Added hull_demo.cpp code for tutorials

samples/cpp/tutorial_code/ShapeDescriptors/hull_demo.cpp

+/**
+ * @function hull_demo.cpp
+ * @brief Demo code to find contours in an image
+ * @author OpenCV team
+ */
+
+#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 );
+}