function exercise2(input_image, ax_outim_h, radius, lightsource, ...
    material)
  
% This function segments the object with the chosen reflectance spectrum.
% Its chromaticity is in 
%
% material
%
% The lightsource chromaticity is in lightsource.
%
% radius is the value behind the button autosegment

axes(ax_outim_h);


% cast the image from 8 bit integer to double
I = im2double(input_image);

% get the size of the image
[y,x,z]=size(I);

% calculate the chromaticities
RGB = I(:,:,1)+I(:,:,2)+I(:,:,3);
r = I(:,:,1)./RGB;
g = I(:,:,2)./RGB;
b = I(:,:,3)./RGB;

% pre calculate point line distance parameters
r0 = lightsource;
a = material - lightsource;
a=a./norm(a);

% 

if lightsource(1) < material(1)
  t1 = lightsource(1)+0.025;
  t2 = material(1)+radius;
else
  t1 = material(1)-radius;
  t2 = lightsource(1)-0.025;
end

% segment by distance to line spanned between material and light source
% iterate over all pixels
for yi = 1:y
  for xi = 1:x
    r1=[r(yi,xi) g(yi,xi) b(yi,xi)];
    dist = norm(cross(a,(r1-r0))); 
    if RGB(yi,xi) > 0.1
      if dist < radius
	if r(yi,xi) > t1 & r(yi,xi) < t2
	  input_image(yi,xi,1) = uint8(255);
	  input_image(yi,xi,2) = uint8(0);
	  input_image(yi,xi,3) = uint8(0);
	end
      end
    end
  end
end
    
% currently the input image is shown
imshow(input_image)
title('segmented image')