function F = defocus(size)
% utilisation: M=defocus(size);
% cree un filtre defocus de diametre size
% size doit etre impair
F=zeros(size,size);
X=[1:size]-(1+size)/2;
rayoncar=(size-1)*(size-1)/4;
for i=X
   for j=X
     F(i+(1+size)/2,j+(1+size)/2) = ((i*i+j*j)<=rayoncar);
   end;
end;
F=F/sum(sum(F));
