function  truss_el_force_value()
% Copyright 2000, J.E. Akin.  All rights reserved.
%  ------------------------------------------------------
%  Color plot truss forces                                 
%  ------------------------------------------------------

% c_x    = x coordinates of nod_per_el line polygon
% c_y    = y coordinates of nod_per_el line polygon
% inc_e  = increment in element numbers on plot, if > 0
% inc_p  = increment in node numbers on plot, if > 0
% loop   = corners for nod_per_el line polygon
% lab_p  = 1, if node points are circled
  lab_p  = 0;
% msh_bc_xyz = Nodal coordinates (with preceeding data)
% msh_typ_nodes   = connectivity list for elements, nt x nod_per_el
% nod_per_el = Nodes per element
% np     = Number of Points
% nt     = Number of elements
% S_1    = Element shrink ratio, 1 = none
  S_1    = 0.8 ;
% S_2    = Gap size, = 1 - S_1
  S_2 = 1 - S_1 ;
% pre_e  = Element items before connectivity list
  pre_e  = 0 ;
% pre_p  = Nodal items before coordinates
  pre_p = 1;
% t_x    = x coordinates of nod_per_el corners
% t_y    = y coordinates of nod_per_el corners
% x_bar  = x-centroid of each element
% xy     = Coordinates of points, np x 2
% y_bar  = y-centroid of each element

%    Read coordinate file and connectivity file
%    integer bc code, real xy pairs for np points (pre_p = 1)

%    Set control data: number of points
  load msh_bc_xyz.tmp ;  
  np = size (msh_bc_xyz,1) ;   % number of nodal points  
  ns = size (msh_bc_xyz,2) - pre_p ;   % dimension of space
  if ( np == 0 )
    error ('Error missing file msh_bc_xyz.tmp')
  end % if error
  fprintf ('Read %g mesh coordinate pairs \n', np)
  if (ns == 3 ) % then assume z=0   
    fprintf('NOTE setting z=0. Use a 3d plot if z~=0. \n')
    ns = 2 ;
  end % if

  x (np)      = 0.       ;       % pre-allocate array x
  y (np)      = 0.       ;       % pre-allocate array y

  % msh_bc_xyz has: pre_p items then: x, y
  x = msh_bc_xyz (1:np, (pre_p+1)) ; % extract x column of xy
  if ( ns >= 2)
    y = msh_bc_xyz (1:np, (pre_p+2)) ; % extract y column of xy
  else
    y (1:np) = 0. ;
  end % if 1D
  
%    Set control data: number elements & nodes_per_element
  load  msh_typ_nodes.tmp ; % nod_per_el nodes per element
  nt = size (msh_typ_nodes,1);           % number of elements 
  if ( nt == 0 )
    error ('Error missing file msh_typ_nodes.tmp')
  end % if error
  nod_per_el = size (msh_typ_nodes,2) - pre_e -1 ; % nodes per elem
  fprintf ('Read %g elements with %g nodes each \n', nt,nod_per_el)

  [M_min, J_min] = min ( msh_typ_nodes ) ;
  if any ( M_min < 1 )
    fprintf ('Mesh seems to have mixed element types, \n')
  end % if

%   get element result magnitude from node 1 to 2
  load truss_el_forces.tmp
  en = size(truss_el_forces,1) ;
  if (nt ~= en )
    fprintf ('Read %g computed axial truss forces. \n', en)
    error('Number of element forces differs from number of elements.\n')
  end % if
  fmax = max (truss_el_forces(:, 3)) ;
  fmin = min (truss_el_forces(:, 3)) ;
  f_scale = max ( abs ([fmin, fmax])) ;
f_scale = f_scale*nt;
  if ( f_scale == 0 )
    error('STOP, all truss forces are zero. \n')
  end % IF

  value (nt)  = 0.       ;
  x_bar (nt)  = 0.       ;       % pre-allocate array x_bar
  y_bar (nt)  = 0.       ;       % pre-allocate array y_bar
  t_nodes (nod_per_el) = 0 ;    % Optional pre-allocation
  t_x (nod_per_el) = 0 ;        % Optional pre-allocation
  t_y (nod_per_el) = 0 ;        % Optional pre-allocation
  c_x (nod_per_el + 1) = 0 ;    % Optional pre-allocation
  c_y (nod_per_el + 1) = 0 ;    % Optional pre-allocation
  loop (nod_per_el + 1) = 0 ;   % Optional pre-allocation
 
%                set constants
  [loop] = get_El_Loop (nod_per_el) ;

%    Initialize plots
  xmax = max (x)      ; xmin = min (x) ;
  ymax = max (y)      ; ymin = min (y) ;
  xdiff = xmax - xmin ; ydiff = ymax - ymin ;
  % ydiff = 0 ;
  if ( ns == 1 )
    ydiff = 0.5 ; % allow for 1-D mesh (with y == 0)
  end % if no y coordinates
  %b xmax = xmax + xdiff/5 ; 
  ymax = ymax + ydiff/5 ;
  %b xmin = xmin - xdiff/5 ; 
  ymin = ymin - ydiff/5 ;
  clf                               % clear graphics
  axis ([xmin, xmax, ymin, ymax])   % set axes
  axis ('equal')                    % true shape style
  hold on                           % hold image for plots
  grid                              % add grid dots
  xlabel (['X for mesh with ', int2str(np),' nodes'])
  ylabel (['Y for mesh with ', int2str(nt),  ...
           ' elements (',int2str(nod_per_el), 'nodes per element)'])
  title (['Planar Truss Element Forces (max = ', ...
           num2str(fmax), ', min = ', num2str(fmin), ')'])
%      Plot input mesh points & label them
  if (lab_p == 1)                   % plot all points
    plot (x, y, 'b.')               % mark each node
  end % if show labels

%    Show 20 nodes and 50 elements, or all for small mesh
  inc_p = floor(np/20) ; inc_e = floor(nt/50) ;
  if (inc_p == 0 ) 
    inc_p = np - 1 ;
  end % if inc_p
  if (inc_e == 0 ) 
    inc_e = nt - 1 ;
  end % if inc_e
%    Show all if a small mesh
  if ( np <= 20 )
    inc_p = 1 ;
  end % if np
  if ( nt <= 50 )
    inc_e = 1 ;
  end % if nt
%b inc_p = 1
%b inc_e = 0

  if (inc_p > 0)      % plot node numbers, if node in mesh
   in_mesh (1:np) = 0 ;               % default to missing
   for i = 1:nt                       % loop all elements
     for j = 1:nod_per_el             % loop its nodes
       I_J =  msh_typ_nodes(i, j+1) ;
       if ( I_J > 0 )
         in_mesh ( I_J ) = 1; % flag as present
       end % if a non-zero node
       % in_mesh (msh_typ_nodes(i, j)) = 1; % flag as present
     end % for its nodes
   end % for all elements
    disp (in_mesh)

   for i = 1:inc_p:np                 % convert to string
     if ( in_mesh (i) > 0 )           % it is present
      p_text = sprintf ('  %g', i);   % offset # from pt
      text (x(i), y(i), p_text)       % plot pt number
     end % if present
   end % for all points
  end % if show labels
  %b disp (' ')

%    Loop over all elements
  for it = 1:nt  ;

%      Extract corner connectivity 
    t_nodes = msh_typ_nodes (it, (pre_e+2):(nod_per_el+pre_e+1)); 
    [N_min, I_min] = min ( t_nodes ) ;
    if ( N_min < 1 )
      fprintf ('Element %g connectivity corrected \n', it)
      t_nodes ( I_min:nod_per_el ) = t_nodes ( I_min - 1 ) ;
      % repeat the last non-zero node
    end % if

%      Extract corner coordinates
    t_x  = x (t_nodes) ;         % x at those nodes, only
    t_y  = y (t_nodes) ;         % y at those nodes, only

%      Get the centroid
    x_bar (it) = sum (t_x' )/nod_per_el ;
    y_bar (it) = sum (t_y' )/nod_per_el ;

%      Plot this polygon
    c_x = t_x (loop) ;       % x for nod_per_el line polygon
    c_y = t_y (loop) ;       % y for nod_per_el line polygon
    c_x = S_1 * c_x + S_2 * x_bar (it) ; % shrink x
    c_y = S_1 * c_y + S_2 * y_bar (it) ; % shrink y

%   get truss force rectangle
    dx = c_x(2) - c_x(1) ; dy = c_y(2) - c_y(1) ;
    L = sqrt(dx*dx + dy*dy) ;
    force = truss_el_forces(it, 3)  ;
    x_off = -dy/L * force/f_scale ;
    y_off =  dx/L * force/f_scale ;
    fx (1:2) = c_x(1:2); 
    fx (3) = fx (2) + x_off ;
    fx (4) = fx (1) + x_off ;
    fy (1:2) = c_y(1:2); 
    fy (3) = fy (2) + y_off ;
    fy (4) = fy (1) + y_off ;
    fill (fx, fy, force)      % plot force color

    plot (c_x, c_y)           % plot nod_per_el lines
  end % for over all elements
  colorbar
 
%    Finish the plots with polygon numbers
  if (inc_e > 0)                     % plot elem number, inclined
    plot (x_bar, y_bar, 'g.')        % centroid of each element
    for i = 1:inc_e:nt               % convert to string
      t_text = sprintf ('   %g', i);  % offset # from pt
      text (x_bar(i), y_bar(i), t_text, 'Rotation', 45) % incline
    end % for all polygons
  end % if show labels
%       -depsc -tiff            % for an eps version
% print -dpsc truss_el_force_value
  hold off
% fprintf ('Created file truss_el_force_value.ps \n')
% end of truss_el_force_value