draw a border around detected object in c#

it is the continuation after a previous post.

in my past post i detected object based on color. and got below result. like in images below

but i want only one region around my detected object. for draw that i take all the points of rectangle start position and ending position. start position is upper-left corner and endng poistion is lower-right corner of the rectangle. Then using this position i draw a convex hull. so steps are 2

• collect all points
• Draw convex hull

so for collecting all the points i add next few line of code

 List pos = new List();
Rectangle[] rects = blobCounter.GetObjectRectangles();
  if (rects.Length > 3)
            {
                foreach (Rectangle objectRect in rects)
                {
                    Point p = objectRect.Location;
                    pos.Add(p); //add start position
                    p.X= objectRect.X + objectRect.Width;
                    p.Y = objectRect.Y + objectRect.Height;
                    pos.Add(p); //add ending position
                }

objectRect.Location return starting location and by sum the height and width we get ending location

Now draw the convexhull, from net a very nice function to draw convex hull, u can see that here . so for draw convexhull i had to write bellow code

  Graphics g = Graphics.FromImage(image3);
                 List hull = null;

                // Get the convex hull.
           hull = MakeConvexHull(pos);//pos is collection of rectangles position

                using (Pen pen = new Pen(Color.FromArgb(160, 255, 160), 5))
                {
                    Point[] hull_points = new Point[ hull.Count];
                    hull.CopyTo(hull_points);
                    g.DrawPolygon(pen, hull_points);
                }
                g.Dispose();

and we get below image
the function make convex hull u can find here in geometryclass. from  that class i only add below code in my class

  public static Point[] g_MinMaxCorners;
        public static Rectangle g_MinMaxBox;
        public static Point[] g_NonCulledPoints;
  public static List MakeConvexHull(List points)
        {
            // Cull.
            points = HullCull(points);

            // Find the remaining point with the smallest Y value.
            // if (there's a tie, take the one with the smaller X value.
            Point best_pt = points[0];
            foreach (Point pt in points)
            {
                if ((pt.Y < best_pt.Y) ||
                   ((pt.Y == best_pt.Y) && (pt.X < best_pt.X)))
                {
                    best_pt = pt;
                }
            }

            // Move this point to the convex hull.
            List hull = new List();
            hull.Add(best_pt);
            points.Remove(best_pt);

            // Start wrapping up the other points.
            float sweep_angle = 0;
            for (; ; )
            {
                // Find the point with smallest AngleValue
                // from the last point.
                int X = hull[hull.Count - 1].X;
                int Y = hull[hull.Count - 1].Y;
                best_pt = points[0];
                float best_angle = 3600;

                // Search the rest of the points.
                foreach (Point pt in points)
                {
                    float test_angle = AngleValue(X, Y, pt.X, pt.Y);
                    if ((test_angle >= sweep_angle) &&
                        (best_angle > test_angle))
                    {
                        best_angle = test_angle;
                        best_pt = pt;
                    }
                }

                // See if the first point is better.
                // If so, we are done.
                float first_angle = AngleValue(X, Y, hull[0].X, hull[0].Y);
                if ((first_angle >= sweep_angle) &&
                    (best_angle >= first_angle))
                {
                    // The first point is better. We're done.
                    break;
                }

                // Add the best point to the convex hull.
                hull.Add(best_pt);
                points.Remove(best_pt);

                sweep_angle = best_angle;

                // If all of the points are on the hull, we're done.
                if (points.Count == 0) break;
            }

            return hull;
        }
        private static float AngleValue(int x1, int y1, int x2, int y2)
        {
            float dx, dy, ax, ay, t;

            dx = x2 - x1;
            ax = Math.Abs(dx);
            dy = y2 - y1;
            ay = Math.Abs(dy);
            if (ax + ay == 0)
            {
                // if (the two points are the same, return 360.
                t = 360f / 9f;
            }
            else
            {
                t = dy / (ax + ay);
            }
            if (dx < 0)
            {
                t = 2 - t;
            }
            else if (dy < 0)
            {
                t = 4 + t;
            }
            return t * 90;
        }

    }

hope that much is useful.

for download code click here