function [H, D] = lpfilter(type, M, N, D0, n)
% LPFILTER Computes frequency domain lowpass filters
% H = LPFILTER(TYPE, M, N, D0, n) creates the transfer function of a
% lowpass filter, H, of the specified TYPE and size (M-by-N). To view the
% filter as an image or mesh plot, it should be centered using H =
% fftshift(H)
% Valid value for TYPE, D0, and n are:
% 'ideal' Ideal lowpass filter with cutoff frequency D0. n need not be
% supplied. D0 must be positive.
% 'btw' Butterworth lowpass filter of order n, and cutoff D0. The
% default value for n is 1.0. D0 must be positive.
% 'gaussian'Gaussian lowpass filter with cutoff (standard deviation) D0.
% n need not be supplied. D0 must be positive.
%
% 得到指定类型的低通滤波器
% Use function dftuv to set up the meshgrid arrays needed for computing the
% required distances.
[U, V] = dftuv(M, N);
% Compute the distances D(U, V)
D = sqrt(U.^2 + V.^2);
% Begin filter computations
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('Unkown filter type.')
end
