function velocity_arrows_sequence ( xy_filename, uv_filename, scale )

%% VELOCITY_ARROWS_SEQUENCE creates velocity plots from a sequence of datasets.
%
%  Discussion:
%
%    This program can be used to turn data files into a sequence of PNG
%    images, suitable for an animation.
%
%    Although this program can handle a single input file, its most useful
%    application is in processing many files, with 'consecutive' names, such
%    as "fred01.txt", "fred02.txt", "fred03.txt" and so on.  The user only
%    has to tell the program to process the first file, and it will automatically
%    continue to process files until it reaches the end of the sequence of names.
%
%    The input files:
%    ===============
%
%    The XY input data file stores the coordinates of the points, which are presumed
%    to be fixed over all the data files, one pair of values per line.
%
%    Each UV input data file is assumed to store the value of the horizontal and
%    vertical velocity at the corresponding point, one pair of values per line.
%
%    Internal processing:
%    ===================
%
%    Once the first file is processed, the routine checks to see if there
%    is a second "consecutive" file, and it so, it is also processed, and 
%    so on, until presumably all files in the sequence have been handled.
%
%  Usage:
%
%    velocity_arrows_sequence ( xy_filename, uv_filename, scale )
%
%    Note that a literal filename must be enclosed in single quotes.
%    Thus, a typical invocation might be
%
%      velocity_arrows_sequence ( 'xy.txt', 'u0001.txt', 1.0 )
%
%    But if you simply say 
%
%      velocity_arrows_sequence
%
%    the program will give you a chance to enter the data interactively.
%
%  Modified:
%
%    28 November 2005
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, string XY_FILENAME, the name of the XY input file.
%
%    Input, string UV_FILENAME, the name of the (first) UV input file.
%
%    Input, real SCALE, the scale factor for the arrow size.  Setting SCALE to 1
%    results in the default vector size.  
%
  fprintf ( 1, '\n' );
  fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE:\n' );
  fprintf ( 1, '  Given one XY file, and a sequence of UV files,\n' );
  fprintf ( 1, '  display the velocity fields\n' );
  fprintf ( 1, '  and save each image in a PNG file.\n' );
%
%  If at least one command line argument, it's the X file name.
%
  if ( nargin < 1 )

    fprintf ( 1, '\n' );
    fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE:\n' );
    xy_filename = input ( '  Enter the XY file, such as ''xy.txt'':' );

  end

  if ( ~file_exist ( xy_filename ) )
    fprintf ( 1, '\n' );
    fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE - Fatal error!\n' );
    fprintf ( 1, '  The file "%s" does not exist.\n', xy_filename );
    return
  end
%
%  If two command line arguments, the second is the UV file name.
%
  if ( nargin < 2 )

    fprintf ( 1, '\n' );
    fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE:\n' );
    uv_filename = input ( ...
      '  Enter the (first) UV file, such as ''uv01.txt'':' );

  end

  if ( ~file_exist ( uv_filename ) )
    fprintf ( 1, '\n' );
    fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE - Fatal error!\n' );
    fprintf ( 1, '  The file "%s" does not exist.\n', uv_filename );
    return
  end
%
%  If three command line arguments, the third is the scale factor.
%
  if ( nargin < 3 )
    fprintf ( 1, '\n' );
    fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE:\n' );
    scale = input ( '  Enter the scale factor:' );
  end
%
%  Save the name of the first input file, and set the name of the first PNG file.
%
  uv_root = uv_filename;

  png_root = file_name_ext_swap ( uv_root, 'png' );
  png_filename = png_root;

  fprintf ( 1, '\n' );
  fprintf ( 1, '  The first PNG file will be %s\n', png_root );
%
%  Read the data in the XY file.
%
  xy = load ( xy_filename );
  x = xy(:,1);
  y = xy(:,2);

  node_num = length ( x );

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

  x_frame = [ x_min - delta, ...
              x_max + delta, ...
              x_max + delta, ...
              x_min - delta, ...
              x_min - delta ];

  y_frame = [ y_min - delta, ...
              y_min - delta, ...
              y_max + delta, ...
              y_max + delta, ...
              y_min - delta ];
%
%  Determine the velocity magnitude scale over the WHOLE range of datasets.
%
  mag_max = 0.0;

  uv_filename = uv_root;

  while ( 1 )
      
    uv = load ( uv_filename );

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

    if ( node_num2 ~= node_num ) 
      fprintf ( 1, '\n' );
      fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE - 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 ( 'VELOCITY_ARROWS_SEQUENCE - Fatal error!' );
    end

    mag = sqrt ( uv(:,1).^2 + uv(:,2).^2 );
    mag_max = max ( mag_max, max ( mag ) );

    uv_filename = file_name_inc ( uv_filename );

    if ( ~file_exist ( uv_filename ) )
      break
    end

  end

  if ( mag_max == 0.0 )
    fprintf ( 1, '\n' );
    fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE - Fatal error!\n' );
    fprintf ( 1, '  The input data is entirely zero.\n' );
    fprintf ( 1, '  No plot is possible.\n' );
    error ( 'VELOCITY_ARROWS_SEQUENCE - The input data is zero!' );
  end
%
%  Create the plots.
%
  uv_filename = uv_root;

  frame_num = 0;
%
%  Loop over all files.
%
  while ( 1 )
      
    uv = load ( uv_filename );
  
    quiver ( x, y, uv(:,1), uv(:,2), scale / mag_max, 'b' );
    axis equal
    hold on

    k = convhull ( x, y );
    plot ( x(k), y(k), 'r' );
    hold on
  
    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_frame, y_frame, 'w' );
         
    hold off

    xlabel ( 'X', 'FontName', 'Helvetica', 'FontWeight', 'bold', ...
      'FontSize', 16 );

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

    title ( uv_filename, 'FontName', 'Helvetica', 'FontWeight', ...
      'bold', 'FontSize', 16 );
%
%  Here's where we take a snapshot of the current image, and save it to 
%  a PNG file.  Doing this was surprisingly complicated, and the help
%  system was not very clear.  Surely there's a more straightforward way!
%
    F = getframe;
    [ XX, map ] = frame2im ( F );
    imwrite ( XX, png_filename, 'PNG' );
    frame_num = frame_num + 1;
%
%  Prepare for next iteration.
%
    uv_filename = file_name_inc ( uv_filename );

    if ( ~file_exist ( uv_filename ) )
      break
    end
    
    png_filename = file_name_inc ( png_filename );

  end

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Processed %d frames.\n', frame_num );
  fprintf ( 1, '\n' );
  fprintf ( 1, 'VELOCITY_ARROWS_SEQUENCE:\n' );
  fprintf ( 1, '  Normal end of execution.\n' );