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How to pass a MatOfKeyPoint and MatOfPoint2f to native code? (OpenCV 4 Android)

I'm currently struggling with the Java Native Interface for Eclipse.

What I have:

With OpenCV, I detected keypoints of a frame and got back an object of type MatOfKeyPoint. (image is of type Mat)

private MatOfKeyPoint mKeypoints;
private FeatureDetector mDetector;
//(...)
mDetector = FeatureDetector.create(FeatureDetector.FAST);
mDetector.detect(image, mKeypoints);

What I want:

My mKeypoints is now of type MatOfKeyPoint. I want to pass this object to native code so I can do calculations faster. After my calculation, the native method should save its results in an object of type MatOfPoint2f

How I tried to do it:

I wrote a method

private native void getSkylinePoints(long addrMatOfKeyPoint, long addrOutputMat);

and used it like this:

getSkylinePoints(mKeypoints.getNativeObjAddr(), 
    mSkylinePoints.getNativeObjAddr());

(mSkylinePoints is of type MatOfPoint2f and is not null)

My c++ code then looks like this:

JNIEXPORT void JNICALL  
Java_ch_ethz_arskyline_detection_FASTDetector_getSkylinePoints(JNIEnv*, 
    jobject, jlong addrMatOfKeyPoint, jlong addrOutputMat)
{
    vector<KeyPoint>& keypoints  = *(vector<KeyPoint>*)addrMatOfKeyPoint;
    vector<Point2f>& output  = *(vector<Point2f>*)addrOutputMat;

        // without this line, it works
    if (!keypoints.empty())
        output.push_back (keypoints[0].pt);

}

I know that vector< KeyPoint> in c++ corresponds to MatOfKeyPoint in Java and vector< Point2f> in c++ corresponds to MatOfPoint2f in Java. I also wrote native functions that pass an object of type Mat (Mat in Java and Mat in c++) and there it works.

The error I get:

Debugging c++ code in eclipse is hard. All the LogCat tells me is:

Tag: libc   Text: fatal signal 6 (SIGABRT) at 0x000358a (code=-6), thread 13757 (Thread-5023)

I think that you can't just do this

vector<KeyPoint>& keypoints  = *(vector<KeyPoint>*)addrMatOfKeyPoint;

as I did with Mat objects:

Mat& background  = *(Mat*)addrBackground;

Does anyone know how to do this? Thanks in advance for any help!

Isa