function [di,dj] = phi_to_xy_increment(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 
% 
%-----

switch phi % assign signs based on "matrix index increments"
 case 0,
  di=  0;
  dj=  1;
 case 45, 
  di= -1;
  dj= +1;
 case 90, 
  di=  1;
  dj=  0;
 case 135
  di= -1;
  dj= -1;
 otherwise
  assert( logical(0), 'unknown value of phi' );
end