y=extract(x)

function y=extract(x)
[s1 s2]=size(x);
nb=s1*s2/4;
mm=1;
for ii=1:2:s1
    for jj=1:2:s1
        yj=x(ii:ii+1,jj:jj+1);
        y(mm,:)=yj(:)';
        mm=mm+1;
    end
end