ldc.m

The pattern recognition matlab toolbox

%LDC Linear Bayes Normal Classifier (BayesNormal_1)
%
%   [W.R,S,M] = LDC(A,R,S,M)
%   W = A*LDC([],R,S,M);
% 
% INPUT
%   A    Dataset
%   R,S  Regularization parameters, 0 <= R,S <= 1
%        (optional; default: no regularization, i.e. R,S = 0)
%   M    Dimension of subspace structure in covariance matrix (default: K,
%        all dimensions)
%
% OUTPUT
%   W    Linear Bayes Normal Classifier mapping
%   R    Value of regularization parameter R as used 
%   S    Value of regularization parameter S as used
%   M    Value of regularization parameter M as used
%
% DESCRIPTION  
% Computation of the linear classifier between the classes of the dataset A
% by assuming normal densities with equal covariance matrices. The joint
% covariance matrix is the weighted (by a priori probabilities) average of
% the class covariance matrices. R and S (0 <= R,S <= 1) are regularization
% parameters used for finding the covariance matrix G by:
%
%      G = (1-R-S)*G + R*diag(diag(G)) + S*mean(diag(G))*eye(size(G,1))
%
% This covariance matrix is then decomposed as G = W*W' + sigma^2 * eye(K),
% where W is a K x M matrix containing the M leading principal components
% and sigma^2 is the mean of the K-M smallest eigenvalues.
%
% The use of soft labels is supported. The classification A*W is computed
% by NORMAL_MAP.
%
% If R, S or M is NaN the regularisation parameter is optimised by REGOPTC.
% The best result are usually obtained by R = 0, S = NaN, M = [], or by
% R = 0, S = 0, M = NaN (which is for problems of moderate or low dimensionality
% faster). If no regularisation is supplied a pseudo-inverse of the
% covariance matrix is used in case it is close to singular.
%
% Note that A*(KLMS([],N)*NMC) performs a similar operation by first
% pre-whitening the data in an N-dimensional space, followed by the
% nearest mean classifier. The regularization controlled by N is different
% from the above in LDC as it entirely removes small variance directions.
%
% To some extend LDC is also similar to FISHERC.
%
% EXAMPLES
% See PREX_PLOTC.
%
% REFERENCES
% 1. R.O. Duda, P.E. Hart, and D.G. Stork, Pattern classification, 2nd edition, 
% John Wiley and Sons, New York, 2001.
% 2. A. Webb, Statistical Pattern Recognition, John Wiley & Sons, New York, 2002.
% 3. C. Liu and H. Wechsler, Robust Coding Schemes for Indexing and Retrieval
% from Large Face Databases, IEEE Transactions on Image Processing, vol. 9, 
% no. 1, 2000, 132-136.
%
%  SEE ALSO
%  MAPPINGS, DATASETS, REGOPTC, NMC, NMSC, LDC, UDC, QUADRC, NORMAL_MAP, FISHERC

% Copyright: R.P.W. Duin, [e-mail]
% Faculty EWI, Delft University of Technology
% P.O. Box 5031, 2600 GA Delft, The Netherlands

% $Id: ldc.m,v 1.9 2008/01/25 10:16:23 duin Exp $

function [W,r,s,dim] = ldc(a,r,s,dim)

	prtrace(mfilename);

	if (nargin < 4)
    prwarning(4,'subspace dimensionality M not given, assuming K');
    dim = [];
  end
	if (nargin < 3) | isempty(s)
		prwarning(4,'Regularisation parameter S not given, assuming 0.');
		s = 0; 
	end
	if (nargin < 2) | isempty(r)
		prwarning(4,'Regularisation parameter R not given, assuming 0.');
		r = 0;
	end

	if (nargin < 1) | (isempty(a))  % No input arguments: 
		W = mapping(mfilename,{r,s,dim}); % return an untrained mapping.
	
	elseif any(isnan([r,s,dim]))    % optimize regularisation parameters
			defs = {0,0,[]};
			parmin_max = [1e-8,9.9999e-1;1e-8,9.9999e-1;1,size(a,2)];
			[W,r,s,dim] = regoptc(a,mfilename,{r,s,dim},defs,[3 2 1],parmin_max,testc([],'soft'),[1 1 0]);
	
	else % training

		islabtype(a,'crisp','soft');
		isvaldfile(a,2,2); % at least 2 object per class, 2 classes

		[m,k,c] = getsize(a);

		% Calculate mean vectors, priors and the covariance matrix G.
		%DXD: for large datasets (high dim, many classes) it is impossible
		%to keep all cov. matrices into memory. We have to loop over the
		%individual classes:
		%[U,G] = meancov(a);
		U = meancov(a);
		w.mean  = +U;
		w.prior = getprior(a);
		%G = reshape(sum(reshape(G,k*k,c)*w.prior',2),k,k);
		[tmpU,G] = meancov(seldat(a,1)); 
		G = w.prior(1)*G;
		for i=2:c
			[tmpU,tmpG] = meancov(seldat(a,i)); 
			G = G + w.prior(i)*tmpG;
		end
		clear tmpG;

		% Regularize 
		if (s > 0) | (r > 0)
			G = (1-r-s)*G + r * diag(diag(G)) + s*mean(diag(G))*eye(size(G,1));
		end	
	
    if (dim < k)
			dim = min(rank(G)-1,dim);
     	[eigvec,eigval] = eig(G); eigval = diag(eigval);
     	[dummy,ind] = sort(-eigval);
			% Estimate sigma^2 as avg. eigenvalue outside subspace.
     	sigma2 = mean(eigval(ind(dim+1:end)));
     	% Subspace basis: first DIM eigenvectors * sqrt(eigenvalues).
     	G = eigvec(:,ind(1:dim)) * diag(eigval(ind(1:dim))) * eigvec(:,ind(1:dim))' + ...
          sigma2 * eye(k);
   	end
		
		w.cov = G;
		W = normal_map(w,getlab(U),k,c);
		W = setcost(W,a);
		
	end
	
	W = setname(W,'Bayes-Normal-1');
	
return