function triangulation_q2l ( input_triangulation_filename )

%% MAIN is the main program for TRIANGULATION_Q2L.
%
%  Discussion:
%
%    TRIANGULATION_Q2L makes a linear triangulation from a quadratic one.
%
%    A quadratic triangulation is assumed to consist of 6-node triangles,
%    as in the following:
%
%    11-12-13-14-15
%     |\    |\    |
%     | \   | \   |
%     6  7  8  9 10
%     |   \ |   \ |
%     |    \|    \|
%     1--2--3--4--5
%
%    This routine rearranges information so as to define the 3-node
%    triangulation:
%
%    11-12-13-14-15
%     |\ |\ |\ |\ |
%     | \| \| \| \|
%     6--7--8--9-10
%     |\ |\ |\ |\ |
%     | \| \| \| \|
%     1--2--3--4--5
%
%  Modified:
%
%    01 January 2007
%
%  Author:
%
%    John Burkardt
%
%  Usage:
%
%    triangulation_q2l ( 'triangle_file' )
%
  timestamp;

  fprintf ( 1, '\n' );
  fprintf ( 1, 'TRIANGULATION_Q2L\n' );
  fprintf ( 1, '  MATLAB version\n' );
  fprintf ( 1, '  Read a "quadratic" triangulation and\n' );
  fprintf ( 1, '  write out a "linear" triangulation.\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  Read a triangulation dataset of TRIANGLE_NUM1\n' );
  fprintf ( 1, '  triangles using 6 nodes.\n' );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  Create a 3 node triangulation by breaking\n' );
  fprintf ( 1, '  every 6 node triangle into 4 smaller ones.\n' );
  fprintf ( 1, '  Write the new linear triangulation to a file.\n' );
%
%  Get the number of command line arguments.
%
  if ( nargin < 1 )

    fprintf ( 1, '\n' );
    fprintf ( 1, 'TRIANGULATION_Q2L:\n' );
    input_triangulation_filename = input ( ...
      '  Please enter the name of the triangulation file.' );

  end
%
%  Read the data.
%
  [ triangle_order1, triangle_num1 ] = itable_header_read ( ...
    input_triangulation_filename );

  if ( triangle_order1 ~= 6 )
    fprintf ( 1, '\n' );
    fprintf ( 1, 'TRIANGULATION_Q2L - Fatal error!\n' );
    fprintf ( 1, '  Data is not for a 6-node triangulation.\n' );
    error ( 'TRIANGULATION_Q2L - Fatal error!' );
  end

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Read the header of ""%s".\n', input_triangulation_filename );
  fprintf ( 1, '\n' );
  fprintf ( 1, '  Triangle order = %d\n', triangle_order1 );
  fprintf ( 1, '  Number of triangles TRIANGLE_NUM1  = %d\n', triangle_num1 );

  triangle_node1(1:triangle_order1,1:triangle_num1) = itable_data_read ( ...
    input_triangulation_filename, triangle_order1, triangle_num1 );

  fprintf ( 1, '\n' );
  fprintf ( 1, '  Read the data in ""%s".\n', input_triangulation_filename );

  i4mat_transpose_print_some ( triangle_order1, triangle_num1, triangle_node1, ...
    1, 1, 6, 10, '  6 by 10 portion TRIANGLE_NODE1:' );
%
%  Create the output file names from the input file names.
%
  output_triangulation_filename = file_name_ext_swap ( ...
    input_triangulation_filename, 'q2l.txt' );
%
%  Set the number of linear triangles:
%
  triangle_num2 = 4 * triangle_num1;
  triangle_order2 = 3;
%
%  Convert the data.
%
  triangle_node2 = triangulation_order6_to_order3 ( triangle_num1, ...
    triangle_node1 );

  i4mat_transpose_print ( triangle_order2, triangle_num2, ...
    triangle_node2, '  TRIANGLE_NODE2' );
%
%  Write out the node and triangle data for the quadratic mesh
%
  header = 1;
  itable_write ( output_triangulation_filename, triangle_order2, ...
    triangle_num2, triangle_node2, header );

  fprintf ( 1, '\n' );
  fprintf ( 1, 'TRIANGULATION_Q2L\n' );
  fprintf ( 1, '  Normal end of execution.\n' );

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