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//convert the image to black and white 
Imgproc.cvtColor(imgSource, imgSource, Imgproc.COLOR_BGR2GRAY);

//convert the image to black and white does (8 bit)
Imgproc.Canny(imgSource, imgSource, 50, 50);

//apply gaussian blur to smoothen lines of dots
Imgproc.GaussianBlur(imgSource, imgSource, new  org.opencv.core.Size(5, 5), 5);

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

double maxArea = -1;
int maxAreaIdx = -1;
Log.d("size",Integer.toString(contours.size()));
MatOfPoint temp_contour = contours.get(0); //the largest is at the index 0 for starting point
MatOfPoint2f approxCurve = new MatOfPoint2f();
MatOfPoint largest_contour = contours.get(0);

List<MatOfPoint> largest_contours = new ArrayList<MatOfPoint>();

for (int idx = 0; idx < contours.size(); idx++) {
    temp_contour = contours.get(idx);
    double contourarea = Imgproc.contourArea(temp_contour);
    //compare this contour to the previous largest contour found
    if (contourarea > maxArea) {
        //check if this contour is a square
        MatOfPoint2f new_mat = new MatOfPoint2f( temp_contour.toArray() );
        int contourSize = (int)temp_contour.total();
        MatOfPoint2f approxCurve_temp = new MatOfPoint2f();
        Imgproc.approxPolyDP(new_mat, approxCurve_temp, contourSize*0.05, true);
        if (approxCurve_temp.total() == 4) {
            maxArea = contourarea;
            maxAreaIdx = idx;
            approxCurve=approxCurve_temp;
            largest_contour = temp_contour;
        }
    }
}

Imgproc.cvtColor(imgSource, imgSource, Imgproc.COLOR_BayerBG2RGB);
sourceImage =Highgui.imread(Environment.getExternalStorageDirectory().
         getAbsolutePath() +"/scan/p/1.jpg");
double[] temp_double;
temp_double = approxCurve.get(0,0);       
Point p1 = new Point(temp_double[0], temp_double[1]);
temp_double = approxCurve.get(1,0);       
Point p2 = new Point(temp_double[0], temp_double[1]);
temp_double = approxCurve.get(2,0);       
Point p3 = new Point(temp_double[0], temp_double[1]);
temp_double = approxCurve.get(3,0);       
Point p4 = new Point(temp_double[0], temp_double[1]);