Why am I getting empty stitched image?

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Hi,

I have two webcams connected to my android tablet - I am capturing the frames and stitching them and display. My java code -

    Log.i(TAG, "Processing .......");

    Mat left = null, right = null;

    //HEIGHT = 640 and WIDTH=480
    left = new Mat(HEIGHT + HEIGHT / 2, WIDTH, CvType.CV_8UC1);
    right = new Mat(HEIGHT + HEIGHT / 2, WIDTH, CvType.CV_8UC1);

    //list of images to be stitiched
    ArrayList<Mat> mats = new ArrayList<>(2);
    mats.add(left);
    mats.add(right);

    // for testing purpose only - if captured frames are proper. 
    // I am able to capture the frames and save them as PNG. It displays fine.
    Bitmap bmpL = Bitmap.createBitmap(left.cols(), left.rows(), Bitmap.Config.ARGB_8888);
    Bitmap bmpR = Bitmap.createBitmap(right.cols(), right.rows(), Bitmap.Config.ARGB_8888);

    while (true) {
       //ensuring frames from both cams
        try {
            synchronized (obL) {
                obL.wait();
            }
            synchronized (obR) {
                obR.wait();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }


        if (frameL == null || frameR == null) continue;

        isProcessing = true;

        //adding frame data to Mats
        left.put(0, 0, frameL);
        Log.d(TAG, "Left: " + left.toString());

        right.put(0, 0, frameR);
        Log.d(TAG, "Right: " + right.toString());

        /* Test Code. Frames are fine and can save them as PNG and see the image
        Utils.matToBitmap(left, bmpL);
        Utils.matToBitmap(right, bmpR);


        saveImage(bmpL, "L");
        saveImage(bmpR,"R");

        bmpL.recycle();
        bmpR.recycle();
        */

        // Below line always produces 0x0 mat  
        // sample info: Mat [ 0*0*CV_8UC1, isCont=false, isSubmat=false, nativeObj=0xffffffffb4b2e7b0, dataAddr=0x0 ]
        Mat result = NativeStitcherWrapper.stitch(mats, false, false);

        isProcessing = false;
    }
}

I am using OpenCV Stitcher.cpp for stitching and here is relevant C++ code being called via JNI

jclass matClass = env->FindClass("org/opencv/core/Mat");
jmethodID getNativeAddr = env->GetMethodID(matClass, "getNativeObjAddr", "()J");

int numImgs = env->GetArrayLength(jInputArray);

vector<Mat> natImgs;
for (int i = 0; i < numImgs; ++i) {
    natImgs.push_back(
            *(Mat *) env->CallLongMethod(
                    env->GetObjectArrayElement(jInputArray, i),
                    getNativeAddr
            )
    );
}

/* The core stitching calls: */
Stitcher stitcher = Stitcher::createDefault();

stitcher.setWaveCorrection((bool) waveCorrect);

Ptr<detail::Blender> blender;

if ((bool) multiBand) {
    blender = new detail::MultiBandBlender();
} else {
    blender = new detail::FeatherBlender();
}

stitcher.setBlender(blender);

Mat result;
Stitcher::Status status = stitcher.stitch(natImgs, result);

stringstream ss;

if (status != Stitcher::OK) {
    __android_log_print(ANDROID_LOG_VERBOSE, APPNAME, "Could not stitch", 1);
}

ss << "status: ";
ss << status;

string str;
str.append("cols: ");

ss << " cols: ";
ss << result.cols;
ss << " rows: ";
ss << result.rows;
string str1 = ss.str();
const char *log = str1.c_str();

__android_log_print(ANDROID_LOG_VERBOSE, APPNAME, log, 1);

// cout << "cols: "<<result.cols&lt;&lt; "="" rows:="" "&lt;<result.rows&lt;&lt;="" endl;<="" p="">

*(Mat *) jResultMat = result;

Appreciate any pointer/suggestions.