C# Text-extracting with OpenCV finds only one line in an image

Question

As result, only just the last line was boxed as seen above.

The code starts from this post below.
Extracting text OpenCV
First, I start it from Diomedes Domínguez's C# code. I modified only one line because original code throws here.

var mask = new Mat(Mat.Zeros(bw.Size(), MatType.CV_8UC1));

System.ArgumentException: 'empty ranges
parameter: ranges'

So I added range.

var mask = new Mat(Mat.Zeros(bw.Size(), MatType.CV_8UC1), new Rect(0, 0, bw.Size().Width, bw.Size().Height));

So here is code what I've tried.

Mat large = new Mat(@"D:\cap_price.PNG");
            Mat rgb = new Mat(), small = new Mat(), grad = new Mat(), bw = new Mat(), connected = new Mat();

            // downsample and use it for processing
            Cv2.PyrDown(large, rgb);
            Cv2.CvtColor(rgb, small, ColorConversionCodes.BGR2GRAY);

            // morphological gradient
            var morphKernel = Cv2.GetStructuringElement(MorphShapes.Ellipse, new OpenCvSharp.Size(3, 3));
            Cv2.MorphologyEx(small, grad, MorphTypes.Gradient, morphKernel);
            // binarize
            Cv2.Threshold(grad, bw, 0, 255, ThresholdTypes.Binary | ThresholdTypes.Otsu);

            // connect horizontally oriented regions
            morphKernel = Cv2.GetStructuringElement(MorphShapes.Rect, new OpenCvSharp.Size(9, 1));
            Cv2.MorphologyEx(bw, connected, MorphTypes.Close, morphKernel);
            // find contours
            var mask = new Mat(Mat.Zeros(bw.Size(), MatType.CV_8UC1));
            Cv2.FindContours(connected, out OpenCvSharp.Point[][] contours, out HierarchyIndex[] hierarchy, RetrievalModes.CComp, ContourApproximationModes.ApproxSimple, new OpenCvSharp.Point(0, 0));
            // filter contours
            var idx = 0;
            foreach (var hierarchyItem in hierarchy)
            {
                OpenCvSharp.Rect rect = Cv2.BoundingRect(contours[idx]);
                var maskROI = new Mat(mask, rect);
                maskROI.SetTo(new Scalar(0, 0, 0));
                // fill the contour
                Cv2.DrawContours(mask, contours, idx, Scalar.White, -1);

                // ratio of non-zero pixels in the filled region
                double r = (double)Cv2.CountNonZero(maskROI) / (rect.Width * rect.Height);
                if (r > .45 /* assume at least 45% of the area is filled if it contains text */
                     &&
                (rect.Height > 8 && rect.Width > 8) /* constraints on region size */
                                                    /* these two conditions alone are not very robust. better to use something 
                                                    like the number of significant peaks in a horizontal projection as a third condition */
                )
                {
                    Cv2.Rectangle(rgb, rect, new Scalar(0, 255, 0), 1);
                }
            }

            //rgb.SaveImage(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "rgb.jpg"));
            rgb.SaveImage(@"D:\rgb.jpg");

I want each text line to be boxed.