function hcell_vector_display ( node_xy_file_name, uv_file_name )

%% HCELL_VECTOR_DISPLAY displays a vector field in the H-Cell.
%
%  Discussion:
%
%    This MATLAB function file reads the H-Cell flow data for a single timestep:
%
%      geometry (XY values at the 6-node triangle nodes) 
%      flow (UV values at nodes)
%
%    and plots the velocity vectors (U,V)(X,Y).
%
%    The file plots either the velocity vector field, or the velocity
%    direction field, depending on the value of the internal logical
%    parameter "normalized".
%
%    The MATLAB routine QUIVER internally scales the vectors, but this
%    can be adjusted by using a value of SCALE that is not 1.
%
%  Usage:
%
%    hcell_vector_display ( node_xy_file_name, uv_file_name )
%
%    A typical invocation might be
%
%      hcell_vector_display ( 'xy6.txt', 'uv000.txt' )
%
%    But if you simply say 
%
%      hcell_vector_display
%
%    the program will give you a chance to enter the file names
%    interactively.
%
%  Modified:
%
%    27 April 2004
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, string NODE_XY_FILE_NAME, the name of a file containing the 
%    coordinates of the nodes.
%
%    Input, string UV_FILE_NAME, the name of a file containing the velocity
%    components for a flow in the H-CELL.
%
  fprintf ( 1, '\n' );
  fprintf ( 1, 'HCELL_VECTOR_DISPLAY:\n' );
  fprintf ( 1, '  MATLAB version\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  Display a vector plot in the HCELL problem.\n' );

  FALSE = 0;
  TRUE = 1;
  scale = 1.0;
  normalized = TRUE;
%
%  Read the coordinates.
%
  if ( nargin < 1 ) 
    fprintf ( 1, '\n' );
    fprintf ( 1, 'HCELL_VECTOR_DISPLAY:\n' );
    node_xy_file_name = input ( 'Enter the name of the XY coordinate file:' );
  end

  node_xy = table_read ( node_xy_file_name );
%
%  Read the velocities.
%
  if ( nargin < 2 )
    fprintf ( 1, '\n' );
    fprintf ( 1, 'HCELL_VECTOR_DISPLAY:\n' );
    uv_file_name = input ( 'Enter the name of the velocity file:' );
  end

  uv = table_read ( uv_file_name );
%
%  Do you want to normalize the velocities?
%
  if ( normalized == TRUE )
    norm = sqrt ( uv(1,:).^2 + uv(2,:).^2 );
    nonzero = find ( norm ~= 0.0 );
    uv(1,nonzero) = uv(1,nonzero) ./ norm(nonzero);
    uv(2,nonzero) = uv(2,nonzero) ./ norm(nonzero);
  end
%
%  Display the vector field.
%
  quiver ( node_xy(1,:), node_xy(2,:), uv(1,:), uv(2,:), scale )
%
%  Add the boundary of the region to the plot.
%
  hcell_boundary_add ( 'r' )
%
%  Draw the invisible bounding box.
%
  hcell_box_add ( 'w' )

  axis equal
  
  if ( normalized == TRUE )
    title ( 'H-Cell Direction Field' )
  else
    title ( 'H-Cell Flow Field' )
  end

  text ( 45.0, 20.0, uv_file_name )

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