Feature Description

Goal

In this tutorial you will learn how to:

• Use the DescriptorExtractor interface in order to find the feature vector correspondent to the keypoints. Specifically:
  • Use SurfDescriptorExtractor and its function compute to perform the required calculations.
  • Use a BruteForceMatcher to match the features vector
  • Use the function drawMatches to draw the detected matches.

Theory

Code

This tutorial code’s is shown lines below. You can also download it from here

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

using namespace cv;

void readme();

/** @function main */
int main( int argc, char** argv )
{
  if( argc != 3 )
   { 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 )
   { 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 with a brute force matcher
  BruteForceMatcher< L2<float> > matcher;
  std::vector< DMatch > matches;
  matcher.match( descriptors_1, descriptors_2, matches );

  //-- Draw matches
  Mat img_matches;
  drawMatches( img_1, keypoints_1, img_2, keypoints_2, matches, img_matches );

  //-- Show detected matches
  imshow("Matches", img_matches );

  waitKey(0);

  return 0;
  }

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

Explanation

Result

1. Here is the result after applying the BruteForce matcher between the two original images: