%IDEAL_RESPONSE_ZP Design an ideal zero phase filter in the frequency 
%   domain using the specifications given in the text file "filename".
%
%   filename: the full path to the text file containing the specifications
%   for the frequency response of the ideal filter. The first two columns
%   of the text file contains the horizontal and the vertical frequency
%   components of the points located on pass band(S) and stop band(S) of
%   frequency response and the third column contains their respected 
%   values. NOTE: since this is a zero phase filter, the user only needs to
%   provide the specifications for one half-plane of the frequency response
%   of the ideal filter.
%
%   Npt: the number of points to be used in the frequency response (64 is
%   recommended).
%   Hd: the frequency response of the output filter.
%   
%   A general suggestion when designing the ideal filter is to truncate the
%   sharp corners located on the bands of the frequency response of the
%   filter. This would allow for a better solution when using
%   filter_design_zp_unc or filter_design_zp_con to design a filter based
%   on this ideal filter. The following example makes use of this
%   suggestion.
%
%   Example
%   -------
%   Design a simple zero phase low pass filter located 45 degreess
%   diagonally in the frequency domain.
%   The filter specifications are given in the following textfile:
%   Content of "zerophase.txt"
%   0     0     1
%   0.1   0     1
%   0.2   0.1   1
%   0.3   0.2   1
%   0.3   0.25  1
%   0.275 0.275 1
%   0.25  0.3   1
%   0.2   0.3   1
%   0.1   0.2   1
%   0     0.1   1
%  -0.1   0     1
%   0.2   0     0
%   0.3   0.1   0
%   0.4   0.2   0
%   0.4   0.3   0
%   0.35  0.35  0
%   0.3   0.4   0
%   0.2   0.4   0
%   0.1   0.3   0
%   0     0.2   0
%  -0.1   0.1   0
%  -0.2   0     0
%   0.5   0     0
%   0     0.5   0
%   0.5   0.5   0
%  -0.5   0     0
%  -0.5   0.5   0
%   -------------------------------------------
%   Hd_zp=ideal_response_zp('zerophase.txt',64);
%
function Hd = ideal_response(filename,Npt)
%filename contains the (full path if not in the same work folder)
%filename of the text file.
%Npt = number of points in ideal response in each dimension.
a = textread(filename);
X = [a(:,1)' -a(:,1)']*2;
Y = [a(:,2)' -a(:,2)']*2;
Z = [a(:,3)' a(:,3)'];
[f1,f2] = freqspace(Npt);
Hd = griddata(X,Y,Z,f1,f2');
[F1,F2] = meshgrid(f1,f2);