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% Optical Imaging and Spectroscopy
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% David J. Brady
% Duke University
% www.opticalimaging.org
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% 
function f2f = f3(x,y)
%image function for convolution backprojection example
f2f = (1-sign(abs(x)-0.5)).*(1-sign(abs(y)-0.5))/4;
f2f = f2f-(1-sign(abs(x+y)/sqrt(2)-0.5)).*(1-sign(abs(x-y)/sqrt(2)-0.5))/4;
f2f=abs(f2f);