OpenCV 3.0 Segmentation Fault (Bag of visual words) - c++

Question

I am trying to set up a bag of visual words using openCV 3.0. I have looked a bit everywhere and all I seem to be able to find is code that is only compatible with versions in the 2.x domain. As of now this is what I have:
#include <opencv2/core/core.hpp>
#include "opencv2/highgui/highgui.hpp"
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/features2d/features2d.hpp>
#include <iostream>
#include <stdio.h>
using namespace std;
using namespace cv;
int main(int argc, const char** argv) {
Ptr<FeatureDetector> features;
Ptr<DescriptorExtractor> descriptors;
Ptr<DescriptorMatcher> matcher;
int MAX_ITER = 100;
int EPS = 2;
TermCriteria tc(MAX_ITER + EPS,1,0.001);
int dictSize = 1000;
int retries = 1;
int flags = KMEANS_PP_CENTERS;
BOWKMeansTrainer bowTrainer(dictSize,tc,retries,flags);
BOWImgDescriptorExtractor bowDE(descriptors,matcher);
Mat img1 = imread("/Users/Lucas/Desktop/pic2.jpg");
Mat img2 = imread("/Users/Lucas/Desktop/2.jpg");
vector<KeyPoint> keypoints,keypoints2;
features->detect(img1, keypoints);
features->detect(img2, keypoints2);
Mat myFeatures;
Mat myFeatures2;
descriptors->compute(img1, keypoints, myFeatures);
descriptors->compute(img2, keypoints2, myFeatures2);
bowTrainer.add(myFeatures);
bowTrainer.add(myFeatures2);
Mat dictionary = bowTrainer.cluster();
bowDE.setVocabulary(dictionary);
cout << dictionary << endl;
return 0;
}
I have put this together by using a few tutorials and snippets, but I am running into an issue. When the program gets to
features->detect(img1, keypoints);
it exits with a segmentation fault 11, whatever that means. Could someone help me and point out what it is I am doing wrong?

Answer 1

you have to create your FeatureDetector, DescriptorExtractor first. atm, you got null-pointer instances (that's your segfault).
#include <opencv2/xfeatures2d.hpp>
...
Ptr<FeatureDetector> features = xfeatures2d::SIFT::create();
Ptr<DescriptorExtractor> descriptors = xfeatures2d::SIFT::create();
Ptr<DescriptorMatcher> matcher = makePtr<BFMatcher>(NORM_L2);
note, that since you have to use SIFT or SURF, you will need the opencv_contrib repo installed for this