learnopencv/ConvexHull/example.cpp

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <cstdlib>

using namespace cv;
using namespace std;

int main(int argc, char** argv) {
  string image_path; // image path for input image

  if(argc < 2)
    image_path = "sample.jpg";
  else 
    image_path = argv[1];

  // declare images 
  Mat src, gray, blur_image, threshold_output;

  // take input image
  src = imread(image_path, 1);
  
 // convert to grayscale 
  cvtColor(src, gray, COLOR_BGR2GRAY);
  
  // add blurring to the input image
  blur(gray, blur_image, Size(3, 3));
  
  // binary threshold the input image
  threshold(gray, threshold_output, 200, 255, THRESH_BINARY);
  
  // show source image
  namedWindow("Source", WINDOW_AUTOSIZE);
  imshow("Source", src);

  // Convex Hull implementation
  Mat src_copy = src.clone();
  
  // contours vector  
  vector< vector<Point> > contours;
  vector<Vec4i> hierarchy;
  
  // find contours for the thresholded image
  findContours(threshold_output, contours, hierarchy, RETR_TREE, 
      CHAIN_APPROX_SIMPLE, Point(0, 0));
  
  // create convex hull vector
  vector< vector<Point> > hull(contours.size());

  // find convex hull for each contour
  for(int i = 0; i < contours.size(); i++)
    convexHull(Mat(contours[i]), hull[i], false);
  
  // create empty black image
  Mat drawing = Mat::zeros(threshold_output.size(), CV_8UC3);
  
  // draw contours and convex hull on the empty black image 
  for(int i = 0; i < contours.size(); i++) {
    Scalar color_contours = Scalar(0, 255, 0); // color for contours : blue
    Scalar color = Scalar(255, 255, 255); // color for convex hull : white
    // draw contours
    drawContours(drawing, contours, i, color_contours, 2, 8, vector<Vec4i>(), 0, Point());
    // draw convex hull
    drawContours(drawing, hull, i, color, 2, 8, vector<Vec4i>(), 0, Point());
  }

  namedWindow("Output", WINDOW_AUTOSIZE);
  imshow("Output", drawing);

  waitKey(0);
  return 0;
}