title: OpenCV Overly Two Images
body: <p>I am new to opencv and I have tried to overly the origanal image with the detected contours in a single window, but I am having difficulty with it. Is it possible to overly this two images in one window. If so, how?</p>
  
  <pre><code> #include "opencv2/highgui/highgui.hpp"
  #include "opencv2/imgproc/imgproc.hpp"
  #include &lt;iostream&gt;
  #include &lt;stdio.h&gt;
  #include &lt;stdlib.h&gt;
  
  using namespace cv;
  using namespace std;
  
  Mat src; Mat src_gray;
  int thresh = 20;
  int max_thresh = 20;
  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( "image18.jpg", 1 );
  
    // Convert image to gray and blur it
    cvtColor( src, src_gray, CV_BGR2GRAY );
     blur( src_gray, src_gray, Size(31,31) );
  
    // Create Window
     char* source_window = "Source";
    namedWindow( source_window, CV_WINDOW_AUTOSIZE );
    imshow( source_window, src );
  
     createTrackbar( " Canny thresh:", "Source", &amp;thresh, max_thresh, thresh_callback );
     thresh_callback( 0, 0 );
  
     waitKey(0);
     return(0);
     }
  
  /** @function thresh_callback */
  void thresh_callback(int, void* )
  {
    Mat canny_output;
    vector&lt;vector&lt;Point&gt; &gt; contours;
    vector&lt;Vec4i&gt; 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&lt; contours.size(); i++ )
       {
         Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
         drawContours( drawing, contours, i, color, 0.5, 1, hierarchy, 0, Point() );
        }
  
     // Show in a window
     namedWindow( "Contours", CV_WINDOW_AUTOSIZE );
     imshow( "Contours", drawing );
      }
  </code></pre>