function hcell_contour_display ( node_xy_file_name, p_file_name )

%% HCELL_CONTOUR_DISPLAY displays a contour plot for the H-Cell problem.
%
%  Discussion:
%
%    This function reads the H-Cell flow data for a single timestep:
%
%      geometry ( X, Y values at the 3-node triangle nodes);
%      pressure ( or any corresponding scalar quantity.)
%
%    It could also handle the pair
%
%      geometry ( X, Y values at the 6-node triangle nodes);
%      velocity norm ( calculated externally and stored in a file)
%
%    The data is arranged to suit a call to MATLAB's CONTOURF routine.
%
%  Usage:
%
%    hcell_contour_display ( node_xy_file_name, p_file_name )
%
%    A typical invocation might be
%
%      hcell_contour_display ( 'xy3.txt', 'p000.txt' )
%
%    But if you simply say 
%
%      hcell_contour_display
%
%    the program will give you a chance to enter the file names
%    interactively.
%
%  Modified:
%
%    30 April 2004
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, NODE_XY_FILE_NAME, the name of the file containing the
%    coordinates of the nodes.
%
%    Input, P_FILE_NAME, the name of the file containing the values of
%    the scalar quantity (such as pressure) at the nodes.
%
  fprintf ( 1, '\n' );
  fprintf ( 1, 'HCELL_CONTOUR_DISPLAY:\n' );
  fprintf ( 1, '  MATLAB version\n' );
  fprintf ( 1, '\n' ); 
  fprintf ( 1, '  Display scalar data contours in the HCELL problem.\n' );
%
%  Do we have the XY file?
%
  if ( nargin < 1 )

    fprintf ( 1, '\n' );
    fprintf ( 1, 'HCELL_CONTOUR_DISPLAY:\n' );
    node_xy_file_name = input ( 'Enter the name of the XY coordinate file:' );

  end
  
  node_xy = table_read ( node_xy_file_name );
  
  [ m1, n1 ] = size ( node_xy );
%
%  Do we have the P file?
%
  if ( nargin < 2 )

    fprintf ( 1, '\n' );
    fprintf ( 1, 'HCELL_CONTOUR_DISPLAY:\n' );
    p_file_name = input ( 'Enter the name of the scalar (pressure?) file:' );

  end
  
  p = table_read ( p_file_name );
  
  [ m2, n2 ] = size ( p );
%
%  Make sure the number of coordinates and values match.
%
  if ( n1 ~= n2 )
    fprintf ( 1, '\n' );
    fprintf ( 1, 'HCELL_CONTOUR_DISPLAY - Fatal error!\n' );
    fprintf ( 1, '  The number of coordinates is %d\n', n1 );
    fprintf ( 1, '  but the number of pressure values is %d\n', n2 );
    return
  end
%
%  Our X, Y data is logically rectangular, with some holes.
%  CONTOURF requires 2D arrays, with no holes.
%  Figure out the number of rows and columns, 
%  construct the data, and fill in the holes.
%
  x = sort ( node_xy(1,1:n1) );
  x_tol = 0.0001 * ( x(n1) - x(1) );
  [ n, x_unique ] = r8vec_sorted_unique ( n1, x, x_tol );
  
  y = sort ( node_xy(2,1:n1) );
  y_tol = 0.0001 * ( y(n1) - y(1) );
  [ m, y_unique ] = r8vec_sorted_unique ( n1, y, y_tol );

  [ x_grid, y_grid ] = meshgrid ( x_unique, y_unique );

  p_min = min ( p(1,1:n2) );
  p_max = max ( p(1,1:n2) );

  z_grid(1:m,1:n) = p_min + ( p_min - p_max );
  z_grid(1:m,1:n) = NaN;
%
%  For each computational node, copy the value of P into the array.
%
  for k = 1 : n1
    i = r8vec_sorted_nearest ( n, x_unique, node_xy(1,k) );
    j = r8vec_sorted_nearest ( m, y_unique, node_xy(2,k) );
    z_grid(j,i) = p(k);
  end
%
%  Set the contour levels evenly spaced between P_MIN and P_MAX.
%
  level_num = 15;
  
  for level = 1 : level_num
    level_value(i) = ( ( 2 * level_num  - 2 * level + 1 ) * p_min  ...
                    + (                  2 * level - 1 ) * p_max ) ...
                    / ( 2 * level_num );
  end

   [ C1, h1 ] = contourf ( x_unique, y_unique, z_grid, 15 );
   colorbar;
   axis equal;
% [ C1, h1 ] = contourf ( x_grid, y_grid, z_grid, level_value );
%
%  Add the boundary of the region to the plot.
%
  hcell_boundary_add ( 'r' )
%
%  Add the invisible bounding box.
%
  hcell_box_add ( 'w' )
%
%  Set up a colormap.
%
  colormap ( jet );
%
%  Label the axes and the plot.
%
  xlabel ( 'X', 'FontName', 'Helvetica', 'FontWeight', 'bold', ...
    'FontSize', 16 );

  ylabel ( 'Y', 'FontName', 'Helvetica', 'FontWeight', 'bold', ...
    'FontSize', 16, 'Rotation', 0 );

  title ( 'Pressure Contours', 'FontName', 'Helvetica', 'FontWeight', ...
    'bold', 'FontSize', 16 );

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