function [rp] = RP(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 RP for just one displacement and angular direction

  Q   = Q_RL(I,phi);  
  num = sum(Q(:));    
  den = prod( size(I) ); % the total number of pixels in the original image

  rp = num/den;
  
else % compute the RP for all angles and average 
  
  all_phis = [ 0, 45, 90, 135 ];
  all_rps = zeros(1,length(all_phis));
  for ii=1:length(all_phis)
    
    a = RP(I,all_phis(ii)); % recursively call the three argument case  
    all_rps(ii) = a;
    
  end
  
  rp = mean(all_rps);
  
end