HoughLinesP generates an empty image

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I'm developing an app wich uses OMR (Optical mark reconize) to read bubblesheets, I'm using the OpenCV API but I'm having some troubbles with HoughLinesP, when I use it that returns an empty image to me. I used the function of Android, Log.e, to see the total, cols and rows of the image before and after the HoughLinesP.

My code is a conversion of a C++ code to Java, both uses OpenCV.

Before I get the following result:

[ORIGINAL IMAGE]: 83840||262||320

After I get the following result:

[LINES IMAGE]: 0||0||1

My code:

private void scanImage(){
    Mat img = Imgcodecs.imread(mediaStorageDir().getPath() + "/" + "test2.jpg", Imgcodecs.CV_LOAD_IMAGE_GRAYSCALE);

    Log.e("[CANAIS]", String.valueOf(img.channels()));

    Size sz = new Size(3,3);
    Imgproc.GaussianBlur(img, img, sz, 0);
    Imgproc.adaptiveThreshold(img, img, 255, Imgproc.ADAPTIVE_THRESH_MEAN_C, Imgproc.THRESH_BINARY, 75, 10);
    Core.bitwise_not(img, img);

    Mat img2 = new Mat();
    Imgproc.cvtColor(img, img2, Imgproc.COLOR_GRAY2RGB);

    Mat img3 = new Mat();
    Imgproc.cvtColor(img, img3, Imgproc.COLOR_GRAY2RGB);

    MatOfInt4 lines = new MatOfInt4();

    Log.e("[ORIGINAL IMAGE]", "" + img.total() + "||" + img.rows() + "||" + img.cols());

    Imgproc.HoughLinesP(img, lines, 1, Math.PI/180,80,400,10);

    Log.e("[LINES IMAGE]", "" + lines.total() + "||" + lines.rows() + "||" + lines.cols());

    for(int i = 0; i < lines.total(); i++){
        Mat l = new Mat();
        l.put(i, 0, lines.get(i, 0));
        Point pr = new Point();
        Point ps = new Point();
        pr.x = Double.valueOf(l.get(0, 0).toString());
        pr.y = Double.valueOf(l.get(1, 0).toString());

        ps.x = Double.valueOf(l.get(2, 0).toString());
        ps.y = Double.valueOf(l.get(3,0).toString());

        Scalar scalar = new Scalar(0,0,255);

        Imgproc.line(img2, pr, ps, scalar, 3, Imgproc.LINE_AA, 0);
    }

    showImage(img2);
    MatOfInt4 mt4 = new MatOfInt4(lines);
    LinkedList<Point> corners = new LinkedList<>();
    for(int i = 0; i < lines.total(); i++){
        for(int x = i + 1; x <lines.total(); x++){
            MatOfInt4 gen = new MatOfInt4();
            MatOfInt4 gen1 = new MatOfInt4();
            gen1.put(x, 0, mt4.get(x, 0));
            gen.put(i, 0, mt4.get(i, 0));
            Point pt = computeIntersect(gen, gen1);
            if(pt.x >= 0 && pt.y >= 0 && pt.x < img.cols() && pt.y < img.rows()){
                corners.addLast(pt);
            }
        }
    }

    Point center = new Point(0,0);
    MatOfPoint mtp = new MatOfPoint(center);
    for(int i = 0; i < corners.size(); i++){
        center.x += corners.get(i).x;
        center.y += corners.get(i).y;
    }

    center.x *= 1./ corners.size();
    center.y *= 1./ corners.size();

    sortCorners(corners, center);

    Rect r = Imgproc.boundingRect(mtp);
    Log.e("[RECT]", r.toString());

    Mat quad = Mat.zeros(r.height, r.width, CvType.CV_8UC3);
    LinkedList<Point> quad_pts = new LinkedList<>();
    quad_pts.addLast(new Point(0,0));
    quad_pts.addLast(new Point(quad.cols(),0));
    quad_pts.addLast(new Point(quad.cols(),quad.rows()));
    quad_pts.addLast(new Point(0,quad.rows()));

    Mat transmtx = Imgproc.getPerspectiveTransform(Converters.vector_Point2f_to_Mat(corners), Converters.vector_Point2f_to_Mat(quad_pts));
    Imgproc.warpPerspective(img3, quad, transmtx, quad.size());

    showImage(quad);


}
private Point computeIntersect(MatOfInt4 a, MatOfInt4 b){
    Point generc = new Point();
    generc.x = -1;
    generc.y = -1 ...