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+%% Copyright (c) 2012 Juan Pablo Carbajal <carbajal@ifi.uzh.ch>
+%%
+%%    This program is free software: you can redistribute it and/or modify
+%%    it under the terms of the GNU General Public License as published by
+%%    the Free Software Foundation, either version 3 of the License, or
+%%    any later version.
+%%
+%%    This program is distributed in the hope that it will be useful,
+%%    but WITHOUT ANY WARRANTY; without even the implied warranty of
+%%    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+%%    GNU General Public License for more details.
+%%
+%%    You should have received a copy of the GNU General Public License
+%%    along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+%% -*- texinfo -*-
+%% @deftypefn {Function File} {[@var{J} @var{sigma}]= } rodmassmatrix (@var{sigma},@var{l}, @var{rho})
+%% Mass matrix of one dimensional rod in 3D.
+%%
+%% Let q be the configuration vector of the rod, with the first three elements of
+%% q being the spatial coordinates (e.g. x,y,z) and the second three elements of
+%% q the rotiational coordinates (e.g. Euler angles), then the kinetical energy
+%% of the rod is given by
+%% T = 1/2 (dqdt)^T kron(J,eye(3)) dqdt
+%%
+%% @var{sigma} is between 0 and 1. Corresponds to the point in the rod that is
+%% being used to indicate the position of the rod in space.
+%% If @var{sigma} is a string then the value corresponding to the center of mass
+%% of the rod. This makes @var{J} a diagonal matrix. If @var{sigma} is a string
+%% the return value of @var{sigma} corresponds to the value pointing to the
+%% center of mass.
+%%
+%% @var{l} is the length of the rod. If omitted the rod has unit length.
+%%
+%% @var{rho} is a function handle to the density of the rod defined in the
+%% interval 0,1. The integral of this density equals the mass and is stored in
+%% @code{@var{J}(1,1)}. If omitted, the default is a uniform rod with unit mass.
+%%
+%% Run @code{demo rodmassmatrix} to see some examples.
+%%
+%% @end deftypefn
+
+function [J varargout] = rodmassmatrix(sigma, l = 1, dens = @(x)1)
+
+  if ischar (sigma)
+    sigma = quadgk (@(x)x.*dens(x), 0,1);
+  end
+
+  u = [-sigma*l (1-sigma)*l];
+
+  m      = quadgk (@(x)dens(sigma+x/l), u(1),u(2))/l;
+  f      = quadgk (@(x)dens(sigma+x/l).*x, u(1),u(2))/l;
+  iner_m = quadgk (@(x)dens(sigma+x/l).*x.^2, u(1),u(2))/l;
+
+  J = [m f; f iner_m];
+
+  if nargout > 0
+    varargout{1} = quadgk (@(x)x.*dens(x), 0,1);
+  end
+
+endfunction
+
+%!demo
+%! barlen  = 2;
+%! [Jc, s] = rodmassmatrix (0, barlen);
+%!
+%! printf ("Inertia matrix from the extrema : \n")
+%! disp (Jc)
+%! printf ("Sigma value to calculate from center of mass : %g \n",s)
+%!
+%! J = rodmassmatrix (s);
+%! printf ("Inertia matrix from the CoM : \n")
+%! disp (J)
+%!
+%! J2 = rodmassmatrix ("com");
+%! tf = all((J2 == J)(:));
+%! disp (["Are J and J2 equal? " "no"*not(tf) "yes"*tf])
+%!
+%! % ----------------------------------------------------------------------------
+%! % This example shows the calculations for rod of length 2. First we place one
+%! % of its extrema in the origin. Then we use the value of sigma provided by
+%! % the function to do the same calculation form the center of mass.
+
+%!demo
+%! % A normalized density function
+%! density = @(x) (0.5*ones(size(x)) + 10*(x<0.1)).*(x>=0 & x<=1)/1.5;
+%! [Jc, s] = rodmassmatrix (0,1,density);
+%!
+%! printf ("Inertia matrix from the extrema : \n")
+%! disp (Jc)
+%! printf ("Sigma value to calculate from center of mass : %g \n",s)
+%! J = rodmassmatrix (s,1,density);
+%!
+%! printf ("Inertia matrix from the CoM : \n")
+%! disp (J)
+%!
+%! figure (1)
+%! clf
+%! x = linspace (0,1,100)';
+%! h = plot (x,density(x),'b-;density;');
+%! set (h,'linewidth',2)
+%! axis tight
+%! v = axis();
+%! hold on
+%! h = plot ([s s],v([3 4]),'k-;CoM;');
+%! set (h, 'linewidth', 2);
+%! hold off
+%! axis auto
+%! % ----------------------------------------------------------------------------
+%! % This example defines a density function with an accumulation of mass near
+%! % one end of the rod. First we place one of its extrema in the origin. Then
+%! % we use the value of sigma provided by the function to do the same
+%! % calculation form the center of mass.

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