opencv/doc/tutorials/imgproc/shapedescriptors/hull/hull.markdown

Convex Hull {#tutorial_hull}
===========

Goal
----

In this tutorial you will learn how to:

-   Use the OpenCV function @ref cv::convexHull

Theory
------

Code
----

This tutorial code's is shown lines below. You can also download it from
[here](https://github.com/Itseez/opencv/tree/master/samples/cpp/tutorial_code/ShapeDescriptors/hull_demo.cpp)
@code{.cpp}
#include "opencv2/highgui.hpp"
#include "opencv2/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* );
@endcode
/\* @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, COLOR_BGR2GRAY ); blur(
        src_gray, src_gray, Size(3,3) );

        /// Create Window 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);

    }

    /\* @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, RETR_TREE,
    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", WINDOW_AUTOSIZE ); imshow( "Hull demo",
    drawing );

    }

Explanation
-----------

Result
------

1.  Here it is:

      ----------- -----------
      |Hull_0|   |Hull_1|
      ----------- -----------