Returning a Mat from native JNI to Java

Returning a Mat from native JNI to Java

Q:

I have a question about mixing OpenCV's C++ and Java API. I'm using the new desktop Java API to do some processing on images, but due to Java limitations I am forced to capture the images in C++. The images are in an OS-dependant format, so I thought it would be best to convert them to the OpenCV format before handing it to the OS-independant Java code. In the android tutorials (Tutorial 2 Advanced) I saw that they instantiate a Mat on the Java side, and use getNativeObjAddr() to pass a pointer to C++, where it is used as a native C++ object. However, since I can not make any assumptions on the dimensions or channels of the images, I can not take this approach. I want Java to be able to receive the Mat object without knowing anything about it beforehand. I have nu clue how to do this correctly. I have two ideas:

1) I create a Java Mat object from the C++ side ((*env)->NewObject), use JNI to call getNativeObjAddr and work with the returned pointer. While this might work, it seems so backwards... Maybe there's a better way?

2) I create a C++ Mat, and return the pointer to Java. I'm hoping there's some kind of functionality that allows me to wrap it in a Java Mat object. The documentation mentions the Mat(long addr) constructor, but there's no further explanation, and I have no idea what it's for. Can it be used to create a Mat object from a pointer to a native Mat object?

TL;DR: How do I get a Mat created in C++ to Java ?

Thank you for reading!

You can create an empty Mat in Java part and then call create method from C++:

// Java
Mat m = new Mat();
jni_func(m.getNativeObjAddr());

// C++
void jni_func(jlong matPtr)
{
    Mat* mat = (Mat*) matPtr;
    mat->create(rows, cols, type);
    memcpy(mat->data, data, mat->step * mat->rows);
}

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Comments

Hah, I feel stupid now... I like this approach, as it doesn't require the bridge to be crossed once we're in the function. Thanks for that!

peanutman  (Apr 23 '13) edit

Returning Mat object from native code to java in OpenCV

http://stackoverflow.com/questions/9935618/returning-mat-object-from-native-code-to-java-in-opencv

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.

Answer:

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;

Rect没有getNativeObjAddr函数，不能类似。

尽量使用 jintArray.

eg: http://lveyo.iteye.com/blog/360622

jintArray faceArray = env-> NewIntArray(4*total);

jint faceBuf[4];  

env->SetIntArrayRegion(faceArray,0,4,faceBuf);  

return faceArray;