Drawing a bounding box around the detected contours of an image

draw_bounding_box.cpp

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <algorithm>
#include <iostream>
#include <iterator>
#include <stdio.h>
#include <stdlib.h>

using namespace cv;
using namespace std;

template <class COCiter, class Oiter>
void flatten (COCiter start, COCiter end, Oiter dest) {
  while (start != end) {
    dest = copy(start->begin(), start->end(), dest);
    ++start;
  }
}

bool replace(string& str, const string& from, const string& to) {
  size_t start_pos = str.find(from);
  if(start_pos == string::npos)
    return false;
  str.replace(start_pos, from.length(), to);
  return true;
}

int main( int argc, char** argv )
{
  char* fname = argv[1];

  /// Read input file
  Mat src = imread( fname, 1 );

  /// Detect edges using canny
  Mat canny_output;
  int thresh  = 20;
  Canny( src, canny_output, thresh, thresh*2, 3 );

  /// Find contours
  vector<vector<Point> > contours;
  vector<Vec4i> hierarchy;
  findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );

  /// Detect bounding box
  vector<Point> flatten_contours;
  flatten(contours.begin(), contours.end(), back_inserter(flatten_contours));
  RotatedRect rrect = minAreaRect(Mat(flatten_contours));

  /// Draw bounding box
  Point2f vertices[4];
  rrect.points(vertices);
  Rect rect = rrect.boundingRect();
  rectangle(src, rect, Scalar(255,0,0));
  cout << fname << " [" << rect.width << " x " << rect.height << " from (" << rect.x << ", " << rect.y << ")]\n";

  /// Uncomment for debugging purposes
  string path = string(fname);
  replace(path, "input", "output");
  imwrite(path, src);
  return(0);
}