Creating Bounding rotated boxes and ellipses for contours

Goal

In this tutorial you will learn how to:

• Use the OpenCV function cv::minAreaRect
• Use the OpenCV function cv::fitEllipse

Theory

Code

This tutorial code's is shown lines below. You can also download it from here

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

using namespace cv;
using namespace std;

Mat src; Mat src_gray;
int thresh = 100;
int max_thresh = 255;
RNG rng(12345);

void thresh_callback(int, void* );

int main( int, char** argv )
{
 src = imread( argv[1], 1 );

 cvtColor( src, src_gray, COLOR_BGR2GRAY );
 blur( src_gray, src_gray, Size(3,3) );

 const char* source_window = "Source";
 namedWindow( source_window, WINDOW_AUTOSIZE );
 imshow( source_window, src );

 createTrackbar( " Threshold:", "Source", &thresh, max_thresh, thresh_callback );
 thresh_callback( 0, 0 );

 waitKey(0);
 return(0);
}

void thresh_callback(int, void* )
{
 Mat threshold_output;
 vector<vector<Point> > contours;
 vector<Vec4i> hierarchy;

 threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );
 findContours( threshold_output, contours, hierarchy, 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( Mat(contours[i]) );
 if( contours[i].size() > 5 )
 { minEllipse[i] = fitEllipse( Mat(contours[i]) ); }
 }

 Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );
 for( size_t i = 0; i< contours.size(); i++ )
 {
 Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
 // contour
 drawContours( drawing, contours, (int)i, color, 1, 8, vector<Vec4i>(), 0, Point() );
 // ellipse
 ellipse( drawing, minEllipse[i], color, 2, 8 );
 // 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, 1, 8 );
 }

 namedWindow( "Contours", WINDOW_AUTOSIZE );
 imshow( "Contours", drawing );
}

Explanation

Result

Here it is: