function direction_arrows ( node_file, velocity_file, scale )

%% DIRECTION_ARROWS makes an arrow plot of a direction field.
%
%  Discussion:
%
%    The data is stored in two files, one containing XY coordinates,
%    and one containing velocity values.  Both files are in the "TABLE" format.
%
%    In particular, each line of the XY coordinate file should contain
%    the X and Y coordinates for a single point.  Similarly, each line of
%    the velocity file should contain the horizontal and vertical velocities
%    for the same point.  Comment lines may be included if they begin with
%    the special symbol "#".
%
%  Modified:
%
%    28 November 2005
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, character NODE_FILE, the name of the node file.
%
%    Input, character VELOCITY_FILE, the name of the velocity file.
%
%    Input, real SCALE, a scale factor.
%
  fprintf ( 1, '\n' );
  fprintf ( 1, 'DIRECTION_ARROWS:\n' );
  fprintf ( 1, '  Arrow plots of a direction field.\n' );
  
  if ( nargin < 1 )
    fprintf ( 1, '\n' );
    node_file = input ( 'Enter name of node file, such as ''nodes6.txt'':  ' );
  end

  xy = load ( node_file );
  x = xy(:,1);
  y = xy(:,2);

  node_num = length ( x );

  if ( node_num < 1 ) 
    fprintf ( 1, '\n' );
    fprintf ( 1, 'DIRECTION_ARROWS - Fatal error!\n' );
    fprintf ( 1, '  The number of nodes in the XY file is less than 1.\n' );
    error ( 'DIRECTION_ARROWS - Fatal error!' );
  end

  if ( nargin < 2 )
    fprintf ( 1, '\n' );
    velocity_file = input ( ...
      'Enter name of velocity file, such as ''velocity6.txt'':  ' );
  end
  
  if ( nargin < 3 )
    fprintf ( 1, '\n' );
    scale = input ( ...
      'Enter a scale factor for the velocities (1.0 for no magnification): ' );
  end
  
  uv = load ( velocity_file );

  node_num2 = length ( uv(:,1) );

  if ( node_num2 ~= node_num ) 
    fprintf ( 1, '\n' );
    fprintf ( 1, 'DIRECTION_ARROWS - Fatal error!\n' );
    fprintf ( 1, '  The number of nodes in %s is %d.\n', ...
      uv_filename, node_num2 );
    fprintf ( 1, '  The number of nodes in %s is %d.\n', ...
      xy_filename, node_num );
    error ( 'DIRECTION_ARROWS - Fatal error!' );
  end

  mag = sqrt ( uv(:,1).^2 + uv(:,2).^2 );
  mag_max = max ( mag );
%
%  As part of the normalization process, we also intend to ignore velocity
%  vectors whose magnitude is less than 1/10000 of the largest velocity.
%
  i = find ( mag ~= 0 & 0.0001 * mag_max <= mag );
  
  uv(i,1) = uv(i,1) ./ mag(i);
  uv(i,2) = uv(i,2) ./ mag(i);
  
  quiver ( x(i), y(i), uv(i,1), uv(i,2), scale, 'b' );
  axis equal
  hold on

  k = convhull ( x, y );
  plot ( x(k), y(k), 'r' );
  hold on
%
%  Plot an invisible frame.  This will help to force all the plots to
%  be the same size.
%
  x_min = min ( x );
  x_max = max ( x );
  y_min = min ( y );
  y_max = max ( y );
  delta = 0.05 * max ( x_max - x_min, y_max - y_min );
  
  plot ( [ x_min - delta, ...
           x_max + delta, ...
           x_max + delta, ...
           x_min - delta, ...
           x_min - delta ], ...
         [ y_min - delta, ...
           y_min - delta, ...
           y_max + delta, ...
           y_max + delta, ...
           y_min - delta ], ...
         'w' );
         
  hold off