function [a] = fourfold_rev1(x)

middle_pixel = (size(x,1)+1)/2;
if x(middle_pixel,middle_pixel) ~= 0
    mount = x ~= 0;
    x = double(x);
    a = zeros(size(x));
    b = uint8(zeros(size(x)));
    for i = 1:4
        a = a + x;
        b = b + uint8(mount);
        x = rot90(x);
        mount = rot90(mount);
    end
    a = a./double(b);
    a = sum((x(mount)-a(mount)).^2)/double(sum(mount(:)));
    a = sqrt(a);
else
    a = 0;
end