1Feature Description {#tutorial_feature_description}
2===================
3
4Goal
5----
6
7In this tutorial you will learn how to:
8
9-   Use the @ref cv::DescriptorExtractor interface in order to find the feature vector correspondent
10    to the keypoints. Specifically:
11    -   Use cv::xfeatures2d::SURF and its function cv::xfeatures2d::SURF::compute to perform the
12        required calculations.
13    -   Use a @ref cv::BFMatcher to match the features vector
14    -   Use the function @ref cv::drawMatches to draw the detected matches.
15
16Theory
17------
18
19Code
20----
21
22This tutorial code's is shown lines below.
23@code{.cpp}
24#include <stdio.h>
25#include <iostream>
26#include "opencv2/core.hpp"
27#include "opencv2/features2d.hpp"
28#include "opencv2/highgui.hpp"
29#include "opencv2/xfeatures2d.hpp"
30
31using namespace cv;
32using namespace cv::xfeatures2d;
33
34void readme();
35
36/* @function main */
37int main( int argc, char** argv )
38{
39  if( argc != 3 )
40   { return -1; }
41
42  Mat img_1 = imread( argv[1], IMREAD_GRAYSCALE );
43  Mat img_2 = imread( argv[2], IMREAD_GRAYSCALE );
44
45  if( !img_1.data || !img_2.data )
46   { return -1; }
47
48  //-- Step 1: Detect the keypoints using SURF Detector, compute the descriptors
49  int minHessian = 400;
50
51  Ptr<SURF> detector = SURF::create();
52  detector->setHessianThreshold(minHessian);
53
54  std::vector<KeyPoint> keypoints_1, keypoints_2;
55  Mat descriptors_1, descriptors_2;
56
57  detector->detectAndCompute( img_1, Mat(), keypoints_1, descriptors_1 );
58  detector->detectAndCompute( img_2, Mat(), keypoints_2, descriptors_2 );
59
60  //-- Step 2: Matching descriptor vectors with a brute force matcher
61  BFMatcher matcher(NORM_L2);
62  std::vector< DMatch > matches;
63  matcher.match( descriptors_1, descriptors_2, matches );
64
65  //-- Draw matches
66  Mat img_matches;
67  drawMatches( img_1, keypoints_1, img_2, keypoints_2, matches, img_matches );
68
69  //-- Show detected matches
70  imshow("Matches", img_matches );
71
72  waitKey(0);
73
74  return 0;
75  }
76
77 /* @function readme */
78 void readme()
79 { std::cout << " Usage: ./SURF_descriptor <img1> <img2>" << std::endl; }
80@endcode
81
82Explanation
83-----------
84
85Result
86------
87
88Here is the result after applying the BruteForce matcher between the two original images:
89
90![](images/Feature_Description_BruteForce_Result.jpg)
91