Fitting rotated boxes and ellipses for contours

RotatedRect fitEllipse(InputArray points)

Fits an ellipse around a set of 2D points.

RotatedRect minAreaRect(InputArray points)
Finds a rotated rectangle of the minimum area enclosing the input 2D point set.

vector<vector<Point>> contours;
findContours( ..., contours, ...); // This will find out the contours and save in contours
cout<<endl<<Mat(contours[index])<<endl; //index = 0,1,2,... used to print the format of contours
ellipse = fitEllipse( Mat(contours[i]) ); // fitting ellipse in contour points
Rect = minAreaRect( Mat(contours[i]) ); 

The code provided below is slight modification of code given in OpenCV documentation.

Steps:

1. Load image
2. Convert to gray-scale.
3. Remove noise by blurring with a Gaussian filter
4. Create new window
5. Add trackbar to the window

• Detect Canny edges
• Find contours
• Find the rotated rectangles and ellipses for each contour
• Draw contours, rotated rectangles and ellipses

Functions:

minAreaRect, fitEllipse, createTrackbar, findContours, Canny, blur, cvtColor, imshow, imread, waitKey.

Example:

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

using namespace cv;
using namespace std;

Mat src; Mat src_gray;
int thresh = 73;
int points = 50;
int max_thresh = 255;
RNG rng(12345);

void thresh_callback(int, void*);

int main( )
{
    /// Load source image and convert it to gray
    src = imread( "1.jpg", 1 );

    /// Convert image to gray and blur it
    cvtColor( src, src_gray, CV_BGR2GRAY );
    blur( src_gray, src_gray, Size(3,3) );

    namedWindow( "Source", CV_WINDOW_AUTOSIZE );    imshow( "Source", src );
    createTrackbar( " Threshold:", "Source", &thresh, max_thresh, thresh_callback );
    
    namedWindow( "Source1", CV_WINDOW_AUTOSIZE );    imshow( "Source1", src );
    createTrackbar( " Threshold:", "Source1", &points, max_thresh, thresh_callback );

    thresh_callback(0,0);

    waitKey(0);
    return(0);
}

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

    /// Detect edges     
    Canny( src_gray, edge, thresh, 3*thresh, 3);
    edge.convertTo(threshold_output, CV_8U);

    /// Find contours
    findContours( threshold_output, contours, hierarchy, CV_RETR_TREE, CV_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() );
    
    cout<<endl<<contours[1]<<endl;
    cout<<endl<<Mat(contours[1])<<endl;

    for( int i = 0; i < contours.size(); i++ )
    { minRect[i] = minAreaRect( Mat(contours[i]) );
    if( contours[i].size() > points )
    { minEllipse[i] = fitEllipse( Mat(contours[i]) ); }
    }

    /// Draw contours + rotated rects + ellipses
    Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );
    for( int 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, 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 );
    }

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

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Result: