function [con] = CON(I, d,phi)
% 
% Written by:
% -- 
% John L. Weatherwax                2005-08-04
% 
% email: wax@alum.mit.edu
% 
% Please send comments and especially bug reports to the
% above email address.
% 
% Epage 
% 
%-----

if( nargin>1 ) % compute the CON for just one displacement and angular direction

  P   = co_occurrence(I, d,phi);
  N_g = size(P,1);

  n=0;
  con = 0.; %=n^2 * sum(diag(P,n)); since n is zero here 
  for n=1:(N_g-1)
    con = con + n^2 * sum(diag(P,+n)); % extract and add the super diagonal
    con = con + n^2 * sum(diag(P,-n)); % extract and add the sub diagonal 
  end
  
else % compute the CON for all angles and average 
  
  d = 1; % assume d=1

  all_phis = [ 0, 45, 90, 135 ];
  all_cons = zeros(1,length(all_phis));
  for ii=1:length(all_phis)
    
    c = CON(I, d,all_phis(ii)); % recursively call the three argument case  
    all_cons(ii) = c;
    
  end
  
  con = mean(all_cons);
  
end