function fem_basis_t3_display ( node_file, element_file )

%% FEM_BASIS_T3_DISPLAY displays a finite element T3 basis.
%
%  Discussion:
%
%    This program reads a data file defining a set of nodes, and a
%    data file defining the triangulation of those nodes using 3 node triangles
%    (or 6 node triangles, as long as the vertices are listed first).
%
%    The program then asks the user interactively to select one of the
%    nodes.  It computes the basis function associated with that node
%    and displays it over the entire mesh.  Of course, the basis function
%    will only be nonzero over a small number of the elements, but it
%    is instructive to see the entire mesh.
%
%    The display is initially "flat", but by using the manipulator
%    on the graphics menu, the user can easily get some dramatic images
%    of the basis function.
%
%  Usage:
%
%    fem_basis_t3_display ( 'node_file.txt', 'element_file.txt' )
%
%  Modified:
%
%    22 August 2006
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, string NODE_FILE, ELEMENT_FILE, the names of the files
%    containing the node and element information.
%
  timestamp;
  fprintf ( 1, '\n' );
  fprintf ( 1, 'FEM_BASIS_T3_DISPLAY:\n' );
  fprintf ( 1, '  MATLAB version\n' );
  fprintf ( 1, '  Display a basis function associated with \n' );
  fprintf ( 1, '  a finite element grid of linear triangles ("T3").\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  This program reads two files:\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  * node_file,     the node file,\n' );
  fprintf ( 1, '  * element_file,  the element file,\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  The user specifies a basis function by node index, and\n' );
  fprintf ( 1, '  the program displays a surface plot of that basis function.\n' );
%
%  Read the data.
%
  p = load ( node_file )';
  [ dim_num, node_num ] = size ( p );

  t = load ( element_file )';
  [ triangle_order, triangle_num ] = size ( t );

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Every basis function is associated with a node.\n' );
  fprintf ( 1, '  To chooose a basis function, you specify a node.\n' );
  if ( triangle_order == 6 )
    fprintf ( 1, '\n' );
    fprintf ( 1, '  This mesh uses 6-node triangles.\n' );
    fprintf ( 1, '  The code will not work for the midside nodes,\n' );
    fprintf ( 1, '  so try NOT to pick those!\n' );
    fprintf ( 1, '\n' );
  end
  fprintf ( 1, '  Nodes range in value from 1 to %d.\n', node_num );
  fprintf ( 1, '\n' );
  
  node_index = input ( 'Enter the index of a node: ' );
%
%  Clear the graphics page.
%
  clf
  
  fprintf ( 1, '\n' );
  
  for triangle = 1 : triangle_num
    
    local = 0.0;
    for j = 1 : 3
      if ( t(j,triangle) == node_index )
        fprintf ( 1, '  Node %d occurs as local node %d in element %d.\n', ...
          node_index, j, triangle );
        local = j;
      end 
    end

    if ( local == 0 )
      x(1,1) = p(1,t(1,triangle)); 
      y(1,1) = p(2,t(1,triangle));
      x(2,1) = p(1,t(1,triangle)); 
      y(2,1) = p(2,t(1,triangle));
      x(1,2) = p(1,t(2,triangle)); 
      y(1,2) = p(2,t(2,triangle));
      x(2,2) = p(1,t(3,triangle)); 
      y(2,2) = p(2,t(3,triangle));
      z(1:2,1:2) = 0.0;
    else
      x(1,1) = p(1,t(local,triangle)); 
      y(1,1) = p(2,t(local,triangle));
      x(2,1) = p(1,t(local,triangle)); 
      y(2,1) = p(2,t(local,triangle));
      j = i4_wrap ( local + 1, 1, 3 );
      x(1,2) = p(1,t(j,triangle)); 
      y(1,2) = p(2,t(j,triangle));
      j = i4_wrap ( local + 2, 1, 3 );
      x(2,2) = p(1,t(j,triangle)); 
      y(2,2) = p(2,t(j,triangle));
      z(1:2,1:2) = 0.0;
      z(1:2,1) = 1.0;
    end
    
    caxis ( [ -0.4, 1.2 ] )
    surface ( x, y, z, 'FaceColor', 'interp' )

  end

  xlabel ( '--X axis--' );
  ylabel ( '--Y axis--' );
  zlabel ( '--Z axis--' );

  title_string = s_escape_tex ( element_file );
  title ( title_string );

  fprintf ( 1, '\n' );
  fprintf ( 1, 'Press return...\n' );
  pause

  fprintf ( 1, '\n' );
  fprintf ( 1, 'FEM_BASIS_T3_DISPLAY:\n' );
  fprintf ( 1, '  Normal end of execution.\n' );

  fprintf ( 1, '\n' );
  timestamp;