Creating Bounding boxes and circles for contours

Table of Contents

• Goal
• Theory
• Code
• Explanation
• Result

Prev Tutorial: Convex Hull
Next Tutorial: Creating Bounding rotated boxes and ellipses for contours

Original author	Ana Huamán
Compatibility	OpenCV >= 3.0

Goal

In this tutorial you will learn how to:

• Use the OpenCV function cv::boundingRect
• Use the OpenCV function cv::minEnclosingCircle

Theory

Code

C++

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

 
#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);
 
void thresh_callback(int, void* );
 
int main( int argc, char** argv )
{
    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;
    }
 
    cvtColor( src, src_gray, 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 );
 
    vector<vector<Point> > contours_poly( contours.size() );
    vector<Rect> boundRect( contours.size() );
    vector<Point2f>centers( contours.size() );
    vector<float>radius( contours.size() );
 
    for( size_t i = 0; i < contours.size(); i++ )
    {
        approxPolyDP( contours[i], contours_poly[i], 3, true );
        boundRect[i] = boundingRect( contours_poly[i] );
        minEnclosingCircle( contours_poly[i], centers[i], radius[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) );
        drawContours( drawing, contours_poly, (int)i, color );
        rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2 );
        circle( drawing, centers[i], (int)radius[i], color, 2 );
    }
 
    imshow( "Contours", drawing );
}

Java

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

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Image;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
 
import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JSlider;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
 
import org.opencv.core.Core;
import org.opencv.core.CvType;
import org.opencv.core.Mat;
import org.opencv.core.MatOfPoint;
import org.opencv.core.MatOfPoint2f;
import org.opencv.core.Point;
import org.opencv.core.Rect;
import org.opencv.core.Scalar;
import org.opencv.core.Size;
import org.opencv.highgui.HighGui;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;
 
class GeneralContours1 {
    private Mat srcGray = new Mat();
    private JFrame frame;
    private JLabel imgSrcLabel;
    private JLabel imgContoursLabel;
    private static final int MAX_THRESHOLD = 255;
    private int threshold = 100;
    private Random rng = new Random(12345);
 
    public GeneralContours1(String[] args) {
        String filename = args.length > 0 ? args[0] : "../data/stuff.jpg";
        Mat src = Imgcodecs.imread(filename);
        if (src.empty()) {
            System.err.println("Cannot read image: " + filename);
            System.exit(0);
        }
 
        Imgproc.cvtColor(src, srcGray, Imgproc.COLOR_BGR2GRAY);
        Imgproc.blur(srcGray, srcGray, new Size(3, 3));
 
        // Create and set up the window.
        frame = new JFrame("Creating Bounding boxes and circles for contours demo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // Set up the content pane.
        Image img = HighGui.toBufferedImage(src);
        addComponentsToPane(frame.getContentPane(), img);
        // Use the content pane's default BorderLayout. No need for
        // setLayout(new BorderLayout());
        // Display the window.
        frame.pack();
        frame.setVisible(true);
        update();
    }
 
    private void addComponentsToPane(Container pane, Image img) {
        if (!(pane.getLayout() instanceof BorderLayout)) {
            pane.add(new JLabel("Container doesn't use BorderLayout!"));
            return;
        }
 
        JPanel sliderPanel = new JPanel();
        sliderPanel.setLayout(new BoxLayout(sliderPanel, BoxLayout.PAGE_AXIS));
 
        sliderPanel.add(new JLabel("Canny threshold: "));
        JSlider slider = new JSlider(0, MAX_THRESHOLD, threshold);
        slider.setMajorTickSpacing(20);
        slider.setMinorTickSpacing(10);
        slider.setPaintTicks(true);
        slider.setPaintLabels(true);
        slider.addChangeListener(new ChangeListener() {
            @Override
            public void stateChanged(ChangeEvent e) {
                JSlider source = (JSlider) e.getSource();
                threshold = source.getValue();
                update();
            }
        });
        sliderPanel.add(slider);
        pane.add(sliderPanel, BorderLayout.PAGE_START);
 
        JPanel imgPanel = new JPanel();
        imgSrcLabel = new JLabel(new ImageIcon(img));
        imgPanel.add(imgSrcLabel);
 
        Mat blackImg = Mat.zeros(srcGray.size(), CvType.CV_8U);
        imgContoursLabel = new JLabel(new ImageIcon(HighGui.toBufferedImage(blackImg)));
        imgPanel.add(imgContoursLabel);
 
        pane.add(imgPanel, BorderLayout.CENTER);
    }
 
    private void update() {
        Mat cannyOutput = new Mat();
        Imgproc.Canny(srcGray, cannyOutput, threshold, threshold * 2);
 
        List<MatOfPoint> contours = new ArrayList<>();
        Mat hierarchy = new Mat();
        Imgproc.findContours(cannyOutput, contours, hierarchy, Imgproc.RETR_TREE, Imgproc.CHAIN_APPROX_SIMPLE);
 
        MatOfPoint2f[] contoursPoly  = new MatOfPoint2f[contours.size()];
        Rect[] boundRect = new Rect[contours.size()];
        Point[] centers = new Point[contours.size()];
        float[][] radius = new float[contours.size()][1];
 
        for (int i = 0; i < contours.size(); i++) {
            contoursPoly[i] = new MatOfPoint2f();
            Imgproc.approxPolyDP(new MatOfPoint2f(contours.get(i).toArray()), contoursPoly[i], 3, true);
            boundRect[i] = Imgproc.boundingRect(new MatOfPoint(contoursPoly[i].toArray()));
            centers[i] = new Point();
            Imgproc.minEnclosingCircle(contoursPoly[i], centers[i], radius[i]);
        }
 
        Mat drawing = Mat.zeros(cannyOutput.size(), CvType.CV_8UC3);
        List<MatOfPoint> contoursPolyList = new ArrayList<>(contoursPoly.length);
        for (MatOfPoint2f poly : contoursPoly) {
            contoursPolyList.add(new MatOfPoint(poly.toArray()));
        }
 
        for (int i = 0; i < contours.size(); i++) {
            Scalar color = new Scalar(rng.nextInt(256), rng.nextInt(256), rng.nextInt(256));
            Imgproc.drawContours(drawing, contoursPolyList, i, color);
            Imgproc.rectangle(drawing, boundRect[i].tl(), boundRect[i].br(), color, 2);
            Imgproc.circle(drawing, centers[i], (int) radius[i][0], color, 2);
        }
 
        imgContoursLabel.setIcon(new ImageIcon(HighGui.toBufferedImage(drawing)));
        frame.repaint();
    }
}
 
public class GeneralContoursDemo1 {
    public static void main(String[] args) {
        // Load the native OpenCV library
        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
 
        // Schedule a job for the event dispatch thread:
        // creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new GeneralContours1(args);
            }
        });
    }
}

Python

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

from __future__ import print_function
import cv2 as cv
import numpy as np
import argparse
import random as rng
 
rng.seed(12345)
 
def thresh_callback(val):
    threshold = val
 
    
    canny_output = cv.Canny(src_gray, threshold, threshold * 2)
    
 
    
    contours, _ = cv.findContours(canny_output, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
    
 
    
    contours_poly = [None]*len(contours)
    boundRect = [None]*len(contours)
    centers = [None]*len(contours)
    radius = [None]*len(contours)
    for i, c in enumerate(contours):
        contours_poly[i] = cv.approxPolyDP(c, 3, True)
        boundRect[i] = cv.boundingRect(contours_poly[i])
        centers[i], radius[i] = cv.minEnclosingCircle(contours_poly[i])
    
 
    
    drawing = np.zeros((canny_output.shape[0], canny_output.shape[1], 3), dtype=np.uint8)
    
 
    
    for i in range(len(contours)):
        color = (rng.randint(0,256), rng.randint(0,256), rng.randint(0,256))
        cv.drawContours(drawing, contours_poly, i, color)
        cv.rectangle(drawing, (int(boundRect[i][0]), int(boundRect[i][1])), \
          (int(boundRect[i][0]+boundRect[i][2]), int(boundRect[i][1]+boundRect[i][3])), color, 2)
        cv.circle(drawing, (int(centers[i][0]), int(centers[i][1])), int(radius[i]), color, 2)
    
 
    
    cv.imshow('Contours', drawing)
    
 
 
parser = argparse.ArgumentParser(description='Code for Creating Bounding boxes and circles for contours tutorial.')
parser.add_argument('--input', help='Path to input image.', default='stuff.jpg')
args = parser.parse_args()
 
src = cv.imread(cv.samples.findFile(args.input))
if src is None:
    print('Could not open or find the image:', args.input)
    exit(0)
 
# Convert image to gray and blur it
src_gray = cv.cvtColor(src, cv.COLOR_BGR2GRAY)
src_gray = cv.blur(src_gray, (3,3))
 
 
 
source_window = 'Source'
cv.namedWindow(source_window)
cv.imshow(source_window, src)
 
max_thresh = 255
thresh = 100 # initial threshold
cv.createTrackbar('Canny thresh:', source_window, thresh, max_thresh, thresh_callback)
thresh_callback(thresh)
 
 
cv.waitKey()

Explanation

The main function is rather simple, as follows from the comments we do the following:

• Open the image, convert it into grayscale and blur it to get rid of the noise.

C++

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

Java

 
        String filename = args.length > 0 ? args[0] : "../data/stuff.jpg";
        Mat src = Imgcodecs.imread(filename);
        if (src.empty()) {
            System.err.println("Cannot read image: " + filename);
            System.exit(0);
        }
 
        Imgproc.cvtColor(src, srcGray, Imgproc.COLOR_BGR2GRAY);
        Imgproc.blur(srcGray, srcGray, new Size(3, 3));

Python

# Load source image
parser = argparse.ArgumentParser(description='Code for Creating Bounding boxes and circles for contours tutorial.')
parser.add_argument('--input', help='Path to input image.', default='stuff.jpg')
args = parser.parse_args()
 
src = cv.imread(cv.samples.findFile(args.input))
if src is None:
    print('Could not open or find the image:', args.input)
    exit(0)
 
# Convert image to gray and blur it
src_gray = cv.cvtColor(src, cv.COLOR_BGR2GRAY)
src_gray = cv.blur(src_gray, (3,3))

• Create a window with header "Source" and display the source file in it.

C++

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

Java

        // Create and set up the window.
        frame = new JFrame("Creating Bounding boxes and circles for contours demo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // Set up the content pane.
        Image img = HighGui.toBufferedImage(src);
        addComponentsToPane(frame.getContentPane(), img);

Python

# Create Window
source_window = 'Source'
cv.namedWindow(source_window)
cv.imshow(source_window, src)

• Create a trackbar on the source_window and assign a callback function to it. In general callback functions are used to react to some kind of signal, in our case it's trackbar's state change. Explicit one-time call of thresh_callback is necessary to display the "Contours" window simultaneously with the "Source" window.

C++

    const int max_thresh = 255;
    createTrackbar( "Canny thresh:", source_window, &thresh, max_thresh, thresh_callback );
    thresh_callback( 0, 0 );

Java

        sliderPanel.add(new JLabel("Canny threshold: "));
        JSlider slider = new JSlider(0, MAX_THRESHOLD, threshold);
        slider.setMajorTickSpacing(20);
        slider.setMinorTickSpacing(10);
        slider.setPaintTicks(true);
        slider.setPaintLabels(true);
        slider.addChangeListener(new ChangeListener() {
            @Override
            public void stateChanged(ChangeEvent e) {
                JSlider source = (JSlider) e.getSource();
                threshold = source.getValue();
                update();
            }
        });

Python

max_thresh = 255
thresh = 100 # initial threshold
cv.createTrackbar('Canny thresh:', source_window, thresh, max_thresh, thresh_callback)
thresh_callback(thresh)

The callback function does all the interesting job.

• Use cv::Canny to detect edges in the images.

C++

 
    Mat canny_output;
    Canny( src_gray, canny_output, thresh, thresh*2 );

Java

 
        Mat cannyOutput = new Mat();
        Imgproc.Canny(srcGray, cannyOutput, threshold, threshold * 2);

Python

    # Detect edges using Canny
    canny_output = cv.Canny(src_gray, threshold, threshold * 2)

• Finds contours and saves them to the vectors contour and hierarchy.

C++

 
    vector<vector<Point> > contours;
    findContours( canny_output, contours, RETR_TREE, CHAIN_APPROX_SIMPLE );

Java

 
        List<MatOfPoint> contours = new ArrayList<>();
        Mat hierarchy = new Mat();
        Imgproc.findContours(cannyOutput, contours, hierarchy, Imgproc.RETR_TREE, Imgproc.CHAIN_APPROX_SIMPLE);

Python

    # Find contours
    contours, _ = cv.findContours(canny_output, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

• For every found contour we now apply approximation to polygons with accuracy +-3 and stating that the curve must be closed. After that we find a bounding rect for every polygon and save it to boundRect. At last we find a minimum enclosing circle for every polygon and save it to center and radius vectors.

C++

 
    vector<vector<Point> > contours_poly( contours.size() );
    vector<Rect> boundRect( contours.size() );
    vector<Point2f>centers( contours.size() );
    vector<float>radius( contours.size() );
 
    for( size_t i = 0; i < contours.size(); i++ )
    {
        approxPolyDP( contours[i], contours_poly[i], 3, true );
        boundRect[i] = boundingRect( contours_poly[i] );
        minEnclosingCircle( contours_poly[i], centers[i], radius[i] );
    }

Java

 
        MatOfPoint2f[] contoursPoly  = new MatOfPoint2f[contours.size()];
        Rect[] boundRect = new Rect[contours.size()];
        Point[] centers = new Point[contours.size()];
        float[][] radius = new float[contours.size()][1];
 
        for (int i = 0; i < contours.size(); i++) {
            contoursPoly[i] = new MatOfPoint2f();
            Imgproc.approxPolyDP(new MatOfPoint2f(contours.get(i).toArray()), contoursPoly[i], 3, true);
            boundRect[i] = Imgproc.boundingRect(new MatOfPoint(contoursPoly[i].toArray()));
            centers[i] = new Point();
            Imgproc.minEnclosingCircle(contoursPoly[i], centers[i], radius[i]);
        }

Python

    # Approximate contours to polygons + get bounding rects and circles
    contours_poly = [None]*len(contours)
    boundRect = [None]*len(contours)
    centers = [None]*len(contours)
    radius = [None]*len(contours)
    for i, c in enumerate(contours):
        contours_poly[i] = cv.approxPolyDP(c, 3, True)
        boundRect[i] = cv.boundingRect(contours_poly[i])
        centers[i], radius[i] = cv.minEnclosingCircle(contours_poly[i])

We found everything we need, all we have to do is to draw.

• Create new Mat of unsigned 8-bit chars, filled with zeros. It will contain all the drawings we are going to make (rects and circles).

C++

    Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );

Java

        Mat drawing = Mat.zeros(cannyOutput.size(), CvType.CV_8UC3);

Python

    drawing = np.zeros((canny_output.shape[0], canny_output.shape[1], 3), dtype=np.uint8)

• For every contour: pick a random color, draw the contour, the bounding rectangle and the minimal enclosing circle with it.

C++

 
    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 );
        rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2 );
        circle( drawing, centers[i], (int)radius[i], color, 2 );
    }

Java

 
        List<MatOfPoint> contoursPolyList = new ArrayList<>(contoursPoly.length);
        for (MatOfPoint2f poly : contoursPoly) {
            contoursPolyList.add(new MatOfPoint(poly.toArray()));
        }
 
        for (int i = 0; i < contours.size(); i++) {
            Scalar color = new Scalar(rng.nextInt(256), rng.nextInt(256), rng.nextInt(256));
            Imgproc.drawContours(drawing, contoursPolyList, i, color);
            Imgproc.rectangle(drawing, boundRect[i].tl(), boundRect[i].br(), color, 2);
            Imgproc.circle(drawing, centers[i], (int) radius[i][0], color, 2);
        }

Python

    # Draw polygonal contour + bonding rects + circles
    for i in range(len(contours)):
        color = (rng.randint(0,256), rng.randint(0,256), rng.randint(0,256))
        cv.drawContours(drawing, contours_poly, i, color)
        cv.rectangle(drawing, (int(boundRect[i][0]), int(boundRect[i][1])), \
          (int(boundRect[i][0]+boundRect[i][2]), int(boundRect[i][1]+boundRect[i][3])), color, 2)
        cv.circle(drawing, (int(centers[i][0]), int(centers[i][1])), int(radius[i]), color, 2)

• Display the results: create a new window "Contours" and show everything we added to drawings on it.

C++

 
    imshow( "Contours", drawing );

Java

 
        imgContoursLabel.setIcon(new ImageIcon(HighGui.toBufferedImage(drawing)));
        frame.repaint();

Python

    # Show in a window
    cv.imshow('Contours', drawing)

Result

Here it is: