Detect the bounds of a Passport page with OpenCV

edit

I am trying to develop a scanner that can scan a page of a Passport with the camera.

So from a Passport page like this:

I'd like to crop out the marked part.

I have written code for edge detection using OpenCV which finds the contours and then approximates the largest quadrilateral. Finally it does a 4 point perspective transformation to get a top view of the image. The edge detection code look like this:

public static List<MatOfPoint> findContours(Mat src){
Mat img = src.clone();
src.release();
//find contours
double ratio = getScaleRatio(img.size());
int width = (int) (img.size().width / ratio);
int height = (int) (img.size().height / ratio);
Size newSize = new Size(width, height);
Mat resizedImg = new Mat(newSize, CvType.CV_8UC4);
Imgproc.resize(img, resizedImg, newSize);

Imgproc.medianBlur(resizedImg, resizedImg, 5);

Mat cannedImg = new Mat(newSize, CvType.CV_8UC1);
Imgproc.Canny(resizedImg, cannedImg, 70, 200, 3, true);
resizedImg.release();

Imgproc.threshold(cannedImg, cannedImg, 200, 255, Imgproc.THRESH_OTSU);

Mat dilatedImg = new Mat(newSize, CvType.CV_8UC1);
Mat morph = Imgproc.getStructuringElement(Imgproc.MORPH_RECT, new Size(3, 3));
Imgproc.dilate(cannedImg, dilatedImg, morph, new Point(-1, -1), 2, 1, new Scalar(1));
cannedImg.release();
morph.release();

ArrayList<MatOfPoint> contours = new ArrayList<>();
Mat hierarchy = new Mat();
Imgproc.findContours(dilatedImg, contours, hierarchy, Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE);
hierarchy.release();

Log.d(TAG, "contours found: " + contours.size());

Collections.sort(contours, new Comparator<MatOfPoint>() {
    @Override
    public int compare(MatOfPoint o1, MatOfPoint o2) {
        return Double.valueOf(Imgproc.contourArea(o2)).compareTo(Imgproc.contourArea(o1));
    }
});

return contours;
}

this code works for single page documents i.e ID cards, credit cards. Where the input is like this:

But doesn't work for passports as the top edge is not as distinctive, for example:

The inputs will be taken from camera preview on Android. Any idea how can I detect the passport page? I am using OpenCV 3.1. Thanks in Advance!

EDIT: I am adding a few sample images.

1.

2.

3.

4.

EDIT 2: I have tried drawing the top border myself so that a quadrilateral could be formed, but in that case the perspective transformation doesn't work as expected. Here's the complete code:

img = data.input.clone();
                    data.input.release();
                    //find contours
                    ratio = img.size().height/FIXED_HEIGHT;
                    width = (int) (img.size().width / ratio);
                    height = (int) (img.size().height / ratio);
                    newSize = new Size(width, height);
                    resizedImg = new Mat(newSize, CvType.CV_8UC4);
                    Imgproc.resize(img, resizedImg, newSize);
                    onNextStep(resizedImg);

                    Imgproc.medianBlur(resizedImg, resizedImg, 5);
                    onNextStep(resizedImg);

                    //Imgproc.line(resizedImg, new Point(0, 0), new Point(resizedImg.cols()-1, 0), new Scalar(0, 0, 0), 1);

                    cannedImg = new Mat(newSize, CvType.CV_8UC1);
                    Imgproc.Canny(resizedImg, cannedImg, 70, 200, 3, true);
                    resizedImg.release();
                    onNextStep(cannedImg);

                    Imgproc.threshold(cannedImg, cannedImg, 70, 255, Imgproc.THRESH_OTSU);
                    onNextStep(cannedImg);

                    dilatedImg = new Mat(newSize, CvType.CV_8UC1);
                    morph = Imgproc.getStructuringElement(Imgproc.MORPH_RECT, new Size(3, 3));
                    Imgproc.dilate(cannedImg, dilatedImg, morph, new Point(-1, -1), 2, 1, new Scalar(1));
                    cannedImg.release();
                    morph.release();
                    onNextStep(dilatedImg);

                    contours = new ArrayList<>();
                    hierarchy = new Mat();
                    Imgproc.findContours(dilatedImg, contours, hierarchy, Imgproc ...