OpenCV - print object name only when homography is drawn

Question

I have an OpenCV program which uses SURF to detect if a template object is detected within the video stream. I am looking to print out the object name when the object is detected but at the moment it seems to be printing whenever a "good" feature match has been found which for the vast majority of the time are false positives.

My program is as follows:

    //Step 1: Detect keypoints using SURF detector
    //Step 2: Calculate descriptors (feature vectors)
    //Step 3: Matching descriptor vectors using FLANN matcher
    //Step 4: Localise the object
  std::vector<Point2f> obj;
  std::vector<Point2f> scene;

  for( int i = 0; i < good_matches.size(); i++ )
  {
    //-- Get the keypoints from the good matches
    obj.push_back( keypoints_object[ good_matches[i].queryIdx ].pt );
    scene.push_back( keypoints_scene[ good_matches[i].trainIdx ].pt );
  }

  Mat H = findHomography( obj, scene, CV_RANSAC );
     std::vector<Point2f> obj_corners(4);
      obj_corners[0] = cvPoint(0,0); obj_corners[1] = cvPoint( img_object.cols, 0 );
      obj_corners[2] = cvPoint( img_object.cols, img_object.rows ); obj_corners[3] = cvPoint( 0, img_object.rows );
      std::vector<Point2f> scene_corners(4);

      perspectiveTransform( obj_corners, scene_corners, H);

      //-- Draw lines between the corners (the mapped object in the scene - image_2 )
      line( img_matches, scene_corners[0] + Point2f( img_object.cols, 0), scene_corners[1] + Point2f( img_object.cols, 0), Scalar(0, 255, 0), 4 );
      line( img_matches, scene_corners[1] + Point2f( img_object.cols, 0), scene_corners[2] + Point2f( img_object.cols, 0), Scalar( 0, 255, 0), 4 );
      line( img_matches, scene_corners[2] + Point2f( img_object.cols, 0), scene_corners[3] + Point2f( img_object.cols, 0), Scalar( 0, 255, 0), 4 );
      line( img_matches, scene_corners[3] + Point2f( img_object.cols, 0), scene_corners[0] + Point2f( img_object.cols, 0), Scalar( 0, 255, 0), 4 );

      if() {
        std::cout << fileNamePostCut << std::endl;
      }
    ...

I'm not sure which condition to state in order to print the object name (fileNamePostCut)

Answer 1

Your goal is to dismiss false positives. You should pursue two approaches:

1. First, use the SIFT ratio test to dismiss unclear matches
2. You are calculating a homography from your matches. Use it as a model for correct matches: cv::findHomography has an optional output called mask. Use it to determine how many matches have actually contributed to the homography (these are called inliers). The more, the better - only print the object's name if you have more than 10 inliers for example.