function dss = diffss(ss)
% DIFFSS  Difference of scale space
%   DSS=DIFFSS(SS) returns a scale space DSS obtained by subtracting
%   consecutive levels of the scale space SS.
%
%   In SIFT, this function is used to compute the difference of
%   Gaussian scale space from the Gaussian scale space of an image.
%
%   See also GAUSSIANSS(), PDF:SIFT.USER.SS.

% AUTORIGHTS
% Copyright (c) 2006 The Regents of the University of California.
% All Rights Reserved.
%
% Created by Andrea Vedaldi
% UCLA Vision Lab - Department of Computer Science
%
% Permission to use, copy, modify, and distribute this software and its
% documentation for educational, research and non-profit purposes,
% without fee, and without a written agreement is hereby granted,
% provided that the above copyright notice, this paragraph and the
% following three paragraphs appear in all copies.
%
% This software program and documentation are copyrighted by The Regents
% of the University of California. The software program and
% documentation are supplied "as is", without any accompanying services
% from The Regents. The Regents does not warrant that the operation of
% the program will be uninterrupted or error-free. The end-user
% understands that the program was developed for research purposes and
% is advised not to rely exclusively on the program for any reason.
%
% This software embodies a method for which the following patent has
% been issued: "Method and apparatus for identifying scale invariant
% features in an image and use of same for locating an object in an
% image," David G. Lowe, US Patent 6,711,293 (March 23,
% 2004). Provisional application filed March 8, 1999. Asignee: The
% University of British Columbia.
%
% IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
% FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
% INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND
% ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN
% ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. THE UNIVERSITY OF
% CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT
% LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
% A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS"
% BASIS, AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE
% MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.

dss.smin = ss.smin ;
dss.smax = ss.smax-1 ;
dss.omin = ss.omin ;
dss.O = ss.O ;
dss.S = ss.S ;
dss.sigma0 = ss.sigma0 ;

for o=1:dss.O
  % Can be done at once, but it turns out to be faster
  % this way
  [M,N,S] = size(ss.octave{o}) ;
  dss.octave{o} = zeros(M,N,S-1) ;
  for s=1:S-1
    dss.octave{o}(:,:,s) = ...
        ss.octave{o}(:,:,s+1) - ss.octave{o}(:,:,s) ;
  end
end
