function  quiver_qp_prin_stress_m (scale, i_opt, inc_g) 
%under construction
%                         default (1.0,   1,     1)
% Copyright 2000, J.E. Akin.  All rights reserved.
%  ------------------------------------------------------
%  Matlab plot of 2-D FE principal stress vectors 
%  at Gauss pts (incremented by inc_g), with mesh
%  i_opt=1 max stress, 2 min stress, 3= max shear
%  ------------------------------------------------------

% 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_g  = increment in Gauss point vector plot, default = 1
% inc_p  = increment in node numbers on plot, if > 0
% msh_typ_nodes   = connectivity list for elements, nt x nod_per_el
% loop   = corners for nod_per_el line polygon
% lab_p  = 1, if node points are circled
  lab_p  = 0;
% length = maximum length of arrows, in scaled x,y units
% nod_per_el = Nodes per element
% np     = Number of Points
% nt     = Number of elements
% pre_e  = Element items before connectivity list
% pre_p  = Nodal items before coordinates
  pre_p  = 1 ;
% msh_bc_xyz = Nodal coordinates (with preceeding data)
% scale  = standard Matlab scale argument, default 1.0
% sign   = flip sign +-
sign=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

%                set constants
  if ( nargin == 0 )
    scale = 1.  ;   % the default scale      
    inc_g = 1   ; sign = 1 ; i_opt=1 ;
  elseif ( nargin == 1 )
    inc_g = 1   ; sign = 1 ; i_opt=1 ;
  elseif ( nargin == 2 )
    inc_g = 1   ;sign = 1 ;
  end % if
  fprintf ('Using a scale of %g and vector increment of %g \n', ...
            scale, inc_g)

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

%    Set control data: number of points
  np = size (msh_bc_xyz,1) ;   % number of nodal points  
  ns = size (msh_bc_xyz,2) - 1;   % dimension of space      
  fprintf ('Read %g mesh coordinates \n', np)
  fprintf ('In %g spatial dimensions \n', ns)
% fprintf ('       x        y \n')
  
%    Set control data: number elements
  load  msh_typ_nodes.tmp ;              % nod_per_el nodes per element
  nt = size (msh_typ_nodes,1) ;          % number of elements in mesh 
  pre_e  = 0 ;
  nod_per_el = size (msh_typ_nodes,2) - pre_e -1; % nodes per element
  fprintf ('Read %g elements connections \n', nt)

%       Read new Gauss locations & flux     components
  load el_qp_xyz_fluxes.tmp ; % x, y, du/dx, du/dy at qp

%       Set control data: number of gauss points
  npg = size(el_qp_xyz_fluxes,1) ; % quadrature pts w vectors
  if ( npg == 0 )
    error ('Error: missing file el_qp_xyz_fluxes.tmp')
  end % if error
  fprintf ('Read %g Gauss x & y coord & flux sets \n', npg)
  nf = size(el_qp_xyz_fluxes,2) - ns;  % flux components
  if ( nf ~= 3 ) % not 2-d stresses
    fprintf ('Read %g stress components instead of 3 \n', nf)
    fprintf ('Use contour_el_fluxes or smooth_el_fluxes instead \n')
    if ( ns == 1 ) % 1-d data
      fprintf ('Use el_qp_1d_flux_graph for 1-D data \n')
    end % if 1-d
    %B error ('Error: These are not 2-d stress data')
  end % if stress data

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

%    Mesh Range Alone
  % msh_bc_xyz has: pre_p items then: x, y
  x = msh_bc_xyz (1:np, (pre_p+1)) ; % extract x column of xy
  y = msh_bc_xyz (1:np, (pre_p+2)) ; % extract y column of xy

  gxmax = max (x)      ; gxmin = min (x) ;
  gymax = max (y)      ; gymin = min (y) ;
  gxdiff = gxmax - gxmin ; gydiff = gymax - gymin ;
  if ( gydiff == 0.0 )
    gydiff = 0.5 ; % allow for 1-D mesh (with y == 0)
  end % if no y coordinates

%%     FE Flux Values
%  d_x(2) = 0. ; d_y(2)  = 0. ; % vector ends on paper
   g_x  = el_qp_xyz_fluxes(1:inc_g:npg,1)  ; % position
   g_y  = el_qp_xyz_fluxes(1:inc_g:npg,2)  ; % position
   g_dx = sign * el_qp_xyz_fluxes(1:inc_g:npg,3)  ; % sigma_x 
   g_dy = sign * el_qp_xyz_fluxes(1:inc_g:npg,4)  ; % sigma_y 
   g_xy = sign * el_qp_xyz_fluxes(1:inc_g:npg,5)  ; % sigma_xy
   g_sq = sqrt ((0.5*(g_dx-g_dy)).^2 + g_xy.^2) ; % max shear
   g_p1 = 0.5*(g_dx+g_dy) + g_sq ; % Max principal stress
   g_p2 = 0.5*(g_dx+g_dy) - g_sq ; % Min principal stress
   %b g_a  = atan2  (-2 * g_xy ./ (g_dx-g_dy)) ./ 2  % radians
   g_a  = atan2  ((g_dx-g_dy), -2 * g_xy ) ./ 2 ; % radians
%b g_a(1:3)
   if ( i_opt == 1 ) 
     big  = max (g_p1)
   elseif ( i_opt == 2 ) 
     big  = max (g_p2)
     g_a  = g_a + 0.7854 ;
   elseif ( i_opt == 3 ) 
     big  = max (g_sq)  ; % magnitude
   end % if max shear
 
   largest = 0.0  ;
%     Scale flux     vectors
   if ( big > largest ) largest = big;

%     Scale everything
% xmax = max([gxmax, fxmax]); xmin = min([gxmin, fxmin]);
% ymax = max([gymax, fymax]); ymin = min([gymin, fymin]);
% xmax = gxmax ; xmin = gxmin ;
% ymax = gymax ; ymin = gymin ;

% xmax = gxmax + gxdiff/10 ; ymax = gymax + gydiff/10 ; % keep
% xmin = gxmin - gxdiff/10 ; ymin = gymin - gydiff/10 ; % keep
  xmax = gxmax + gxdiff/20 ; ymax = gymax + gydiff/20 ;
  xmin = gxmin - gxdiff/20 ; ymin = gymin - gydiff/20 ;

  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 ', int2str(nt),' Elements'])
xlabel (['X for ', int2str(nt),' Elements with ', ...
        int2str(nod_per_el), ' nodes'])
  ylabel (['Y; for ', int2str(np),' Nodes'])
  if ( inc_g == 1 )
   if ( i_opt == 1 )
    title  (['FEA 2-D Max Principal Stress at ', int2str(npg), ...
            ' Gauss Points, max = ', num2str(largest)]) 
   else
    title  (['FEA 2-D Min Principal Stress at ', int2str(npg), ...
            ' Gauss Points, max = ', num2str(largest)]) 
   end % if 
  else
   if ( i_opt == 1 )
    title  (['FEA 2-D Max Principal Stress at ', int2str(npg), ...
            ' Gauss Points, inc = ', int2str(inc_g), ...
            ', max = ', num2str(largest)]) % add title
   else
    title  (['FEA 2-D MIn Principal Stress at ', int2str(npg), ...
            ' Gauss Points, inc = ', int2str(inc_g), ...
            ', max = ', num2str(largest)]) % add title
   end % if 
  end % if alternate title

%      Select element type details
  [loop] = get_El_Loop (nod_per_el) ;

%%      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 10 elements
  inc_p = floor(np/10) ; inc_e = floor(nt/5) ;
  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 <= 10 )
    inc_e = 1 ;
  end % if nt

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

%      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
    plot (c_x, c_y, 'b-')     % plot nod_per_el lines, in blue
  end % for over all elements
 
%    Finish the plots with polygon numbers
% inc_e = 0 %b
  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

%  ---------------- Now add the vectors to the mesh --------------

  if ( i_opt == 1 ) % max prin
    VX = g_p1 .* cos(g_a) ;
    VY = g_p1 .* sin(g_a) ;
    quiver (g_x, g_y, VX, VY)
    quiver (g_x, g_y,-VX,-VY)
  elseif ( i_opt == 2 ) % min prin
    VX = g_p2 .* cos(g_a) ;
    VY = g_p2 .* sin(g_a) ;
    quiver (g_x, g_y, VX, VY)
    quiver (g_x, g_y,-VX,-VY)
  end % if i_opt
  
%%     Gauss point locations
%     plot (g_x, g_y, 'g+')
%
%%    Plot flux     arrows at Gauss point
%    for ig = 1:npg
%%          FE values
%      d_x(1) = g_x(ig)  ; d_y(1) = g_y(ig) ; % vector start point
%      %b d_x(2) = g_x(ig) + g_dx(ig)*g_sq(ig) ; % vector end position
%      %b d_y(2) = g_y(ig) + g_dy(ig)*g_sq(ig) ; % vector end position
%      d_x(2) = g_dx(ig) ; % vector end position
%      d_y(2) = g_dy(ig) ; % vector end position
%      plot (d_x, d_y, 'k-')       % black line
%      plot (d_x(2), d_y(2), 'r.') % red end point
%
%    end % for Gauss pts

%       -depsc -tiff            % for an eps version
  % print -dpsc quiver_qp_prin_stress_m
  hold off
% fprintf ('Created file quiver_qp_prin_stress_m.ps \n')
end % quiver_qp_prin_stress_m