Detect all Countour not an only Rectangle

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I have snippet of code which can help you to find the largest contour area of rectangle.

protected Bitmap findEdges(Bitmap rotatedBitmap) {

 Mat rgba = new Mat();
 Utils.bitmapToMat(rotatedBitmap, rgba);

 Mat edges = new Mat(rgba.size(), CV_8UC1);
 Imgproc.cvtColor(rgba, edges, Imgproc.COLOR_RGB2GRAY, 4);
 Canny(edges, edges, 80, 100);

 Bitmap resultBitmap = Bitmap.createBitmap(edges.cols(), edges.rows(), Bitmap.Config.ARGB_8888);
 Utils.matToBitmap(edges, resultBitmap);

 // https://github.com/daisygao/ScannerLites
 Mat rgbMat = new Mat();
 Mat grayMat = new Mat();
 Mat cannyMat;
 Mat linesMat = new Mat();
 BitmapFactory.Options o = new BitmapFactory.Options();


 // define the destination image size: A4 - 200 PPI
 int w_a4 = 1654, h_a4 = 2339;

 // TODO: 29/08/2016  May need to check sample size https://developer.android.com/training/displaying-bitmaps/load-bitmap.html
 o.inSampleSize = 4;
 o.inDither = false;


 ByteArrayOutputStream baos = new ByteArrayOutputStream();
 rotatedBitmap.compress(Bitmap.CompressFormat.PNG, 100, baos);
 byte[] b = baos.toByteArray();

 try {
     // resize
     rotatedBitmap = BitmapFactory.decodeByteArray(b, 0, b.length);
 } catch (OutOfMemoryError error) {
     error.printStackTrace();
 }


 int w = rotatedBitmap.getWidth();

 int h = rotatedBitmap.getHeight();
 int min_w = 800;
 double scale = Math.min(10.0, w * 1.0 / min_w);
 int w_proc = (int)(w * 1.0 / scale);
 int h_proc = (int)(h * 1.0 / scale);
 Bitmap srcBitmap = Bitmap.createScaledBitmap(rotatedBitmap, w_proc, h_proc, false);

 Utils.bitmapToMat(srcBitmap, rgbMat); //convert original bitmap to Mat, R G B.

 // grayscale
 Imgproc.cvtColor(rgbMat, grayMat, Imgproc.COLOR_RGB2GRAY); //rgbMat to gray grayMat

 // canny
 cannyMat = getCanny(grayMat);

 // HoughLinesP

 Imgproc.HoughLinesP(cannyMat, linesMat, 1, Math.PI / 280, w_proc / 12, w_proc / 12, 20);

 // Calculate horizontal lines and vertical lines
 Log.e("opencv", "lines.cols " + linesMat.cols() + " w_proc/3: " + w_proc / 3);
 List < Line > horizontals = new ArrayList < > ();
 List < Line > verticals = new ArrayList < > ();
 for (int x = 0; x < linesMat.rows(); x++) {
     double[] vec = linesMat.get(x, 0);
     double x1 = vec[0],
         y1 = vec[1],
         x2 = vec[2],
         y2 = vec[3];
     Point start = new Point(x1, y1);
     Point end = new Point(x2, y2);
     Line line = new Line(start, end);
     if (Math.abs(x1 - x2) > Math.abs(y1 - y2)) {
         horizontals.add(line);
     } else {
         verticals.add(line);
     }

     // for visualization in debug mode
     if (BuildConfig.DEBUG) {}
 }

 Log.e("HoughLines", "completed HoughLines");
 Log.e("HoughLines", "linesMat size: " + linesMat.size());
 Log.e("Lines Detected", Integer.toString(linesMat.rows()));


 // if we don't have at least 2 horizontal lines or vertical lines
 if (horizontals.size() < 2) {
     if (horizontals.size() == 0 || horizontals.get(0)._center.y > h_proc / 2) {
         horizontals.add(new Line(new Point(0, 0), new Point(w_proc - 1, 0)));
     }
     if (horizontals.size() == 0 || horizontals.get(0)._center.y <= h_proc / 2) {
         horizontals.add(new Line(new Point(0, h_proc - 1), new Point(w_proc - 1, h_proc - 1)));
     }
 }
 if (verticals.size() < 2) {
     if (verticals.size() == 0 || verticals.get(0)._center.x > w_proc / 2) {
         verticals.add(new Line(new Point(0, 0), new Point(h_proc - 1, 0)));
     }
     if (verticals.size() == 0 || verticals.get(0)._center.x <= w_proc / 2) {
         verticals.add(new Line(new Point(w_proc - 1, 0), new Point(w_proc - 1, h_proc - 1)));
     }
 }

 Collections.sort(horizontals ...