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I am developing an Android application that needs to do some image processing. I am calling c++ code through JNI from java for it. I am using OpenCV for image processing. First, I confess that I just started using OpenCv and learning image processing. What I am trying to do now is trying to return open CV mat data from the native C++ to Java.

I am not a C++ developer as well. I am following this link- http://answers.opencv.org/question/12090/returning-a-mat-from-native-jni-to-java/.

This is my native C++ code

#include <jni.h>
#include <string>

#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/features2d/features2d.hpp>

using namespace std;
using namespace cv;

extern "C"
JNIEXPORT jstring



JNICALL
Java_media_memento_memento_SphereCameraActivity_stitchPhotos(
 JNIEnv *env,
 jobject /* this */, jlong addrMat, jlong addrNewMat) 




 Mat& mat = *(Mat*)addrMat;
 Mat& newMat = *(Mat*)addrNewMat;

 newMat.create(mat.rows, mat.rows, mat.type());
 memcpy(newMat.data, mat.data , mat.step * mat.rows);

 std::string hello = "Stitching the photo in C++";
 return env->NewStringUTF(hello.c_str());

As you can see, this line

memcpy(newMat.data, mat.data , mat.step * mat.rows);

will pass the new value to the mat in the java. This is my Java code.

 Mat mat;

 bitmap.getHeight();
 bitmap.getWidth();
 mat = new Mat(bitmap.getWidth(), bitmap.getHeight(), CvType.CV_8UC3);
 mat.put(0, 0, jpeg);
 Mat newMat = new Mat();
 stitchPhotos(mat.getNativeObjAddr(), newMat.getNativeObjAddr());

According to my code, when I call native function from Java, newMat variable in java should get the other mat variable value. Right? But when I run that code, my application crashes.
It is giving me this error

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
 Process: media.memento.memento, PID: 4823
 java.lang.UnsupportedOperationException: Provided data element number (39397) should be multiple of the Mat channels count (3)
 at org.opencv.core.Mat.put(Mat.java:1011)
 at media.memento.memento.SphereCameraActivity$MyCameraListener$1.onBitmapReady(SphereCameraActivity.java:276)
 at com.otaliastudios.cameraview.CameraUtils$1$1.run(CameraUtils.java:95)
 at android.os.Handler.handleCallback(Handler.java:751)
 at android.os.Handler.dispatchMessage(Handler.java:95)
 at android.os.Looper.loop(Looper.java:154)
 at android.app.ActivityThread.main(ActivityThread.java:6119)
 at java.lang.reflect.Method.invoke(Native Method)
 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

What is wrong with my code and how can I fix it?

The Java call stack is pointing to this line before the stitchPhotos() call even happens:

mat.put(0, 0, jpeg);

Based on the error message, jpeg has the wrong dimensions. It appears that put() expects to receive an array that is a multiple of 3 in size (since mat is declared as type CV_8UC3, which is 8 bit, unsigned, with 3 channels per pixel).

P.S. An easier way to copy one Mat to another in C++ is copyTo, or the assignment operator. Examples:

Mat& mat = *(Mat*)addrMat;
Mat& newMat = *(Mat*)addrNewMat;
mat.copyTo(newMat);
newMat = mat;