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% STK_VARIANCE_EVAL [experimental]
%
% CALL: V = stk_variance_eval (NOISEMODEL, X, DIFF)
%
% EXPERIMENTAL FUNCTION WARNING
%
% This function is currently considered experimental, since the whole
% 'noise model classes' thing is.
%
% Classes derived from stk_gaussiannoise_ must implement this to define the
% value of the variance at any given point x.
%
% In the future, it might be a good idea to make such a function part of
% the "standard API" of all models in STK, together with similar
% functions such as:
%
% * M = stk_mean_eval (MODEL, X, DIFF)
% * V = stk_variance_eval (NOISEMODEL, X, DIFF)
% * Q = stk_quantile_eval (NOISEMODEL, X, DIFF)
% * ...
%
% STK users that wish to experiment with this function, and more
% generally with noise model objects, are welcome to do so, but should
% be aware that the API is very likely to change in the future.
% Please send questions, comments and suggestions about this part of the
% toolbox to the STK mailing list.
% Copyright Notice
%
% Copyright (C) 2018 CentraleSupelec
%
% Author: Julien Bect <julien.bect@centralesupelec.fr>
% Copying Permission Statement
%
% This file is part of
%
% STK: a Small (Matlab/Octave) Toolbox for Kriging
% (http://sourceforge.net/projects/kriging)
%
% STK 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.
%
% STK 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 STK. If not, see <http://www.gnu.org/licenses/>.
function v = stk_variance_eval (gn, x, diff)
% Defaut value for 'diff' (arg #3): -1
if nargin < 3, diff = -1; end
switch diff
case {-1, 1}
% -1 means "no derivative"
% +1 means "derivative wrt log_dispersion"
v = exp (gn.log_dispersion) * feval (gn.variance_function, double (x));
otherwise
stk_error ('diff should be either -1 or +1', 'IncorrectArgument');
end
end % function
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