Android Color background document not detects Document Edges

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I am working Auto Edge detection On CameraFrame,It Does not Detect Document Edges.Please help Me out. My Code

    try {
        return findLargestRectangle(inputFrame.rgba());
    } catch (Exception e) {
        e.printStackTrace();
    }
    return inputFrame.gray(); 



private Mat findLargestRectangle(Mat original_image) {


    Mat imgSource = original_image;
    //Mat untouched = original_image.clone();
  //Clone the Image
    Mat CloneImage =original_image.clone();

    //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 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;
    MatOfPoint temp_contour = contours.get(0); //the largest is at the index 0 for starting point
    MatOfPoint2f approxCurve = new MatOfPoint2f();
    MatOfPoint2f maxCurve = new MatOfPoint2f();
    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();
            Imgproc.approxPolyDP(new_mat, approxCurve, contourSize*0.05, true);
            if (approxCurve.total() == 4) {
                maxCurve = approxCurve;
                maxArea = contourarea;
                maxAreaIdx = idx;
                largest_contours.add(temp_contour);
            }
        }
    }
         Scalar GREEN = new Scalar(0,255,0);

        Imgproc.line(CloneImage, new Point(maxCurve.get(3, 0)), new Point(maxCurve.get(2, 0)), GREEN, 4);
        Imgproc.line(CloneImage, new Point(maxCurve.get(2, 0)), new Point(maxCurve.get(1, 0)), GREEN, 4);
        Imgproc.line(CloneImage, new Point(maxCurve.get(1, 0)), new Point(maxCurve.get(0, 0)), GREEN, 4);
        Imgproc.line(CloneImage, new Point(maxCurve.get(0, 0)), new Point(maxCurve.get(3, 0)), GREEN, 4);
        double temp_double[] = maxCurve.get(0, 0);
        Point p1 = new Point(temp_double[0], temp_double[1]);
        Imgproc.circle(CloneImage, new Point(p1.x, p1.y), 20, new Scalar(255, 0, 0), 5); //p1 is colored red
        String temp_string = "Point 1: (" + p1.x + ", " + p1.y + ")";

        temp_double = maxCurve.get(1, 0);
        Point p2 = new Point(temp_double[0], temp_double[1]);
        Imgproc.circle(CloneImage, new Point(p2.x, p2.y), 20, new Scalar(0, 255, 0), 5); //p2 is colored green
        temp_string += "\nPoint 2: (" + p2.x + ", " + p2.y + ")";

        temp_double = maxCurve.get(2, 0);
        Point p3 = new Point(temp_double[0], temp_double[1]);
        Imgproc.circle(CloneImage, new Point(p3.x, p3.y), 20, new Scalar(0, 0, 255), 5); //p3 is colored blue
        temp_string += "\nPoint 3: (" + p3.x + ", " + p3.y + ")";

        temp_double = maxCurve.get(3, 0);
        Point p4 = new Point(temp_double[0], temp_double[1]);

  //  if(Imgproc.contourArea(contours.get(contourIdx))>100) {
        Imgproc.circle(CloneImage, new Point(p4.x, p4.y), 20, new Scalar(0, 255, 255), 5); //p4 is colored violet
   // }
        temp_string += "\nPoint 4: (" + p4.x + ", " + p4.y + ")";
/*    if(isCapture) {
        Rect roi = new Rect(40, 100, 100, 120);
        Mat cropped = new Mat(CloneImage, roi);



        Bitmap bmp = Bitmap.createBitmap(200, 400, Bitmap.Config.ARGB_8888);
        Mat tmp = new Mat (200,200,CvType.CV_8UC1,new Scalar(4));
        try {
            //Imgproc ...