function y = gcyl(r) % autocorrelation of cyl(r) y = zeros(size(r)); r = abs(r); idx = find(r<1); x = r(idx); y(idx) = (2.0/pi)*(acos(x)-x.*sqrt(1-x.*x));
function y = gcyl(r) % autocorrelation of cyl(r) y = zeros(size(r)); r = abs(r); idx = find(r<1); x = r(idx); y(idx) = (2.0/pi)*(acos(x)-x.*sqrt(1-x.*x));