C++ opencv Convex Hull

#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;

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

/** @function main */
int main(int argc, char** argv)
{
VideoCapture cap(0);
while (1)
{
cap >> src;

/// 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);

if (waitKey(30) == 27)
break;
}
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++)
{
drawContours(drawing, hull, i, Scalar(0,255,0), 1, 8, vector<Vec4i>(), 0, Point());
}

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