Convex Hull

Table of Contents

• Goal
• Theory
• Code
• Explanation
• Result

Prev Tutorial: Finding contours in your image
Next Tutorial: Creating Bounding boxes and circles for contours

Original author	Ana Huamán
Compatibility	OpenCV >= 3.0

Goal

In this tutorial you will learn how to:

• Use the OpenCV function cv::convexHull

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> >hull( contours.size() );
    for( size_t i = 0; i < contours.size(); i++ )
    {
        convexHull( contours[i], hull[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, (int)i, color );
        drawContours( drawing, hull, (int)i, color );
    }
 
    imshow( "Hull demo", 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.MatOfInt;
import org.opencv.core.MatOfPoint;
import org.opencv.core.Point;
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 Hull {
    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 Hull(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("Convex Hull 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);
 
        List<MatOfPoint> hullList = new ArrayList<>();
        for (MatOfPoint contour : contours) {
            MatOfInt hull = new MatOfInt();
            Imgproc.convexHull(contour, hull);
 
            Point[] contourArray = contour.toArray();
            Point[] hullPoints = new Point[hull.rows()];
            List<Integer> hullContourIdxList = hull.toList();
            for (int i = 0; i < hullContourIdxList.size(); i++) {
                hullPoints[i] = contourArray[hullContourIdxList.get(i)];
            }
            hullList.add(new MatOfPoint(hullPoints));
        }
 
        Mat drawing = Mat.zeros(cannyOutput.size(), CvType.CV_8UC3);
        for (int i = 0; i < contours.size(); i++) {
            Scalar color = new Scalar(rng.nextInt(256), rng.nextInt(256), rng.nextInt(256));
            Imgproc.drawContours(drawing, contours, i, color);
            Imgproc.drawContours(drawing, hullList, i, color );
        }
 
        imgContoursLabel.setIcon(new ImageIcon(HighGui.toBufferedImage(drawing)));
        frame.repaint();
    }
}
 
public class HullDemo {
    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 Hull(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
 
    # Detect edges using Canny
    canny_output = cv.Canny(src_gray, threshold, threshold * 2)
 
    # Find contours
    contours, _ = cv.findContours(canny_output, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
 
    # Find the convex hull object for each contour
    hull_list = []
    for i in range(len(contours)):
        hull = cv.convexHull(contours[i])
        hull_list.append(hull)
 
    # Draw contours + hull results
    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, i, color)
        cv.drawContours(drawing, hull_list, i, color)
 
    # Show in a window
    cv.imshow('Contours', drawing)
 
# Load source image
parser = argparse.ArgumentParser(description='Code for Convex Hull 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 Window
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

Result

Here it is: