function value = q_measure ( n, z, triangle_order, triangle_num, triangle_node )

%*****************************************************************************80
%
%% Q_MEASURE determines the triangulated pointset quality measure Q.
%
%  Discussion:
%
%    The Q measure evaluates the uniformity of the shapes of the triangles
%    defined by a triangulated pointset.
%
%    For a single triangle T, the value of Q(T) is defined as follows:
%
%      TAU_IN = radius of the inscribed circle,
%      TAU_OUT = radius of the circumscribed circle,
%
%      Q(T) = 2 * TAU_IN / TAU_OUT
%        = ( B + C - A ) * ( C + A - B ) * ( A + B - C ) / ( A * B * C )
%
%    where A, B and C are the lengths of the sides of the triangle T.
%
%    The Q measure computes the value of Q(T) for every triangle T in the
%    triangulation, and then computes the standard deviation of this
%    set of values:
%
%      Q_MEASURE = min ( all T in triangulation ) Q(T)
%
%    In an ideally regular mesh, all triangles would have the same
%    equilateral shape, for which Q = 1.  In a good mesh, 0.5 < Q.
%
%    Given the 2D coordinates of a set of N nodes, stored as Z(1:2,1:N),
%    a triangulation is a list of NT triples of node indices that form
%    triangles.  Generally, a maximal triangulation is expected, namely,
%    a triangulation whose image is a planar graph, but for which the
%    addition of any new triangle would mean the graph was no longer planar.
%    A Delaunay triangulation is a maximal triangulation which maximizes
%    the minimum angle that occurs in any triangle.
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license.
%
%  Modified:
%
%    08 November 2005
%
%  Author:
%
%    John Burkardt
%
%  Reference:
%
%    Max Gunzburger and John Burkardt,
%    Uniformity Measures for Point Samples in Hypercubes.
%
%    Per-Olof Persson and Gilbert Strang,
%    A Simple Mesh Generator in MATLAB,
%    SIAM Review,
%    Volume 46, Number 2, pages 329-345, June 2004.
%
%  Parameters:
%
%    Input, integer N, the number of points.
%
%    Input, real Z(DIM_NUM,N), the points.
%
%    Input, integer TRIANGLE_ORDER, the order of the triangles.
%
%    Input, integer TRIANGLE_NUM, the number of triangles.
%
%    Input, integer TRIANGLE_NODE(TRIANGLE_ORDER,TRIANGLE_NUM), the triangulation.
%
%    Output, real VALUE, the Q quality measure.
%
  if ( triangle_num < 1 )
    value = -1.0;
    return
  end
%
%  Compute the mean value of Q.
%
  q_min = r8_huge ( );

  for triangle = 1 : triangle_num

    a_index = triangle_node(1,triangle);
    b_index = triangle_node(2,triangle);
    c_index = triangle_node(3,triangle);

    ab_length = sqrt ( ...
        ( z(1,a_index) - z(1,b_index) )^2 ...
      + ( z(2,a_index) - z(2,b_index) )^2 );

    bc_length = sqrt ( ...
        ( z(1,b_index) - z(1,c_index) )^2 ...
      + ( z(2,b_index) - z(2,c_index) )^2 );

    ca_length = sqrt ( ...
        ( z(1,c_index) - z(1,a_index) )^2 ...
      + ( z(2,c_index) - z(2,a_index) )^2 );

    q = ( bc_length + ca_length - ab_length ) ...
      * ( ca_length + ab_length - bc_length ) ...
      * ( ab_length + bc_length - ca_length ) ...
      / ( ab_length * bc_length * ca_length );

    q_min = min ( q_min, q );

  end

  value = q_min;

  return
end