function H=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('Unknown filter type');
end