Shape (Triangle, Rectangle, Circle, Line) Detection in CSharp

//Load the image from file
Image<Bgr, Byte> img = new Image<Bgr, byte>(fileNameTextBox.Text).Resize(400, 400, true);

//Convert the image to grayscale and filter out the noise
Image<Gray, Byte> gray = img.Convert<Gray, Byte>().PyrDown().PyrUp();

Gray cannyThreshold = new Gray(180);
Gray cannyThresholdLinking = new Gray(120);
Gray circleAccumulatorThreshold = new Gray(120);

CircleF[] circles = gray.HoughCircles(
    cannyThreshold,
    circleAccumulatorThreshold,
    5.0, //Resolution of the accumulator used to detect centers of the circles
    10.0, //min distance 
    5, //min radius
    0 //max radius
    )[0]; //Get the circles from the first channel

Image<Gray, Byte> cannyEdges = gray.Canny(cannyThreshold, cannyThresholdLinking);
LineSegment2D[] lines = cannyEdges.HoughLinesBinary(
    1, //Distance resolution in pixel-related units
    Math.PI / 45.0, //Angle resolution measured in radians.
    20, //threshold
    30, //min Line width
    10 //gap between lines
    )[0]; //Get the lines from the first channel

#region Find triangles and rectangles
List<Triangle2DF> triangleList = new List<Triangle2DF>();
List<MCvBox2D> boxList = new List<MCvBox2D>();

using (MemStorage storage = new MemStorage()) //allocate storage for contour approximation
   for (Contour<Point> contours = cannyEdges.FindContours(); contours != null; contours = contours.HNext)
   {
      Contour<Point> currentContour = contours.ApproxPoly(contours.Perimeter * 0.05, storage);

      if (contours.Area > 250) //only consider contours with area greater than 250
      {
         if (currentContour.Total == 3) //The contour has 3 vertices, it is a triangle
         {
            Point[] pts = currentContour.ToArray();
            triangleList.Add(new Triangle2DF(
               pts[0],
               pts[1],
               pts[2]
               ));
         }
         else if (currentContour.Total == 4) //The contour has 4 vertices.
         {
            #region determine if all the angles in the contour are within the range of [80, 100] degree
            bool isRectangle = true;
            Point[] pts = currentContour.ToArray();
            LineSegment2D[] edges = PointCollection.PolyLine(pts, true);

            for (int i = 0; i < edges.Length; i++)
            {
               double angle = Math.Abs(
                  edges[(i + 1) % edges.Length].GetExteriorAngleDegree(edges[i]));
               if (angle < 80 || angle > 100)
               {
                  isRectangle = false;
                  break;
               }
            }
            #endregion

            if (isRectangle) boxList.Add(currentContour.GetMinAreaRect());
         }
      }
   }
#endregion

originalImageBox.Image = img;

#region draw triangles and rectangles
Image<Bgr, Byte> triangleRectangleImage = img.CopyBlank();
foreach (Triangle2DF triangle in triangleList)
   triangleRectangleImage.Draw(triangle, new Bgr(Color.DarkBlue), 2);
foreach (MCvBox2D box in boxList)
   triangleRectangleImage.Draw(box, new Bgr(Color.DarkOrange), 2);
triangleRectangleImageBox.Image = triangleRectangleImage;
#endregion

#region draw circles
Image<Bgr, Byte> circleImage = img.CopyBlank();
foreach (CircleF circle in circles)
   circleImage.Draw(circle, new Bgr(Color.Brown), 2);
circleImageBox.Image = circleImage;
#endregion

#region draw lines
Image<Bgr, Byte> lineImage = img.CopyBlank();
foreach (LineSegment2D line in lines)
   lineImage.Draw(line, new Bgr(Color.Green), 2);
lineImageBox.Image = lineImage;
#endregion