Question

I’m implementing this simple example just to get familiar with OpenCv and SIFT.

1  #include <opencv2/core/core.hpp>
2  #include <opencv2/features2d/features2d.hpp>
3  #include <opencv2/highgui/highgui.hpp>
4
5  using namespace std;
6 
7  int main(int argc, char** argv) {
8
9     const cv::Mat input = cv::imread("/tmp/image.jpg", 0); //Load as grayscale
10
11    cv::SiftFeatureDetector detector;
12    std::vector<cv::KeyPoint> keypoints;
13    detector.detect(input, keypoints);
14
15    // Add results to image and save.
16    cv::Mat output;
17    cv::drawKeypoints(input, keypoints, output);
18    cv::imwrite("/tmp/SIFT_RESULT.jpg", output);
19
20    return 0;
21
22 }

I was expecting it to be quite straightforward, but it’s throwing the following errors:

• undefined reference to 'cv::SIFT::CommonParams::CommonParams()' at line 11
• undefined reference to 'cv::FeatureDetector::detect(cv::Mat const&, std::vector<cv::KeyPoint, std::allocator<cv::KeyPoint> >&, cv::Mat const&) const' at line 13
• undefined reference to 'cv::drawKeypoints(cv::Mat const&, std::vector<cv::KeyPoint, std::allocator<cv::KeyPoint> > const&, cv::Mat&, cv::Scalar_<double> const&, int)' at line 17

Can you please tell me what’s going wrong?
Is it a problem in the code, or am I missing some headers?

---

Complete Build Output:

make all 
Building file: ../main.cpp
Invoking: GCC C++ Compiler
g++ -I/usr/local/include/opencv -I/usr/include/c++/4.5.2 -O0 -g3 -Wall -c -fmessage-length=0 -std=c++0x -MMD -MP -MF"main.d" -MT"main.d" -o "main.o" "../main.cpp"
Finished building: ../main.cpp

Building target: BioInfo
Invoking: GCC C++ Linker
g++ -L/usr/local/lib -o "BioInfo"  ./imageProcessingClass.o ./main.o   -lopencv_core -lopencv_highgui
./main.o: In function `main':
/.../Debug/../main.cpp:38: undefined reference to `cv::SIFT::CommonParams::CommonParams()'
/.../Debug/../main.cpp:38: undefined reference to `cv::SIFT::DetectorParams::DetectorParams()'
/.../Debug/../main.cpp:38: undefined reference to `cv::SiftFeatureDetector::SiftFeatureDetector(cv::SIFT::DetectorParams const&, cv::SIFT::CommonParams const&)'
/.../Debug/../main.cpp:40: undefined reference to `cv::FeatureDetector::detect(cv::Mat const&, std::vector<cv::KeyPoint, std::allocator<cv::KeyPoint> >&, cv::Mat const&) const'
/.../Debug/../main.cpp:44: undefined reference to `cv::drawKeypoints(cv::Mat const&, std::vector<cv::KeyPoint, std::allocator<cv::KeyPoint> > const&, cv::Mat&, cv::Scalar_<double> const&, int)'
./main.o: In function `~SiftFeatureDetector':
/usr/local/include/opencv2/features2d/features2d.hpp:1502: undefined reference to `vtable for cv::SiftFeatureDetector'
/usr/local/include/opencv2/features2d/features2d.hpp:1502: undefined reference to `cv::FeatureDetector::~FeatureDetector()'
collect2: ld returned 1 exit status
make: *** [BioInfo] Error 1

Build Finished

Answer 1

It seems you have forgotten to link with the OpenCV libraries. Also remember if you’re linking against OpenCV 2.4 there is a new library for “non-free” algorithms named opencv_nonfree that you need to link against for SIFT.