Returning Mat object from native code to java in OpenCV

Question

I have an OpenCV Android app. Most of its code is in Java but I have one function that is in C. The function gets a Mat object and returns a new one.

My question is how do I return a Mat from the native code to Java? Couldn't find any example of that.

Thanks.

stacker · Accepted Answer · 2012-04-07 09:06:18Z

27

+25

Today I had to return a Mat from native code. I started with "Tutorial 2 Advanced - 2. Mix Java+Native OpenCV" it already passes two Mat (Images captured from camera) objects to the native code. But I wanted to return extracted feature, thus I added jlong addrDescriptor to the signature:

extern "C" {
JNIEXPORT void JNICALL Java_org_opencv_samples_tutorial4_Sample4View_FindFeatures(JNIEnv* env, jobject thiz, jlong addrGray, jlong addrRgba, jlong addrDescriptor)
{
    Mat* pMatGr=(Mat*)addrGray;
    Mat* pMatRgb=(Mat*)addrRgba;
    Mat* pMatDesc=(Mat*)addrDescriptor;
    vector<KeyPoint> v;

    //OrbFeatureDetector detector(50);
    OrbFeatureDetector detector;
    OrbDescriptorExtractor  extractor;
    detector.detect(*pMatGr, v);
    extractor.compute( *pMatGr, v, *pMatDesc );
    circle(*pMatRgb, Point(100,100), 10, Scalar(5,128,255,255));
    for( size_t i = 0; i < v.size(); i++ ) {
        circle(*pMatRgb, Point(v[i].pt.x, v[i].pt.y), 10, Scalar(255,128,0,255));
    }
    }
}

In the java part I added the Mat

private Mat descriptor;
descriptor = new Mat();

The method getNativeObjAddr() does the trick. The Mat is allocated in java and its address is passed to the native code, thus there isn't any explicit returning.

FindFeatures(mGraySubmat.getNativeObjAddr(), mRgba.getNativeObjAddr(), descriptor.getNativeObjAddr());
Log.i("desc:"  , descriptor.dump());

The Mat was filled with the required data and is directly accessible in the java code after the JNI invokation returns.

Somwhere else in the code the Mat is released:

if ( descriptor != null) 
  descriptor.release();
descriptor = null;

answered Apr 7, 2012 at 9:06

stacker

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5 Comments

Saqib Razaq Over a year ago

Hi stacker, changed my native code to this: Mat* image=(Mat*)addrDescriptor; Mat processing(image.size(), CV_8UC3); cv::medianBlur(*image, processing, 5); Now i get the following error message : jni/objDetector.cpp:23: error: request for member 'size' in 'image', which is of non-class type 'cv::Mat

stacker Over a year ago

@SaqibRazaq the signature is void medianBlur(const Mat& src, Mat& dst, int ksize) you need to invoke it with a reference to processing. try &processing. Or define it as Mat& processing(image.size(), CV_8UC3);

stacker Over a year ago

@SaqibRazaq If you want to return Mat &dst, then you should declare it in your java code and add it to the invocation of JNI method the same way you added image.

Romantic Electron Over a year ago

@stacker what do you mean by Somwhere else in the code the Mat is released:

stacker Over a year ago

@RomanticElectron that the code (last 3 lines of the post) should be invoked to release memory after the work is done.

K Lam · Accepted Answer · 2018-03-16 04:09:30Z

7

in C++

jlong funC(){
Mat *mat = new Mat();
//...
return (jlong)mat;
}

in java:

long = addr;// addr is return from c method funC()
Mat mat = new Mat(addr);

Attention: You must new Mat() in C,if you code is : Mat mat();mat object memory will be collect when funC() end.

answered Mar 16, 2018 at 4:09

K Lam

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