function [H, D] = lpfilter(type,M,N,D0,n)
[U, V] = dftuv(M, N);
D = sqrt(U.^2 + V.^2);
switch type
    case 'ideal'
        H = double(D <= D0);
    case 'btw'
        if nargin == 4
            n = 1;
        end
        H = 1./(1+(D./D0).^(2*n));
    case 'gaussian'
        H = exp(-(D.^2)./(2*(D0^2)));
    otherwise
        error('Unknow Filter Type.')
end