opencv/samples/cpp/tutorial_code/features2D/SURF_Homography.cpp

/**
 * @file SURF_Homography
 * @brief SURF detector + descriptor + FLANN Matcher + FindHomography
 * @author A. Huaman
 */

#include <stdio.h>
#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/calib3d/calib3d.hpp"

using namespace cv;

void readme();

/**
 * @function main
 * @brief Main function
 */
int main( int argc, char** argv )
{
  if( argc != 3 )
  { readme(); return -1; }

  Mat img_1 = imread( argv[1], CV_LOAD_IMAGE_GRAYSCALE );
  Mat img_2 = imread( argv[2], CV_LOAD_IMAGE_GRAYSCALE );
  
  if( !img_1.data || !img_2.data )
  { std::cout<< " --(!) Error reading images " << std::endl; return -1; }

  //-- Step 1: Detect the keypoints using SURF Detector
  int minHessian = 400;

  SurfFeatureDetector detector( minHessian );

  std::vector<KeyPoint> keypoints_1, keypoints_2;

  detector.detect( img_1, keypoints_1 );
  detector.detect( img_2, keypoints_2 );

  //-- Step 2: Calculate descriptors (feature vectors)
  SurfDescriptorExtractor extractor;

  Mat descriptors_1, descriptors_2;

  extractor.compute( img_1, keypoints_1, descriptors_1 );
  extractor.compute( img_2, keypoints_2, descriptors_2 );

  //-- Step 3: Matching descriptor vectors using FLANN matcher
  FlannBasedMatcher matcher;
  std::vector< DMatch > matches;
  matcher.match( descriptors_1, descriptors_2, matches );

  double max_dist = 0; double min_dist = 100;

  //-- Quick calculation of max and min distances between keypoints
  for( int i = 0; i < descriptors_1.rows; i++ )
  { double dist = matches[i].distance;
    if( dist < min_dist ) min_dist = dist;
    if( dist > max_dist ) max_dist = dist;
  }

  printf("-- Max dist : %f \n", max_dist );
  printf("-- Min dist : %f \n", min_dist );
  
  //-- Draw only "good" matches (i.e. whose distance is less than 3*min_dist )
  std::vector< DMatch > good_matches;

  for( int i = 0; i < descriptors_1.rows; i++ )
  { if( matches[i].distance < 3*min_dist )
    { good_matches.push_back( matches[i]); }
  }  

  Mat img_matches;
  drawMatches( img_1, keypoints_1, img_2, keypoints_2, 
               good_matches, img_matches, Scalar::all(-1), Scalar::all(-1), 
               vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS ); 


  //-- Localize the object from img_1 in img_2 
  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_1[ good_matches[i].queryIdx ].pt );
    scene.push_back( keypoints_2[ good_matches[i].trainIdx ].pt ); 
  }

  Mat H = findHomography( obj, scene, CV_RANSAC );

  //-- Get the corners from the image_1 ( the object to be "detected" )
  Point2f obj_corners[4] = { cvPoint(0,0), cvPoint( img_1.cols, 0 ), cvPoint( img_1.cols, img_1.rows ), cvPoint( 0, img_1.rows ) };
  Point scene_corners[4];

  //-- Map these corners in the scene ( image_2)
  for( int i = 0; i < 4; i++ )
  {
    double x = obj_corners[i].x; 
    double y = obj_corners[i].y;

    double Z = 1./( H.at<double>(2,0)*x + H.at<double>(2,1)*y + H.at<double>(2,2) );
    double X = ( H.at<double>(0,0)*x + H.at<double>(0,1)*y + H.at<double>(0,2) )*Z;
    double Y = ( H.at<double>(1,0)*x + H.at<double>(1,1)*y + H.at<double>(1,2) )*Z;
    scene_corners[i] = cvPoint( cvRound(X) + img_1.cols, cvRound(Y) );
  }  
   
  //-- Draw lines between the corners (the mapped object in the scene - image_2 )
  line( img_matches, scene_corners[0], scene_corners[1], Scalar(0, 255, 0), 2 );
  line( img_matches, scene_corners[1], scene_corners[2], Scalar( 0, 255, 0), 2 );
  line( img_matches, scene_corners[2], scene_corners[3], Scalar( 0, 255, 0), 2 );
  line( img_matches, scene_corners[3], scene_corners[0], Scalar( 0, 255, 0), 2 );

  //-- Show detected matches
  imshow( "Good Matches & Object detection", img_matches );

  waitKey(0);

  return 0;
}

/**
 * @function readme
 */
void readme()
{ std::cout << " Usage: ./SURF_descriptor <img1> <img2>" << std::endl; }