System.loadLibrary(Core.NATIVE_LIBRARY_NAME);

// Load the image
String path = "/home/bikz05/Desktop/geubs.png";
Mat original = Highgui.imread(path);
Mat image = new Mat();
Imgproc.cvtColor(original, image, Imgproc.COLOR_BGR2GRAY);

// Threshold the image
Mat threshold = new Mat();
Imgproc.threshold(image, threshold, 127, 255, 1);

// Find the contours
List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
Imgproc.findContours(threshold, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);

// Get contour index with largest area
double max_area = -1;
int index = 0;
for(int i=0; i< contours.size();i++) {
    if (Imgproc.contourArea(contours.get(i)) > max_area) {
        max_area = Imgproc.contourArea(contours.get(i));
        index = i;
    }
}

// Approximate the largest contour
MatOfPoint2f approxCurve = new MatOfPoint2f();
MatOfPoint2f oriCurve = new MatOfPoint2f( contours.get(index).toArray() );
Imgproc.approxPolyDP(oriCurve, approxCurve, 6.0, true);

// Draw contour points on the original image
Point [] array = approxCurve.toArray();
for(int i=0; i < array.length;i++) {
    Core.circle(original, array[i], 2, new Scalar(0, 0 ,255), 2);
}