C++ opencv Threshold

#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(" Canny thresh:", "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 canny_output;
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;

/// Detect edges using canny
Canny(src_gray, canny_output, thresh, thresh * 2, 3);
/// Find contours
findContours(canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));

/// Draw contours
Mat drawing = Mat::zeros(canny_output.size(), CV_8UC3);
for (int i = 0; i< contours.size(); i++)
{
drawContours(drawing, contours, i, Scalar(255,255,255), 1, 8, hierarchy, 0, Point());
}

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