http://benjithian.sg/2012/12/simple-convex-hull-opencv/ Simple Convex hull. Simple stuff.

SimpleConvexHull

/*
http://benjithian.sg/2012/12/simple-convex-hull-opencv/
Simple Convex hull. Simple stuff.
*/
#include <stdio.h>
#include <curl/curl.h>
#include <sstream> 
#include <iostream>
#include <vector> 
#include <opencv2/opencv.hpp>
 
//curl writefunction to be passed as a parameter
size_t write_data(char *ptr, size_t size, size_t nmemb, void *userdata) {
    std::ostringstream *stream = (std::ostringstream*)userdata;
    size_t count = size * nmemb;
    stream->write(ptr, count);
    return count;
}

//Some of the Convex Hull Code from http://docs.opencv.org/doc/tutorials/imgproc/shapedescriptors/hull/hull.html 
std::vector<std::vector<cv::Point> > contourImg(cv::Mat src) {
  
	std::vector<std::vector<cv::Point> > contours;
	std::vector<cv::Vec4i> hierarchy;		
	cv::findContours(src, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, cv::Point(0, 0) );

	/// Find the convex hull object for each contour
	std::vector<std::vector<cv::Point> >hull( contours.size() );
	for( int i = 0; i < contours.size(); i++ )
		{  
			cv::convexHull( cv::Mat(contours[i]), hull[i], false ); // false as i want clockwise
		}
	int largestarea = 0; // to hold the convex with largest area
	int areaCount = 0; // hold the hull number with the largest area
	for(int i = 0; i < hull.size();i++)
	{
		std::vector<cv::Point> hull1 = hull[i];
		
		
		int area = 0;
		// I use trapezium formula to find the area of each 2 points
		// the top trapezium will the + and the bottom trapezium will be - 
		// the difference will be the area
		for ( int j = 0 ; j < hull1.size()-1 ; j++ )
		{
			cv::Point point1 = hull1[j]; // current point 
			cv::Point point2 = hull1[j+1]; // next point
			int area1 = (point1.y + point2.y) * (point1.x-point2.x); // area
			area += area1; // add them up
		}
		if ( largestarea < area ) { largestarea = area;areaCount=i; } //find largest area
	}
	std::vector<std::vector<cv::Point> >hull1( 1 ); //create a vector to hold the hull with the largest area
	hull1.push_back(hull[areaCount]);// insert the largest hull
	return hull1;
}

//function to retrieve the image as Cv::Mat data type
cv::Mat curlImg()
{
	CURL *curl;
	CURLcode res;
	std::ostringstream stream;
	curl = curl_easy_init();
    curl_easy_setopt(curl, CURLOPT_URL, "http://192.168.0.108:8080/shot.jpg"); //the JPEG Frame url
	curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data); // pass the writefunction
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &stream); // pass the stream ptr when the writefunction is called
	res = curl_easy_perform(curl); // start curl
	std::string output = stream.str(); // convert the stream into a string
	curl_easy_cleanup(curl); // cleanup
	std::vector<char> data = std::vector<char>( output.begin(), output.end() ); //convert string into a vector
	cv::Mat data_mat = cv::Mat(data); // create the cv::Mat datatype from the vector
	cv::Mat image = cv::imdecode(data_mat,1); //read an image from memory buffer
	
	return image;
}
int main(void)
{
	cv::namedWindow( "Image output", CV_WINDOW_AUTOSIZE );
	int i = 1;
	cv::Mat image = curlImg(); // get the image frame
	while(1)
	{
		if ( i == 1 ) // Update the background every 100*33ms
		{
			i = 100;
			image = curlImg(); // get the image frame
			cv::cvtColor(image,image, CV_BGR2GRAY);//Convert to GreyScale
		}
			i--;
		char c = cvWaitKey(33); // sleep for 33ms or till a key is pressed (put more then ur camera framerate mine is 30ms)
		cv::Mat image2 = curlImg(); // the image that is constantly being updated
		cv::Mat image3 = image2.clone();
		cv::cvtColor(image2,image2, CV_BGR2GRAY);//Convert to GreyScale
		cv::absdiff(image,image2,image2);// Absolute differences between the 2 images
		cv::blur(image2,image2,cv::Size(10,10)); // Try to elimite stray patch
		cv::threshold(image2,image2,5,255,CV_THRESH_BINARY); // set threshold to ignore small differences you can also use inrange function		
		std::vector<std::vector<cv::Point> > hull = contourImg(image2);
	   /// Draw contours + hull results
		for( int i = 0; i< hull.size(); i++ )
		 {
			cv::Scalar color = cv::Scalar( 255, 0, 0 ); // blue
			cv::drawContours( image3, hull, i, color, 3, 8, std::vector<cv::Vec4i>(), 0, cv::Point() ); // draw the hull
		 }

		//cv::Mat image3 = contourImg(image2);
		cv::imshow("Image output",image3); // display image
		c = cvWaitKey(33);
		if ( c == 27 ) break; // break if ESC is pressed		
	}
	cv::destroyWindow("Image output");
}