CODEWITHSUNDEEP
javaandroidopencvjava-native-interface
Sophia
Sophia

Reputation: 35

Return array of Mat object to Java

I am new to opencv with android. I want to get back an array of Mat objects from JNI. I have created an application that sends some Mat objects from java to JNI. I can return int,long and other types from JNI. How would i return Mat object or array of Mat objects? In this example it returns a long object 

 JNIEXPORT jlong JNICALL
 Java_org_opencv_samples_fd_DetectionBasedTracker_nativeCreateObject
       (JNIEnv *, jclass, jstring, jint);

Upvotes: 0

Views: 573

Answers (1)

Sergei Bubenshchikov
Sergei Bubenshchikov

Reputation: 5371

On java side you need to define native method like this:

public class DetectionBasedTracker {

    public static native Mat[] nativeCreateObject(String name, int count);
}

Your need to take information about Mat object from java and return new object as jobject and return array as jobjectArray:

JNIEXPORT jobjectArray JNICALL
Java_org_opencv_samples_fd_DetectionBasedTracker_nativeCreateObject
(JNIEnv *env, jclass cls, jstring str, jint count){

    // cls argument - is DetectionBasedTracker.class

    // take class info
    jclass matCls = env->FindClass("your/package/Mat");
    if (env->ExceptionOccurred())
        return NULL;

    // take constructor by signature
    const char* constructorSignature = "(Ljava/lang/String;)V";
    jmethodID constructor = env->GetMethodID(matCls, "<init>", constructorSignature);
    if (env->ExceptionOccurred())
        return NULL;

    // create java objects array
    jobjectArray matArray = env->NewObjectArray((jsize)count, matCls, NULL);
    for(jsize i = 0; i < count; i++){
        // create new object
        jobject mat = env->NewObject(matCls, constructor, /* constructor args */ str);
        // put object into array
        env->SetObjectArrayElement(matArray , i, mat);
    }

    return matArray;
}

To learn more about constructorSignature arguments look at Java VM Type Signatures tabel

Note: this is just a sample of creating Java objects via JNI. Using JNI only for creating instances of java objects - it's a bad practice.

Upvotes: 2

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