OpenCV brief descriptors less than keypoints - opencv

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.

Answer 1

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.

Answer 2

In, particular, FAST draws a diameter-7 ring around each test-point to determine if it is a keypoint, but BRIEF uses 256 points around the test-point. I don't know if BRIEF uses a square area or a circular area but, either way, it is bigger, and so FAST may find keypoints that are too close to the boundary of the image for BRIEF to be able to calculate the description.