%MAPPINGS Info on the mapping class construction of PRTools% % This is not a command, just an information file% % In the Pattern Recognition Toolbox PRTools there are many commands to train % and use mappings between spaces of different (or equal) dimensionalities. For % example:% %    if    A  is a m x k dataset (m objects in a k-dimensional space)%    and   W  is a k x n mapping (map from k to n dimensions)%    then A*W is a m x n dataset (m objects in a n-dimensional space)% % Mappings can be linear (e.g. a rotation) as well as nonlinear (e.g. a neural % network). Typically they can be used for classifiers. In that case a k x n % mapping maps a k-feature data vector on the output space of an n-class % classifier (exception: 2-class classifiers like discriminant functions may be % implemented by a mapping to a 1-dimensional space like the distance to the % discriminant, n = 1).% % Mappings are of the datatype 'mapping' (class(W) is 'mapping'), have a size % of [k,n] if they map from k to n dimensions. Mappings can be instructed to % assign labels to the output columns, e.g. the class names. These labels can % be retrieved by% %    labels = getlab(W); before the mapping, or%    labels = getlab(A*W); after the dataset A is mapped by W.% % Mappings can be learned from examples, (labeled) objects stored in a dataset % A (see datasets), for instance by training a classifier:% %    W3 = ldc(A);       the normal densities based linear classifier%    W2 = knnc(A,3);    the 3-nearest neighbor rule%    W1 = svc(A,'p',2); the support vector classifier based on  a 2-nd order%                       polynomial kernel% % Untrained or empty mappings are sometimes very useful. Here the dataset is % replaced by an empty set or entirely skipped: %    V1 = ldc; V2 = knnc([],a); V3 = svc([],'p'2);% Such mappings can be trained later by%    W1 = A*V1; W2 = A*V2; W3 = A*V3;% The mapping of a testset B by B*W1 is now equivalent to B*(A*V1) or even, % irregulary but very handy to A*V1*B (or even A*ldc*B). Note that expressions % are evaluated from left to right, so B*A*V1 may result  in an error as the % multiplication of the two datasets (B*A) is executed first.% % Users can add new mappings or classifiers by a single routine that should% support the following type of calls:%% W = newmapm([], par1, par2, ...); Defines the untrained, empty mapping.% W = newmapm(A, par1, par2, ...); Defines the map trained by dataset A.% B = newmapm(A, W); Defines the mapping of  A on W, resulting in B.%% For an example list the routine subsc.m.%% Some trainable mappings do not depend on class labels and can be interpreted % as finding a space that approximates as good as possible the original dataset % given some conditions and measures. Examples are the Karhunen-Loeve Mapping % (klm) which may be used for PCA and the Support Vector Mapping (svm) by which % nonlinear, kernel PCA mappings can be computed% % In addition to trainable mappings, there are fixed mappings, which operation % is not computed from a trainingset but defined by at most a few parameters. % Most of them can be set by cmapm. Other ones are sigm and invsigm.% % The result D of a mapping of a testset on a trained classifier, %%    D = B*W1%% is again a dataset, storing for each object in B the output values of the % classifier. These values, usually between -inf and inf  can be interpreted as % similarities: the larger, the more similar with the corresponding class. These % number can be mapped on the [0,1] interval by sigm:%%    D = B*W1*sigm%% The values in a single row (object) don't necessarily sum to one. This can be % achieved by the fixed mapping normm:%%    D = B*W1*sigm*normm%% which is equivalent to B*W1*classc. Effectively a mapping W is converted into % a classifier by W*classc, which maps objects on the normalized [0,1] output % space. Usually a mapping that can be converted into a classifier in this way, is% scaled such by a multiplicative constant that these numbers optimally represent% (in the maximum likelihood sense) the posterior probabilities for the training% data. The resulting output dataset D has column labels for the classes and row% labels for the objects. The class labels of the maximum values for each object% can be retrieved by%%    labels = D*classd; or labels = classd(D);%% A global classification error follows from%%    e = D*testd; or e = testd(D);%% Mappings can be combined in the following ways:% % sequential: W = W1 * W2 * W3 (equal inner dimensions)% stacked   : W = [W1, W2, W3] (equal numbers of 'rows' (input dimensions))% parallel  : W = [W1; W2 ;W3] (unrestricted)% % The output size of the parallel mapping is irregulary equal to (k1+k2+k3) x % (n1+n2+n3) as the output combining of columns is undefined. In a stacked or % parallel mapping columns with the same label can be combined by various % combiners like maxc, meanc and prodc. If the classifiers W1, W2 and W3 are % trained for the same n classes, their output labels are the same and may be% combined by W = prodc([W1;W2;W3]) into a (k1+k2+k3) x n classifier.% % W for itself, or display(W) lists the size and type of a classifier as well % as the routine or section in @mapping/mtimes used for computing a mapping A*W.% The construction of a combined mapping may be inspected by parsc(W). % % A mapping may be given an outputselection by W = W(:,J), in which J is a set % of indices pointing to the desired classes.%%    B = A*W(:,J); is equivalent to B = A*W; B = B(:,J);%% Input selection is not possible for a mapping.% % A classifier may be given a reject option by rejectc.%% User defined mappings% ---------------------%% Users may define their own mapping (e.g. mapexm) using the mapping constructor.% In this constructor call the first parameter should be the name of the routine% that handles the mapping of an incoming dataset. If the mapping is trained% by W = mapexm(A,pars) and W is set in mapexm by W = mapping('mapexm', ...), % then PRTools calls mapexm(A,W) for the evaluation of A*W. An untrained mapping% should be defined inside mapexm by W = mapping('mapexm',{pars}), to be called% by W = A*mapexm([],pars). See subsc.m for an example.%% Fixed mappings are defined as W = mapping('mapexm','fixed',{parameters}).% The parameters are the ones needed for calling mapexm, so A*W is evaluated% as mapexm(A,p1,p2,...) if {parameters} = {p1,p2}. Fixed mappings are not called% for training and have usually an undetermined size. Size checking is thereby not% done. An example is cmapm.%% Combiner mappings are defined as W = mapping('mapexm','combiner',{parameters}).% They can be functions of other mappings. An example is is classc. The call% V = ldc(A)*classc is thereby evaluated as V = classc(ldc(A)), resulting% in a new mapping V. Typically this will be a trained mapping or a fixed mapping.%% A trained mapping is computed for a training set, e.g. W = ldc(A). It thereby% differs from a fixed mapping. Consequently it has determined sizes for the% dimensions of the input and the output space. The first is typically the% feature size of A. For classifiers the dimension of the output space is% typically the number of classes. Two-class classifiers may return an output% space of just one dimension, i.e. the distance to the separation function.%% An untrained mapping just stores the mapping or classifier to be used for% later training, e.g. W = klm([],0.95). Now A*W is evaluated as klm(A,0.95).% Untrained mappings may also be combined, both sequentially as well as% stacked. An example is W = klm([],0.95) * fisherc. The training command% A*W is now evaluated as klm(A,0.95)*fisherc(A*klm(A,0.95)). Another example% is W = [nmc fisherc qdc]*maxc. B*(A*W) is now evaluated as % maxc([B*nmc(A) B*fisherc(A) B*qdc(A)]).%% Differences between the four types of mappings can now be summarized as:%% A*W     -  fixed     : results in a dataset, no size checking%         -  combiner  : treated as fixed%         -  untrained : results in a mapping%         -  trained   : results in a dataset, size checking%% A*(V*W) - % evaluated as V = A*ldc (training), followed by W = classc(V).