Convex Hull — OpenCV 2.4.13.4 documentation

### Navigation

- index
- next |
- previous |
- OpenCV 2.4.13.4 documentation »
- OpenCV Tutorials »
- imgproc module. Image Processing »

# Convex Hull

## Goal

In this tutorial you will learn how to:

- Use the OpenCV function convexHull

## Theory

## Code

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

     #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);

     /// Function header
     void thresh_callback(int, void* );

    /** @function main */
    int main( int argc, char** argv )
     {
       /// Load source image and convert it to gray
       src = imread( argv[1], 1 );

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

       /// Create Window
       char* source_window = "Source";
       namedWindow( source_window, CV_WINDOW_AUTOSIZE );
       imshow( source_window, src );

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

       waitKey(0);
       return(0);
     }

     /** @function thresh_callback */
     void thresh_callback(int, void* )
     {
       Mat src_copy = src.clone();
       Mat threshold_output;
       vector<vector<Point> > contours;
       vector<Vec4i> hierarchy;

       /// Detect edges using Threshold
       threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );

       /// Find contours
       findContours( threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );

       /// Find the convex hull object for each contour
       vector<vector<Point> >hull( contours.size() );
       for( int i = 0; i < contours.size(); i++ )
          {  convexHull( Mat(contours[i]), hull[i], false ); }

       /// Draw contours + hull results
       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) );
            drawContours( drawing, contours, i, color, 1, 8, vector<Vec4i>(), 0, Point() );
            drawContours( drawing, hull, i, color, 1, 8, vector<Vec4i>(), 0, Point() );
          }

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

## Explanation

## Result

1. Here it is:

## Help and Feedback

You did not find what you were looking for?

- Ask a question on the Q&A forum.
- If you think something is missing or wrong in the documentation, please file a bug report.

### Navigation

- index
- next |
- previous |
- OpenCV 2.4.13.4 documentation »
- OpenCV Tutorials »
- imgproc module. Image Processing »