function [ro, go, bo] = remove_ellipse(ri, gi, bi)
% REMOVE_ELLIPSE  Removes the ellipse from a headtracking 
%                 RGB or greyscale image.
%                 [R2,G2,B2]=REMOVE_ELLIPSE(R1,G1,B1) removes
%                     a pure red ellipse (255,0,0) from an RGB image
%                 IMG2=REMOVE_ELLIPSE(IMG1) removes
%                     a pure white ellipse (255) from a greyscale image
%                 IMG2=REMOVE_ELLIPSE(IMG1,COLOR) removes
%                     a pure white ellipse (COLOR) from a greyscale image

%            author:  Stan Birchfield, birchfield@cs.stanford.edu
%            date:    February 10, A.D. 1998
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

sz = size(ri);

%%% Greyscale
if (nargin==1 | nargin==2) & nargout==1,

	ro=ri;
	if nargin==2,
		ellipse_color = gi;
	else
		ellipse_color = 255;
	end

	for j=2:sz(1)-1,
  		for i=2:sz(2)-1,
    			if ri(j, i) == ellipse_color,
      			small = img(j-1:j+1, i-1:i+1);
	 			mask = small~=ellipse_color;
      			ro(j, i) = sum(sum(small .* mask)) / ...
                 		 	sum(sum(mask));
    			end
  		end
	end

%%% RGB
elseif nargin==3 & nargout==3,

	ro=ri;
	go=gi;
	bo=bi;
	
	for j=2:sz(1)-1,
  		for i=2:sz(2)-1,
    			if ri(j,i)==255 & gi(j,i)==0 & bi(j,i)==0,
      			small_r = ri(j-1:j+1, i-1:i+1);
      			small_g = gi(j-1:j+1, i-1:i+1);
      			small_b = bi(j-1:j+1, i-1:i+1);
	 			mask = small_r~=255 | small_g~=0 | small_b~=0;
      			ro(j, i) = sum(sum(small_r .* mask)) / sum(sum(mask));
      			go(j, i) = sum(sum(small_g .* mask)) / sum(sum(mask));
      			bo(j, i) = sum(sum(small_b .* mask)) / sum(sum(mask));
    			end
  		end
  	end
else
	error('Number of arguments is invalid')
end