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+## Copyright (C) 2004-2011 David Legland <david.legland@grignon.inra.fr>
+## Copyright (C) 2004-2011 INRA - CEPIA Nantes - MIAJ (Jouy-en-Josas)
+## Copyright (C) 2012 Adapted to Octave by Juan Pablo Carbajal <carbajal@ifi.uzh.ch>
+## All rights reserved.
+## 
+## Redistribution and use in source and binary forms, with or without
+## modification, are permitted provided that the following conditions are met:
+## 
+##     1 Redistributions of source code must retain the above copyright notice,
+##       this list of conditions and the following disclaimer.
+##     2 Redistributions in binary form must reproduce the above copyright
+##       notice, this list of conditions and the following disclaimer in the
+##       documentation and/or other materials provided with the distribution.
+## 
+## THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ''AS IS''
+## AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+## IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+## ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
+## ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+## DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+## SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+## CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+## OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+## OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{K} = } ellipse2cov (@var{elli})
+## @deftypefnx {Function File} {@var{K} = } ellipse2cov (@var{ra}, @var{rb})
+## @deftypefnx {Function File} {@var{K} = } ellipse2cov (@dots{}, @var{theta})
+## Calculates covariance matrix from ellipse.
+##
+## If only one input is given, @var{elli} must define an ellipse as described in
+## @command{ellipses2d}.
+## If two inputs are given, @var{ra} and @var{rb} define the half-lenght of the
+## axes.
+## If a third input is given, @var{theta} must be the angle of rotation of the
+## ellipse in radians, and in counter-clockwise direction.
+##
+## The output @var{K} contains the covariance matrix define by the ellipse.
+##
+## Run @code{demo ellipse2cov} to see an example.
+##
+## @seealso{ellipses2d, cov2ellipse, drawEllipse}
+## @end deftypefn
+
+function K = ellipse2cov (elli, varargin);
+
+  ra    = 1;
+  rb    = 1;
+  theta = 0;
+  switch numel (varargin)
+    case 0
+    ## ellipse format
+      if numel (elli) != 5
+        print_usage ();
+      end
+      ra    = elli(1,3);
+      rb    = elli(1,4);
+      theta = elli(1,5)*pi/180;
+
+    case 1
+    ## ra,rb
+      if numel (elli) != 1
+        print_usage ();
+      end
+      ra = elli;
+      rb = varargin{1};
+
+    case 2
+    ## ra,rb, theta
+      if numel (elli) != 1
+        print_usage ();
+      end
+      ra    = elli;
+      rb    = varargin{1};
+      theta = varargin{2};
+
+    otherwise
+      print_usage ();
+  end
+
+  T = createRotation (theta)(1:2,1:2);
+  K = T*diag([ra rb])*T';
+
+endfunction
+
+%!demo
+%! elli = [0 0 1 3 -45];
+%!
+%! # Create 2D normal random variables with covarinace defined by elli.
+%! K = ellipse2cov (elli)
+%! L = chol(K,'lower');
+%! u = randn(1e3,2)*L';
+%!
+%! Kn = cov (u)
+%!
+%! figure(1)
+%! plot(u(:,1),u(:,2),'.r');
+%! hold on;
+%! drawEllipse(elli,'linewidth',2);
+%! hold off
+%! axis tight