How to detect lines using HoughLines in OpenCV (Java)?

Question

I\'m currently trying to detect whether a level crossing barrier has been deployed on an image using HoughLines in OpenCV. I thought my code would

Answer 1

The lines are returned in the lines matrix, which has two columns in which each line returns the rho and theta parameters of the polar coordinates. These coordinates refer to the distance between the top-left corner of the image and the line rotation in radians, respectively. So, in order to use the show method you must create a Mat that will represent the lines using the line coordinates.

  public Mat getHoughTransform(Mat image, double rho, double theta, int threshold) {
    Mat result = Image.getInstance().getImage().clone();
    Mat lines = new Mat();
    Imgproc.HoughLines(image, lines, rho, theta, threshold);

    for (int i = 0; i < lines.cols(); i++) {
        double data[] = lines.get(0, i);
        double rho1 = data[0];
        double theta1 = data[1];
        double cosTheta = Math.cos(theta1);
        double sinTheta = Math.sin(theta1);
        double x0 = cosTheta * rho1;
        double y0 = sinTheta * rho1;
        Point pt1 = new Point(x0 + 10000 * (-sinTheta), y0 + 10000 * cosTheta);
        Point pt2 = new Point(x0 - 10000 * (-sinTheta), y0 - 10000 * cosTheta);
        Imgproc.line(result, pt1, pt2, new Scalar(0, 0, 255), 2);
    }
    return result;
}

And this is a easier way to do it. It involve the Imgproc.HoughLinesP method.

public Mat getHoughPTransform(Mat image, double rho, double theta, int threshold) {
    Mat result = Image.getInstance().getImage().clone();
    Mat lines = new Mat();
    Imgproc.HoughLinesP(image, lines, rho, theta, threshold);

    for (int i = 0; i < lines.cols(); i++) {
        double[] val = lines.get(0, i);
        Imgproc.line(result, new Point(val[0], val[1]), new Point(val[2], val[3]), new Scalar(0, 0, 255), 2);
    }
    return result;
}