function gd=gabdual(g,a,M,varargin)
%GABDUAL Canonical dual window of Gabor frame
% Usage: gd=gabdual(g,a,M);
% gd=gabdual(g,a,M,L);
% gd=gabdual(g,a,M,'lt',lt);
%
% Input parameters:
% g : Gabor window.
% a : Length of time shift.
% M : Number of channels.
% L : Length of window. (optional)
% lt : Lattice type (for non-separable lattices).
% Output parameters:
% gd : Canonical dual window.
%
% GABDUAL(g,a,M) computes the canonical dual window of the discrete Gabor
% frame with window g and parameters a, M.
%
% The window g may be a vector of numerical values, a text string or a
% cell array. See the help of GABWIN for more details.
%
% If the length of g is equal to M, then the input window is assumed
% to be an FIR window. In this case, the canonical dual window also has
% length of M. Otherwise the smallest possible transform length is chosen
% as the window length.
%
% GABDUAL(g,a,M,L) returns a window that is the dual window for a system
% of length L. Unless the dual window is a FIR window, the dual window
% will have length L.
%
% GABDUAL(g,a,M,'lt',lt) does the same for a non-separable lattice
% specified by lt. Please see the help of MATRIX2LATTICETYPE for a
% precise description of the parameter lt.
%
% If a>M then the dual window of the Gabor Riesz sequence with window
% g and parameters a and M will be calculated.
%
% Examples:
% ---------
%
% The following example shows the canonical dual window of the Gaussian
% window:
%
% a=20;
% M=30;
% L=300;
% g=pgauss(L,a*M/L);
% gd=gabdual(g,a,M);
%
% % Simple plot in the time-domain
% figure(1);
% plot(gd);
%
% % Frequency domain
% figure(2);
% magresp(gd,'dynrange',100);
%
% See also: gabtight, gabwin, fir2long, dgt
%
% Url: http://ltfat.github.io/doc/gabor/gabdual.html
% Copyright (C) 2005-2023 Peter L. Soendergaard <peter@sonderport.dk> and others.
% This file is part of LTFAT version 2.6.0
%
% This program is free software: you can redistribute it and/or modify
% it under the terms of the GNU General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
%
% This program is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with this program. If not, see <http://www.gnu.org/licenses/>.
% AUTHOR : Peter L. Søndergaard.
% TESTING: TEST_DGT
% REFERENCE: REF_GABDUAL.
%% ---------- Assert correct input.
if nargin<3
error('%s: Too few input parameters.',upper(mfilename));
end;
definput.keyvals.L=[];
definput.keyvals.lt=[0 1];
definput.keyvals.nsalg=0;
[flags,kv,L]=ltfatarghelper({'L'},definput,varargin);
%% ------ step 2: Verify a, M and L
if isempty(L)
if isnumeric(g)
% Use the window length
Ls=length(g);
else
% Use the smallest possible length
Ls=1;
end;
% ----- step 2b : Verify a, M and get L from the window length ----------
L=dgtlength(Ls,a,M,kv.lt);
else
% ----- step 2a : Verify a, M and get L
Luser=dgtlength(L,a,M,kv.lt);
if Luser~=L
error(['%s: Incorrect transform length L=%i specified. Next valid length ' ...
'is L=%i. See the help of DGTLENGTH for the requirements.'],...
upper(mfilename),L,Luser);
end;
end;
%% ----- step 3 : Determine the window
[g,info]=gabwin(g,a,M,L,kv.lt,'callfun',upper(mfilename));
if L<info.gl
error('%s: Window is too long.',upper(mfilename));
end;
R=size(g,2);
% -------- Are we in the Riesz sequence of in the frame case
scale=1;
if a>M*R
% Handle the Riesz basis (dual lattice) case.
% Swap a and M, and scale differently.
scale=a/M;
tmp=a;
a=M;
M=tmp;
end;
% -------- Compute -------------
if kv.lt(2)==1
% Rectangular case
if (info.gl<=M) && (R==1)
% Diagonal of the frame operator
d = gabframediag(g,a,M,L);
gd=g./long2fir(d,info.gl);
else
% Long window case
% Just in case, otherwise the call is harmless.
g=fir2long(g,L);
gd=comp_gabdual_long(g,a,M)*scale;
end;
else
% Non-separable case
g=fir2long(g,L);
if (kv.nsalg==1) || (kv.nsalg==0 && kv.lt(2)<=2)
mwin=comp_nonsepwin2multi(g,a,M,kv.lt,L);
gdfull=comp_gabdual_long(mwin,a*kv.lt(2),M)*scale;
% We need just the first vector
gd=gdfull(:,1);
else
[s0,s1,br] = shearfind(L,a,M,kv.lt);
if s1 ~= 0
p1 = comp_pchirp(L,s1);
g = p1.*g;
end
b=L/M;
Mr = L/br;
ar = a*b/br;
if s0 == 0
gd=comp_gabdual_long(g,ar,Mr);
else
p0=comp_pchirp(L,-s0);
g = p0.*fft(g);
gd=comp_gabdual_long(g,L/Mr,L/ar)*L;
gd = ifft(conj(p0).*gd);
end
if s1 ~= 0
gd = conj(p1).*gd;
end
end;
if (info.gl<=M) && (R==1)
gd=long2fir(gd,M);
end;
end;
% --------- post process result -------
if isreal(g) && (kv.lt(2)==1 || kv.lt(2)==2)
% If g is real and the lattice is either rectangular or quinqux, then
% the output is known to be real.
gd=real(gd);
end;
if info.wasrow
gd=gd.';
end;