Opencv SIFT未定义引用问题

Question

In function keypoint_calc(cv::Mat)': FilePath/MyFirstOpenCVJNI-master/jni/first-opencvjni.cpp:35: undefined reference to cv::xfeatures2d::SIFT::create(int, int, double, double, double)'

./obj/local/arm64-v8a/objs/first-opencvjni/first-opencvjni.o: In function sift_descriptor(cv::Mat, std::vector<cv::KeyPoint, std::allocator<cv::KeyPoint> >)': FilePath/MyFirstOpenCVJNI-master/jni/first-opencvjni.cpp:45: undefined reference to cv::xfeatures2d::SIFT::create(int, int, double, double, double)'

#include <jni.h>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc_c.h>
#include <opencv/cv.h>
#include <opencv2/opencv.hpp>
#include <GLES2/gl2.h>
#include <android/log.h>

#include <opencv2/flann/flann.hpp>
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/calib3d/calib3d.hpp"
#include "opencv2/nonfree/nonfree.hpp"
#include "opencv2/nonfree/features2d.hpp"
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/xfeatures2d.hpp>
#include <iostream>

#define  LOG_TAG    "cpp-file"

are my includes.

And on those mentioned lines i have following code.

cv::Ptr<cv::xfeatures2d::SIFT> feature_detector = cv::xfeatures2d::SIFT::create(200);

and

cv::Ptr<cv::xfeatures2d::SIFT> feature_detector = cv::xfeatures2d::SIFT::create(200);

I'm a noob to JNI any help would be highly appreciated.

Answer 1

Undefined reference is reported by the linker . Your project ( Android.mk or CMakeLists.txt ) should include the OpenCV libraries. SIFT is patented, therefore it is not part of the free prebuilt OpenCV Android distribution. You can either build libnonfree.so separately and use it as a prebuilt shared library in your app, or compile SIFT as a static library.

Detailed instructions can be found here and here , but they may require adaptation for the latest (3.2) version, and the new NDK release. If you don't care about the latest version, you can also download prebuilt binaries .

Answer 2

After days of struggle finally able to solve the contrib module, after some work around came across a pretty nice self explainable article to build the opencv with Extra modules. The link for the same is here

And in case some one wants to use an already built opencv master with contrib extra modules, they can feel free to use it from my repo, the link for the same is this , This article works only on windows, just for the sake of information as i've failed to follow up the same on a mac.