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.
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;