function ICOPP=rgb2icopp(I)
  % RGB2ICOPP converts an RGB image to intensity (gray!), Blue-Yellow 
  %   opponent, and Red-Green opponent (element-wise, no local surround).
  %   (see, e.g., http://en.wikipedia.org/wiki/Opponent_process)
  % 
  % @author: B. Schauerte
  % @date:   2009

  % Copyright 2009 B. Schauerte. All rights reserved.
  % 
  % Redistribution and use in source and binary forms, with or without 
  % modification, are permitted provided that the following conditions are 
  % met:
  % 
  %    1. Redistributions of source code must retain the above copyright 
  %       notice, this list of conditions and the following disclaimer.
  % 
  %    2. Redistributions in binary form must reproduce the above copyright 
  %       notice, this list of conditions and the following disclaimer in 
  %       the documentation and/or other materials provided with the 
  %       distribution.
  % 
  % THIS SOFTWARE IS PROVIDED BY B. SCHAUERTE ''AS IS'' AND ANY EXPRESS OR 
  % IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 
  % WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 
  % DISCLAIMED. IN NO EVENT SHALL B. SCHAUERTE OR CONTRIBUTORS BE LIABLE 
  % FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
  % CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 
  % SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR 
  % BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, 
  % WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
  % OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  % ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  % 
  % The views and conclusions contained in the software and documentation
  % are those of the authors and should not be interpreted as representing 
  % official policies, either expressed or implied, of B. Schauerte.

  r=I(:,:,1);
  g=I(:,:,2);
  b=I(:,:,3);
  R=r - ((g + b) / 2);
  G=g - ((r + b) / 2);
  B=b - ((r + g) / 2);
  Y=((r + g) / 2) - (abs(r - g) / 2) - b;
  ICOPP(:,:,1)=rgb2gray(I);
  ICOPP(:,:,2)=R - G;
  ICOPP(:,:,3)=B - Y;