How to resolve the error mentioned below (Camera Cailbration | Calib3d.java)

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import java.io.File; import java.util.ArrayList; import org.opencv.calib3d.Calib3d; import org.opencv.core.Core; import org.opencv.core.CvType; import org.opencv.core.Mat; import org.opencv.core.MatOfPoint2f; import org.opencv.core.MatOfPoint3f; import org.opencv.core.Point3; import org.opencv.core.Size; import org.opencv.core.TermCriteria; import org.opencv.highgui.Highgui; import org.opencv.imgproc.Imgproc;

public class CalibChessBoard { int flagsCorner = Calib3d.CALIB_CB_ADAPTIVE_THRESH | Calib3d.CALIB_CB_FAST_CHECK | Calib3d.CALIB_CB_NORMALIZE_IMAGE; int flagsCalib = Calib3d.CALIB_ZERO_TANGENT_DIST | Calib3d.CALIB_FIX_PRINCIPAL_POINT | Calib3d.CALIB_FIX_K4 | Calib3d.CALIB_FIX_K5; TermCriteria criteria = new TermCriteria(TermCriteria.EPS + TermCriteria.MAX_ITER, 40, 0.001); Size winSize = new Size(5, 5), zoneSize = new Size(-1, -1); Size patternSize; ArrayList objectPoints, imagePoints = new ArrayList(); ArrayList vCorners; ArrayList vImg; Mat cameraMatrix = Mat.eye(3, 3, CvType.CV_64F); Mat distCoeffs = Mat.zeros(8, 1, CvType.CV_64F); ArrayList rvecs = new ArrayList(); ArrayList tvecs = new ArrayList();

CalibChessBoard() {
}

CalibChessBoard(Size patternSize) {
    this.patternSize = patternSize;
}

boolean getCorners(Mat gray, MatOfPoint2f corners) {
    if (!Calib3d.findChessboardCorners(gray, patternSize,
            corners, flagsCorner))
        return false;
    Imgproc.cornerSubPix(gray, corners, winSize, zoneSize,
                               criteria);
    return true;
}

MatOfPoint3f getCorner3f() {
    MatOfPoint3f corners3f = new MatOfPoint3f();
    double squareSize = 50;
    Point3[] vp = new Point3[(int) (patternSize.height * 
                                        patternSize.width)];
    int cnt = 0;
    for (int i = 0; i < patternSize.height; ++i)
        for (int j = 0; j < patternSize.width; ++j, cnt++)
            vp[cnt] = new Point3(j * squareSize, 
                                 i * squareSize, 0.0d);
    corners3f.fromArray(vp);
    return corners3f;
}

public static void main(String[] args) {
    test0();
}

static void test0() {
    CalibChessBoard cb = new CalibChessBoard(new Size(8, 6));
    cb.getAllCornors("/the/photo/folder");
    cb.calibrate();
}

void calibrate() {
    double errReproj = Calib3d.calibrateCamera(objectPoints, 
            imagePoints,vImg.get(0).size(), cameraMatrix, 
            distCoeffs, rvecs, tvecs,flagsCalib);
    System.out.println("done, \nerrReproj = " + errReproj);
    System.out.println("cameraMatrix = \n" + cameraMatrix.dump());
    System.out.println("distCoeffs = \n" + distCoeffs.dump());
}

void getAllCornors(String path) {
    vImg = new ArrayList();
    objectPoints = new ArrayList();
    imagePoints = new ArrayList();
    MatOfPoint3f corners3f = getCorner3f();
    for (File f : new File(path).listFiles()) {
        Mat mat = Highgui.imread(f.getPath(), 
                       Highgui.CV_LOAD_IMAGE_COLOR);
        if (mat == null || mat.channels() != 3)
            continue;
        System.out.println("fn = " + f.getPath());
        System.out.println("mat.channels() = " + mat.channels() 
                + ", " + mat.cols() + ", " + mat.rows());
        Mat gray = new Mat();
        Imgproc.cvtColor(mat, gray, Imgproc.COLOR_BGR2GRAY);
        MatOfPoint2f corners = new MatOfPoint2f();
        if (!getCorners(gray, corners))
            continue;
        objectPoints.add(corners3f);
        imagePoints.add(corners);
        vImg.add(mat);
    }
}

static {
    System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
}

}

reference:"https://computervisionandj..."

In this code there is a issue in calibrate method. cannot resolve size in vImg.get(0).size(). How to resolve this bug in this code.