( i == 0 )
        {
            for( int j = 0; j <= 10; j++ )
            {
                double angle = (j+5)*CV_PI/21;
                line(img, Point(cvRound(dx+100+j*10-80*cos(angle)),
                    cvRound(dy+100-90*sin(angle))),
                    Point(cvRound(dx+100+j*10-30*cos(angle)),
                    cvRound(dy+100-30*sin(angle))), white, 1, 8, 0);
            }
        }

        ellipse( img, Point(dx+150, dy+100), Size(100,70), 0, 0, 360, white, -1, 8, 0 );
        ellipse( img, Point(dx+115, dy+70), Size(30,20), 0, 0, 360, black, -1, 8, 0 );
        ellipse( img, Point(dx+185, dy+70), Size(30,20), 0, 0, 360, black, -1, 8, 0 );
        ellipse( img, Point(dx+115, dy+70), Size(15,15), 0, 0, 360, white, -1, 8, 0 );
        ellipse( img, Point(dx+185, dy+70), Size(15,15), 0, 0, 360, white, -1, 8, 0 );
        ellipse( img, Point(dx+115, dy+70), Size(5,5), 0, 0, 360, black, -1, 8, 0 );
        ellipse( img, Point(dx+185, dy+70), Size(5,5), 0, 0, 360, black, -1, 8, 0 );
        ellipse( img, Point(dx+150, dy+100), Size(10,5), 0, 0, 360, black, -1, 8, 0 );
        ellipse( img, Point(dx+150, dy+150), Size(40,10), 0, 0, 360, black, -1, 8, 0 );
        ellipse( img, Point(dx+27, dy+100), Size(20,35), 0, 0, 360, white, -1, 8, 0 );
        ellipse( img, Point(dx+273, dy+100), Size(20,35), 0, 0, 360, white, -1, 8, 0 );
    }
    //show the faces
    namedWindow( "image", 1 );
    imshow( "image", img );
    //Extract the contours so that
    vector<vector<Point> > contours0;
    findContours( img, contours0, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);

    contours.resize(contours0.size());
    for( size_t k = 0; k < contours0.size(); k++ )
        approxPolyDP(Mat(contours0[k]), contours[k], 3, true);

    namedWindow( "contours", 1 );
    createTrackbar( "levels+3", "contours", &levels, 7, on_trackbar );

    on_trackbar(0,0);
    waitKey();

    return 0;
}