OpenCV brief descriptors less than keypoints

Question

I'm using OpenCV to detect features and compute descriptors.

For feature detection I'm using FAST:

cv::Ptr<cv::FeatureDetector> _detector = cv::FastFeatureDetector::create(_configuration.threshold,
                                            _configuration.nonmaxSuppression);

For descriptors I'm using BRIEF:

cv::Ptr<cv::DescriptorExtractor> _descriptor_extractor = cv::xfeatures2d::BriefDescriptorExtractor::create();

After that, I'd like to order keypoints based on their response and store just a certain number of them:

typedef std::map<float,cv::KeyPoint,std::greater<float> > ResponseKeypointMap;

// keypoint buffer
std::vector<cv::KeyPoint> keypoints;
cv::Mat descriptors;

// detect keypoints
_detector->detect(rgb_image_, keypoints);

const int keypoints_size = keypoints.size();
if(!keypoints_size){
    std::cerr << "warning: [PointDetector] found 0 keypoints!\n";
    return;
}

ResponseKeypointMap keypoints_map;
for(int i=0; i < keypoints_size; ++i){
  keypoints_map.insert(std::make_pair(keypoints[i].response,keypoints[i]));
}

int iterations = std::min(_configuration.max_keypoints_size,keypoints_size);
std::vector<cv::KeyPoint> filtered_keypoints;
filtered_keypoints.resize(iterations);
int k=0;

for(ResponseKeypointMap::iterator it = keypoints_map.begin();
    it != keypoints_map.end();
    ++it){
  filtered_keypoints[k] = it->second;
  k++;

  if(k>=iterations)
    break;
}

std::cerr << "filtered keypoints size: " << filtered_keypoints.size() << std::endl;

_descriptor_extractor->compute(rgb_image_, filtered_keypoints, descriptors);
std::cerr << "Computed " << descriptors.rows << "x" << descriptors.cols << " descriptors" << std::endl;

I don't know why I'm giving 100 keypoints to the DescriptorExtractor, but I'm recieving 55 descriptors.

I'd be very grateful if you could explain me what is happening.

Thanks.

Aleksey Petrov Aleksey Petrov · Accepted Answer · 2018-02-22T10:45:29

According to OpenCV documentation https://docs.opencv.org/2.4/modules/features2d/doc/common_interfaces_of_descriptor_extractors.html

DescriptorExtractor::compute(const Mat& image, vector& keypoints, Mat& descriptors)
...
keypoints – Input collection of keypoints. Keypoints for which a descriptor cannot be computed are removed and the remaining ones may be reordered. Sometimes new keypoints can be added, for example: SIFT duplicates a keypoint with several dominant orientations (for each orientation).
...

So, after execution of compute method, your filtered_keypoints vector is altered and you have new pair of keypoints and descriptors, both of size 55.

OpenCV brief descriptors less than keypoints

2 Answers