not able to detect filled circles in OMR sheet with AForge Library

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C#

WPF

private void ProcessImage(Bitmap bitmap)
        {
            // lock image
            BitmapData bitmapData = bitmap.LockBits(new System.Drawing.Rectangle(0, 0, bitmap.Width, bitmap.Height),
            ImageLockMode.ReadWrite, bitmap.PixelFormat);

           
            BlobCounter blobCounter = new BlobCounter();

            blobCounter.FilterBlobs = true;
            blobCounter.MinHeight = 1;
            blobCounter.MinWidth = 1;

            blobCounter.ProcessImage(bitmapData);
            Blob[] blobs = blobCounter.GetObjectsInformation();
            bitmap.UnlockBits(bitmapData);
            
            SimpleShapeChecker shapeChecker = new SimpleShapeChecker();
           Bitmap newBitmap = new Bitmap(bitmap.Width, bitmap.Height);
            Graphics g = Graphics.FromImage(newBitmap);
            g.DrawImage(bitmap, 0, 0);
          
            System.Drawing.Pen redPen = new System.Drawing.Pen(System.Drawing.Color.Red, 1);
            System.Drawing.Pen yellowPen = new System.Drawing.Pen(System.Drawing.Color.Yellow, 1);
            System.Drawing.Pen greenPen = new System.Drawing.Pen(System.Drawing.Color.Green, 1);
            System.Drawing.Pen bluePen = new System.Drawing.Pen(System.Drawing.Color.Blue, 1);

            for (int i = 0, n = blobs.Length; i < n; i++)
            {
                List<IntPoint> edgePoints = blobCounter.GetBlobsEdgePoints(blobs[i]);
                AForge.Point center;
                float radius;

                if (shapeChecker.IsCircle(edgePoints, out center, out radius))
                {
                    g.DrawEllipse(redPen,
                    (float)(center.X - radius), (float)(center.Y - radius),
                    (float)(radius * 2), (float)(radius * 2));
                    X = center.X;
                    Y = center.Y;
                    
                    //MessageBox.Show("X = " +X,"Y= " +Y);
                    CreateDataTable();
                    MessageBox.Show("");
                    d++;
                }
                else
                
                {
                    List<IntPoint> corners;
                   
                    if (shapeChecker.IsQuadrilateral(edgePoints, out corners))
                    {
                        if (shapeChecker.CheckPolygonSubType(corners) == PolygonSubType.Rectangle)
                        {
                            g.DrawPolygon(bluePen, ToPointsArray(corners));
                          //CreateDataTable();
                        
                        }
                   }
                }
            }

            redPen.Dispose();
            greenPen.Dispose();
            bluePen.Dispose();
            yellowPen.Dispose();
            g.Dispose();
            System.Windows.Media.Imaging.BitmapSource b = System.Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap(bitmap.GetHbitmap(),
                   IntPtr.Zero,
                   System.Windows.Int32Rect.Empty,
                    BitmapSizeOptions.FromWidthAndHeight(bitmap.Width, bitmap.Height));
            imgupload.Source = b;
        }

I m using AForge Library ti detect circles and rectangles from OMR sheet.
It will detect only not filled circles. if i want to detect filled circles then it will not work perfectly..