Segmentation Edges - opencv

Question

i am working on project where i need to detect the contour of differents type of document.
Currently i am able to segment and detect contours using findcontours and everything works fine in most cases.
But, if the document is white color and the background similar to white color, i can't detect the contour.
For example, in this image
or this xhttp://i.stack.imgur.com/9exrg.jpg
i can't detect the white paper.
Here is the code i am using to segment the image in order to detect perfect edge (edges with no holes) / Perfectly straight.
public static Mat process(Mat original){
Mat src = original.clone();
Mat hsvMat = new Mat();
Mat gray = new Mat();
Mat sobx = new Mat();
Mat soby = new Mat();
Mat grad_abs_val_approx = new Mat();
Imgproc.cvtColor(src, hsvMat, Imgproc.COLOR_BGR2HSV);
List<Mat> hsv_channels = new ArrayList<Mat>(3);
Core.split(hsvMat, hsv_channels);
Mat hue = hsv_channels.get( 0 );
Mat sat = hsv_channels.get( 1 );
Mat val = hsv_channels.get( 2 );
Imgproc.GaussianBlur(val, gray, new Size(9, 9), 2, 2);
Mat imf = new Mat();
gray.convertTo(imf, CV_32FC1, 0.5f, 0.5f);
Imgproc.Sobel(imf, sobx, -1, 1, 0);
Imgproc.Sobel(imf, soby, -1, 0, 1);
sobx = sobx.mul(sobx);
soby = soby.mul(soby);
Mat sumxy = new Mat();
Core.add(sobx,soby, sumxy);
Core.pow(sumxy, 0.5, grad_abs_val_approx);
sobx.release();
soby.release();
sumxy.release();;
Mat filtered = new Mat();
Imgproc.GaussianBlur(grad_abs_val_approx, filtered, new Size(9, 9), 2, 2);
final MatOfDouble mean = new MatOfDouble();
final MatOfDouble stdev = new MatOfDouble();
Core.meanStdDev(filtered, mean, stdev);
Mat thresholded = new Mat();
Imgproc.threshold(filtered, thresholded, mean.toArray()[0] + stdev.toArray()[0], 1.0, Imgproc.THRESH_TOZERO);
Mat converted = new Mat();
thresholded.convertTo(converted, CV_8UC1);
return converted;
}
Using the code above leads to the following result :
As you can notice, the edges is not Perfectly straight (and there is holes). Edges are barely visible and Findcontours fails to detect the contours.
I have tried alls solutions/suggestions described here
therefore, here is my questions :
1) what's wrong with my code ?
2) how can i preprocess the image in order to detect perfect edge (edges with no holes) / Perfectly straight for contour detection ?
Many thanks in advance for your assistance.

Answer 1

When you see that the edges are not easily detected, you can try other complementary approaches.
For example, on this image, the simple matlab code (which you can implement using OpenCV):
I=imread('page.png');
r=I(:,:,1);
g=I(:,:,2);
b=I(:,:,3);
imshow(b>g);
Produces the following result, which you can use using your edge detection code: