title: Drawing rectangle around detected object using SURF
body: <p>I am trying to detect an object from the following code involving surf detector, I do not want to draw matches, I want to draw a rectangle around the detected object, but somehow I am unable to get correct Homography, please can anyone point out where I am going wrong.</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;iostream&gt;
  #include "opencv2/core/core.hpp"
  #include "opencv2/features2d/features2d.hpp"
  #include "opencv2/highgui/highgui.hpp"
  #include "opencv2/imgproc/imgproc.hpp"
  #include "opencv2/calib3d/calib3d.hpp"
  
  using namespace cv;
  
  int main()
  {
      Mat object = imread( "sample.jpeg", CV_LOAD_IMAGE_GRAYSCALE );
  
      if( !object.data )
      {
          std::cout&lt;&lt; "Error reading object " &lt;&lt; std::endl;
          return -1;
      }
  
      //Detect the keypoints using SURF Detector
      int minHessian = 500;
  
      SurfFeatureDetector detector( minHessian );
      std::vector&lt;KeyPoint&gt; kp_object;
  
      detector.detect( object, kp_object );
  
      //Calculate descriptors (feature vectors)
      SurfDescriptorExtractor extractor;
      Mat des_object;
  
      extractor.compute( object, kp_object, des_object );
  
      FlannBasedMatcher matcher;
  
      VideoCapture cap(0);
  
      namedWindow("Good Matches",0);
      cvResizeWindow("Good Matches",800,800);
  
      std::vector&lt;Point2f&gt; obj_corners(4);
  
      //Get the corners from the object
      obj_corners[0] = (cvPoint(0,0));
      obj_corners[1] = (cvPoint(object.cols,0));
      obj_corners[2] = (cvPoint(object.cols,object.rows));
      obj_corners[3] = (cvPoint(0, object.rows));
  
      char key = 'a';
      int framecount = 0;
      while (key != 27)
      {
          Mat frame;
          cap &gt;&gt; frame;
  
          if (framecount &lt; 5)
          {
              framecount++;
              continue;
          }
  
          Mat des_image, img_matches;
          std::vector&lt;KeyPoint&gt; kp_image;
          std::vector&lt;vector&lt;DMatch &gt; &gt; matches;
          std::vector&lt;DMatch &gt; good_matches;
          std::vector&lt;Point2f&gt; obj;
          std::vector&lt;Point2f&gt; scene;
          std::vector&lt;Point2f&gt; scene_corners(4);
          Mat H;
          Mat image;
  
          cvtColor(frame, image, CV_RGB2GRAY);
  
          detector.detect( image, kp_image );
         extractor.compute( image, kp_image, des_image );
  
          matcher.knnMatch(des_object, des_image, matches, 2);
  
         for(int i = 0; i &lt; min(des_image.rows-1,(int) matches.size()); i++) //THIS LOOP IS SENSITIVE TO SEGFAULTS
        {
              if((matches[i][0].distance &lt; 0.6*(matches[i][1].distance)) &amp;&amp; ((int) matches[i].size()&lt;=2 &amp;&amp; (int) matches[i].size()&gt;0))
           {
                good_matches.push_back(matches[i][0]);
              }
         }
  
          //Draw only "good" matches
       //  drawMatches( object, kp_object, image, kp_image, good_matches, img_matches, Scalar::all(-1), Scalar::all(-1), vector&lt;char&gt;(),DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS);
  
          if (good_matches.size() &gt;= 4)
          {
              for( int i = 0; i &lt; good_matches.size(); i++ )
              {
                  //Get the keypoints from the good matches
                  obj.push_back( kp_object[ good_matches[i].queryIdx ].pt );
                  scene.push_back( kp_image[ good_matches[i].trainIdx ].pt );
             }
  
              H = findHomography( obj, scene, CV_RANSAC );
  
              perspectiveTransform( obj_corners, scene_corners, H);
  
              //Draw lines between the corners (the mapped object in the scene image )
              line( image, scene_corners[0] + Point2f( object.cols, 0), scene_corners[1] + Point2f( object.cols, 0), Scalar(0, 255, 0), 4 );
              line( image, scene_corners[1] + Point2f( object.cols, 0), scene_corners[2] + Point2f( object.cols, 0), Scalar( 0, 255, 0), 4 );
              line( image, scene_corners[2] + Point2f( object.cols, 0), scene_corners[3] + Point2f( object.cols, 0), Scalar( 0, 255, 0), 4 );
              line( image, scene_corners[3] + Point2f( object.cols, 0), scene_corners[0] + Point2f( object.cols, 0), Scalar( 0, 255, 0), 4 );
          }
  
          //Show detected matches
          imshow( "Good Matches", image );
  
          key = waitKey(1);
      }
      return 0;
  }
  </code></pre>