/// <summary> /// Detect ellipses in image /// </summary> /// <param name="image">Binary image with white foreground</param> /// <returns>List of ellipses found</returns> public DetectedEllipse[] DetectEllipses(Emgu.CV.Image <Emgu.CV.Structure.Gray, byte> image) { List <DetectedEllipse> ellipses = new List <DetectedEllipse>(); Emgu.CV.Contour <System.Drawing.Point> c = image.FindContours(); while (c != null) { if (c.Count() >= _min_contour_count) { Emgu.CV.Structure.Ellipse e = FitByContour(c); double rating = GoodnessOfFit(e, c); ellipses.Add(new DetectedEllipse(c, e, rating)); } c = c.HNext; } return(ellipses.ToArray()); }
/// <summary> /// Evaluate goodness of fit. /// </summary> /// <remarks> /// Calculates the average distance of all contour points to the ellipse. The distance of point to /// an ellipse is given by calculating the point on the ellipse that is closest to the query. This calculation /// is performed by transforming the point into a coordinate system where the ellipse is in main-pose /// (x-axis points toward a, y-axis points toward b, the origin is the center of the ellipse). Additionally, /// the coordinate system is crafted in such a way (non-uniform scaling) that the ellipse becomes a circle. /// Then, the closest point is simply the closest point on the circle. This point is transformed back into /// the world coordinate system where the distance between the query and the returned point is computed. /// </remarks> /// <param name="e">Ellipse</param> /// <param name="c">Contour points</param> /// <returns>Goodness of fit</returns> private double GoodnessOfFit(Emgu.CV.Structure.Ellipse e, Emgu.CV.Contour <System.Drawing.Point> c) { Matrix m = EllipseDetector.GetAffineFrame(e); Matrix inv = m.Inverse(); List <double> distances = new List <double>(); double avg_distance = 0.0; foreach (System.Drawing.Point p in c) { // Bring the point into the ellipse coordinate frame Vector x = p.ToParsley().ToHomogeneous(1.0); Vector r = (inv.Multiply(x.ToColumnMatrix())).GetColumnVector(0); // Find the closest point on the circle to r // From the above scaling construction the resulting circle has radius b Vector closest = r.ToNonHomogeneous().Normalize().Scale(e.MCvBox2D.size.Height); // Transform the closest point back Vector closest_in_world = (m.Multiply(closest.ToHomogeneous(1.0).ToColumnMatrix())).GetColumnVector(0); // Calculate the squared distance between the query and the point. avg_distance += (closest_in_world - x).ToNonHomogeneous().SquaredNorm(); } return(Math.Sqrt(avg_distance / c.Count())); }