OpenCV sharpen the edges (edges with no holes)

Question

I am trying to detect the biggest/larger rectangular shape and draw bounding box to the detected area. In my use case, very often (and not always) the object that represent

Answer 1

Thanks for yours comments and suggestion. The code provided by @NEJC works perfectly and cover 80% of my use case.

Nevertheless, it does not works with similar case like this case not solved by the current code and i don't know why.

Perhaps someone have an idea/clue/solution ?

I continue to improve the code and try to find a more generic solution that can cover more case. I will post it if i ever i find.

In any case, below is the working code based on @NEJC solution and notes.

public static Mat process(Mat original){
    Mat src = original.clone();
    Mat hsvMat = new Mat();
    Mat saturation = 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 hsv_channels = new ArrayList(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(sat, saturation, new Size(9, 9), 2, 2);
    Mat imf = new Mat();
    saturation.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 thresholded_bin = new Mat();
    Imgproc.threshold(filtered, thresholded_bin, mean.toArray()[0] + stdev.toArray()[0], 1.0, Imgproc.THRESH_BINARY_INV);
    Mat converted = new Mat();
    thresholded_bin.convertTo(converted, CV_8UC1);
    */

    Mat converted = new Mat();
    thresholded.convertTo(converted, CV_8UC1);
    return converted;
}