【matlab】频域滤波函数lpfilt（）

function [ H, D ] = lpfilter( type,M,N,D0,n )  %LPFILTER creates the transfer function of a lowpass filter.  %   Detailed explanation goes here  %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