function [rln] = RLN(I, 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 RLN for just one displacement and angular direction

  Q    = Q_RL(I,phi);  
  tmp  = sum( Q, 1 );  
  num  = sum( tmp.^2 );
  den  = sum(Q(:));    
  
  rln = num/den;
  
else % compute the RLN for all angles and average 
  
  all_phis = [ 0, 45, 90, 135 ];
  all_rlns = zeros(1,length(all_phis));
  for ii=1:length(all_phis)
    
    a = RLN(I,all_phis(ii)); % recursively call the three argument case  
    all_rlns(ii) = a;
    
  end
  
  rln = mean(all_rlns);
  
end