.. vim: set fileencoding=utf-8 :
.. Laurent El Shafey <Laurent.El-Shafey@idiap.ch>
.. Wed Mar 14 12:31:35 2012 +0100
..
.. Copyright (C) 2011-2014 Idiap Research Institute, Martigny, Switzerland

.. testsetup:: *

   import numpy
   numpy.set_printoptions(precision=3, suppress=True)

   import bob.learn.misc

   import os
   import tempfile
   current_directory = os.path.realpath(os.curdir)
   temp_dir = tempfile.mkdtemp(prefix='bob_doctest_')
   os.chdir(temp_dir)

============
 User guide
============

This section includes the machine/trainer guides for learning techniques
available in this package.

Machines
--------

Machines are one of the core components of |project|. They represent
statistical models or other functions defined by parameters that can be learnt
or set by using Trainers.

K-means machines
================

`k-means <http://en.wikipedia.org/wiki/K-means_clustering>`_ is a clustering
method which aims to partition a set of observations into :math:`k` clusters.
The `training` procedure is described further below. Here, we explain only how
to use the resulting machine. For the sake of example, we create a new
:py:class:`bob.learn.misc.KMeansMachine` as follows:

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> machine = bob.learn.misc.KMeansMachine(2,3) # Two clusters with a feature dimensionality of 3
   >>> machine.means = numpy.array([[1,0,0],[0,0,1]], 'float64') # Defines the two clusters

Then, given some input data, it is possible to determine to which cluster the
data is the closest as well as the min distance.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> sample = numpy.array([2,1,-2], 'float64')
   >>> print(machine.get_closest_mean(sample)) # Returns the index of the closest mean and the distance to it at the power of 2
   (0, 6.0)


Gaussian machines
=================

The :py:class:`bob.learn.misc.Gaussian` represents a `multivariate diagonal
Gaussian (or normal) distribution
<http://en.wikipedia.org/wiki/Multivariate_normal_distribution>`_. In this
context, a *diagonal* Gaussian refers to the covariance matrix of the
distribution being diagonal. When the covariance matrix is diagonal, each
variable in the distribution is independent of the others.

Objects of this class are normally used as building blocks for more complex
:py:class:`bob.learn.misc.GMMMachine` or GMM objects, but can also be used
individually. Here is how to create one multivariate diagonal Gaussian
distribution:

.. doctest::

  >>> g = bob.learn.misc.Gaussian(2) #bi-variate diagonal normal distribution
  >>> g.mean = numpy.array([0.3, 0.7], 'float64')
  >>> g.mean
  array([ 0.3,  0.7])
  >>> g.variance = numpy.array([0.2, 0.1], 'float64')
  >>> g.variance
  array([ 0.2,  0.1])

Once the :py:class:`bob.learn.misc.Gaussian` has been set, you can use it to
estimate the log-likelihood of an input feature vector with a matching number
of dimensions:

.. doctest::

  >>> log_likelihood = g(numpy.array([0.4, 0.4], 'float64'))

As with other machines you can save and re-load machines of this type using
:py:meth:`bob.learn.misc.Gaussian.save` and the class constructor
respectively.

Gaussian mixture models
=======================

The :py:class:`bob.learn.misc.GMMMachine` represents a Gaussian `mixture model
<http://en.wikipedia.org/wiki/Mixture_model>`_ (GMM), which consists of a
mixture of weighted :py:class:`bob.learn.misc.Gaussian`\s.

.. doctest::

  >>> gmm = bob.learn.misc.GMMMachine(2,3) # Mixture of two diagonal Gaussian of dimension 3

By default, the diagonal Gaussian distributions of the GMM are initialized with
zero mean and unit variance, and the weights are identical. This can be updated
using the :py:attr:`bob.learn.misc.GMMMachine.means`,
:py:attr:`bob.learn.misc.GMMMachine.variances` or
:py:attr:`bob.learn.misc.GMMMachine.weights`.

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> gmm.weights = numpy.array([0.4, 0.6], 'float64')
  >>> gmm.means = numpy.array([[1, 6, 2], [4, 3, 2]], 'float64')
  >>> gmm.variances = numpy.array([[1, 2, 1], [2, 1, 2]], 'float64')
  >>> gmm.means
  array([[ 1.,  6.,  2.],
       [ 4.,  3.,  2.]])

Once the :py:class:`bob.learn.misc.GMMMachine` has been set, you can use it to
estimate the log-likelihood of an input feature vector with a matching number
of dimensions:

.. doctest::

  >>> log_likelihood = gmm(numpy.array([5.1, 4.7, -4.9], 'float64'))

As with other machines you can save and re-load machines of this type using
:py:meth:`bob.learn.misc.GMMMachine.save` and the class constructor respectively.

Gaussian mixture models Statistics
==================================

The :py:class:`bob.learn.misc.GMMStats` is a container for the sufficient
statistics of a GMM distribution.

Given a GMM, the sufficient statistics of a sample can be computed as
follows:

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> gs = bob.learn.misc.GMMStats(2,3)
  >>> sample = numpy.array([0.5, 4.5, 1.5])
  >>> gmm.acc_statistics(sample, gs)
  >>> print(gs) # doctest: +SKIP

Then, the sufficient statistics can be accessed (or set as below), by
considering the following attributes.

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> gs = bob.learn.misc.GMMStats(2,3)
  >>> log_likelihood = -3. # log-likelihood of the accumulated samples
  >>> T = 1 # Number of samples used to accumulate statistics
  >>> n = numpy.array([0.4, 0.6], 'float64') # zeroth order stats
  >>> sumpx = numpy.array([[1., 2., 3.], [4., 5., 6.]], 'float64') # first order stats
  >>> sumpxx = numpy.array([[10., 20., 30.], [40., 50., 60.]], 'float64') # second order stats
  >>> gs.log_likelihood = log_likelihood
  >>> gs.t = T
  >>> gs.n = n
  >>> gs.sum_px = sumpx
  >>> gs.sum_pxx = sumpxx

Joint Factor Analysis
=====================

Joint Factor Analysis (JFA) [1]_ [2]_ is a session variability modelling
technique built on top of the Gaussian mixture modelling approach. It utilises
a within-class subspace :math:`U`, a between-class subspace :math:`V`, and a
subspace for the residuals :math:`D` to capture and suppress a significant
portion of between-class variation.

An instance of :py:class:`bob.learn.misc.JFABase` carries information about
the matrices :math:`U`, :math:`V` and :math:`D`, which can be shared between
several classes.  In contrast, after the enrolment phase, an instance of
:py:class:`bob.learn.misc.JFAMachine` carries class-specific information about
the latent variables :math:`y` and :math:`z`.

An instance of :py:class:`bob.learn.misc.JFABase` can be initialized as
follows, given an existing GMM:

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> jfa_base = bob.learn.misc.JFABase(gmm,2,2) # dimensions of U and V are both equal to 2
  >>> U = numpy.array([[1, 2], [3, 4], [5, 6], [7, 8], [9, 10], [11, 12]], 'float64')
  >>> V = numpy.array([[6, 5], [4, 3], [2, 1], [1, 2], [3, 4], [5, 6]], 'float64')
  >>> d = numpy.array([0, 1, 0, 1, 0, 1], 'float64')
  >>> jfa_base.u = U
  >>> jfa_base.v = V
  >>> jfa_base.d = d

Next, this :py:class:`bob.learn.misc.JFABase` can be shared by several
instances of :py:class:`bob.learn.misc.JFAMachine`, the initialization being
as follows:

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> m = bob.learn.misc.JFAMachine(jfa_base)
  >>> m.y = numpy.array([1,2], 'float64')
  >>> m.z = numpy.array([3,4,1,2,0,1], 'float64')


Once the :py:class:`bob.learn.misc.JFAMachine` has been configured for a
specific class, the log-likelihood (score) that an input sample belongs to the
enrolled class, can be estimated, by first computing the GMM sufficient
statistics of this input sample, and then calling the
:py:meth:`bob.learn.misc.JFAMachine.forward` on the sufficient statistics.

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> gs = bob.learn.misc.GMMStats(2,3)
  >>> gmm.acc_statistics(sample, gs)
  >>> score = m.forward(gs)

As with other machines you can save and re-load machines of this type using
:py:meth:`bob.learn.misc.JFAMachine.save` and the class constructor
respectively.


Inter-Session Variability
=========================

Similarly to Joint Factor Analysis, Inter-Session Variability (ISV) modelling
[3]_ [2]_ is another session variability modelling technique built on top of
the Gaussian mixture modelling approach. It utilises a within-class subspace
:math:`U` and a subspace for the residuals :math:`D` to capture and suppress a
significant portion of between-class variation. The main difference compared to
JFA is the absence of the between-class subspace :math:`V`.

Similarly to JFA, an instance of :py:class:`bob.learn.misc.JFABase` carries
information about the matrices :math:`U` and :math:`D`, which can be shared
between several classes, whereas an instance of
:py:class:`bob.learn.misc.JFAMachine` carries class-specific information about
the latent variable :math:`z`.

An instance of :py:class:`bob.learn.misc.ISVBase` can be initialized as
follows, given an existing GMM:

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> isv_base = bob.learn.misc.ISVBase(gmm,2) # dimension of U is equal to 2
  >>> isv_base.u = U
  >>> isv_base.d = d

Next, this :py:class:`bob.learn.misc.ISVBase` can be shared by several
instances of :py:class:`bob.learn.misc.ISVMachine`, the initialization being
as follows:

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> m = bob.learn.misc.ISVMachine(isv_base)
  >>> m.z = numpy.array([3,4,1,2,0,1], 'float64')

Once the :py:class:`bob.learn.misc.ISVMachine` has been configured for a
specific class, the log-likelihood (score) that an input sample belongs to the
enrolled class, can be estimated, by first computing the GMM sufficient
statistics of this input sample, and then calling the
:py:meth:`bob.learn.misc.ISVMachine.forward` on the sufficient statistics.

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> gs = bob.learn.misc.GMMStats(2,3)
  >>> gmm.acc_statistics(sample, gs)
  >>> score = m.forward(gs)

As with other machines you can save and re-load machines of this type using
:py:meth:`bob.learn.misc.ISVMachine.save` and the class constructor
respectively.


Total Variability (i-vectors)
=============================

Total Variability (TV) modelling [4]_ is a front-end initially introduced for
speaker recognition, which aims at describing samples by vectors of low
dimensionality called ``i-vectors``. The model consists of a subspace :math:`T`
and a residual diagonal covariance matrix :math:`\Sigma`, that are then used to
extract i-vectors, and is built upon the GMM approach.

An instance of the class :py:class:`bob.learn.misc.IVectorMachine` carries
information about these two matrices. This can be initialized as follows:

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> m = bob.learn.misc.IVectorMachine(gmm, 2)
  >>> m.t = numpy.array([[1.,2],[4,1],[0,3],[5,8],[7,10],[11,1]])
  >>> m.sigma = numpy.array([1.,2.,1.,3.,2.,4.])


Once the :py:class:`bob.learn.misc.IVectorMachine` has been set, the
extraction of an i-vector :math:`w_{ij}` can be done in two steps, by first
extracting the GMM sufficient statistics, and then estimating the i-vector:

.. doctest::
  :options: +NORMALIZE_WHITESPACE

  >>> gs = bob.learn.misc.GMMStats(2,3)
  >>> gmm.acc_statistics(sample, gs)
  >>> w_ij = m.forward(gs)

As with other machines you can save and re-load machines of this type using
:py:meth:`bob.learn.misc.IVectorMachine.save` and the class constructor
respectively.


Probabilistic Linear Discriminant Analysis (PLDA)
=================================================

Probabilistic Linear Discriminant Analysis [5]_ [6]_ is a probabilistic model
that incorporates components describing both between-class and within-class
variations. Given a mean :math:`\mu`, between-class and within-class subspaces
:math:`F` and :math:`G` and residual noise :math:`\epsilon` with zero mean and
diagonal covariance matrix :math:`\Sigma`, the model assumes that a sample
:math:`x_{i,j}` is generated by the following process:

.. math::

   x_{i,j} = \mu + F h_{i} + G w_{i,j} + \epsilon_{i,j}

Information about a PLDA model (:math:`\mu`, :math:`F`, :math:`G` and
:math:`\Sigma`) are carried out by an instance of the class
:py:class:`bob.learn.misc.PLDABase`.

.. doctest::

   >>> ### This creates a PLDABase container for input feature of dimensionality 3,
   >>> ### and with subspaces F and G of rank 1 and 2 respectively.
   >>> pldabase = bob.learn.misc.PLDABase(3,1,2)

Class-specific information (usually from enrollment samples) are contained in
an instance of :py:class:`bob.learn.misc.PLDAMachine`, that must be attached
to a given :py:class:`bob.learn.misc.PLDABase`. Once done, log-likelihood
computations can be performed.

.. doctest::

   >>> plda = bob.learn.misc.PLDAMachine(pldabase)
   >>> samples = numpy.array([[3.5,-3.4,102], [4.5,-4.3,56]], dtype=numpy.float64)
   >>> loglike = plda.compute_log_likelihood(samples)


Trainers
--------

In the previous section, the concept of a `machine` was introduced. A `machine`
is fed by some input data, processes it and returns an output. Machines can be
learnt using trainers in |project|.

K-means
=======

**k-means** [7]_ is a clustering method, which aims to partition a set of
observations into :math:`k` clusters. This is an `unsupervised` technique. As
for **PCA** [1]_, which is implemented in the :py:class:`bob.learn.linear.PCATrainer`
class, the training data is passed in a 2D :py:class:`numpy.ndarray` container.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> data = numpy.array([[3,-3,100], [4,-4,98], [3.5,-3.5,99], [-7,7,-100], [-5,5,-101]], dtype='float64')

The training procedure will learn the `means` for the
:py:class:`bob.learn.misc.KMeansMachine`. The number :math:`k` of `means` is given
when creating the `machine`, as well as the dimensionality of the features.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> kmeans = bob.learn.misc.KMeansMachine(2, 3) # Create a machine with k=2 clusters with a dimensionality equal to 3

Then training procedure for `k-means` is an **Expectation-Maximization**-based
[8]_ algorithm. There are several options that can be set such as the maximum
number of iterations and the criterion used to determine if the convergence has
occurred. After setting all of these options, the training procedure can then
be called.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> kmeansTrainer = bob.learn.misc.KMeansTrainer()
   >>> kmeansTrainer.max_iterations = 200
   >>> kmeansTrainer.convergence_threshold = 1e-5

   >>> kmeansTrainer.train(kmeans, data) # Train the KMeansMachine
   >>> print(kmeans.means)
   [[ -6.   6.  -100.5]
    [  3.5 -3.5   99. ]]


Maximum likelihood for Gaussian mixture model
=============================================

A Gaussian **mixture model** (GMM) [9]_ is a common probabilistic model. In
order to train the parameters of such a model it is common to use a
**maximum-likelihood** (ML) approach [10]_. To do this we use an
**Expectation-Maximization** (EM) algorithm [8]_. Let's first start by creating
a :py:class:`bob.learn.misc.GMMMachine`. By default, all of the Gaussian's have
zero-mean and unit variance, and all the weights are equal. As a starting
point, we could set the mean to the one obtained with **k-means** [7]_.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> gmm = bob.learn.misc.GMMMachine(2,3) # Create a machine with 2 Gaussian and feature dimensionality 3
   >>> gmm.means = kmeans.means # Set the means to the one obtained with k-means

The |project| class to learn the parameters of a GMM [9]_ using ML [10]_ is
:py:class:`bob.learn.misc.ML_GMMTrainer`. It uses an **EM**-based [8]_ algorithm
and requires the user to specify which parameters of the GMM are updated at
each iteration (means, variances and/or weights). In addition, and as for
**k-means** [7]_, it has parameters such as the maximum number of iterations
and the criterion used to determine if the parameters have converged.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> trainer = bob.learn.misc.ML_GMMTrainer(True, True, True) # update means/variances/weights at each iteration
   >>> trainer.convergence_threshold = 1e-5
   >>> trainer.max_iterations = 200
   >>> trainer.train(gmm, data)
   >>> print(gmm) # doctest: +SKIP


MAP-adaptation for Gaussian mixture model
=========================================

|project| also supports the training of GMMs [9]_ using a **maximum a
posteriori** (MAP) approach [11]_. MAP is closely related to the ML [10]_
technique but it incorporates a prior on the quantity that we want to estimate.
In our case, this prior is a GMM [9]_. Based on this prior model and some
training data, a new model, the MAP estimate, will be `adapted`.

Let's consider that the previously trained GMM [9]_ is our prior model.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> print(gmm) # doctest: +SKIP

The training data used to compute the MAP estimate [11]_ is again stored in a
2D :py:class:`numpy.ndarray` container.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> dataMAP = numpy.array([[7,-7,102], [6,-6,103], [-3.5,3.5,-97]], dtype='float64')

The |project| class used to perform MAP adaptation training [11]_ is
:py:class:`bob.learn.misc.MAP_GMMTrainer`. As with the ML estimate [10]_, it uses
an **EM**-based [8]_ algorithm and requires the user to specify which parts of
the GMM are adapted at each iteration (means, variances and/or weights). In
addition, it also has parameters such as the maximum number of iterations and
the criterion used to determine if the parameters have converged, in addition
to this there is also a relevance factor which indicates the importance we give
to the prior.  Once the trainer has been created, a prior GMM [9]_ needs to be
set.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> relevance_factor = 4.
   >>> trainer = bob.learn.misc.MAP_GMMTrainer(relevance_factor, True, False, False) # mean adaptation only
   >>> trainer.convergence_threshold = 1e-5
   >>> trainer.max_iterations = 200
   >>> trainer.set_prior_gmm(gmm)
   True
   >>> gmmAdapted = bob.learn.misc.GMMMachine(2,3) # Create a new machine for the MAP estimate
   >>> trainer.train(gmmAdapted, dataMAP)
   >>> print(gmmAdapted) # doctest: +SKIP


Joint Factor Analysis
=====================

The training of the subspace :math:`U`, :math:`V` and :math:`D` of a Joint
Factor Analysis model, is performed in two steps. First, GMM sufficient
statistics of the training samples should be computed against the UBM GMM. Once
done, we get a training set of GMM statistics:

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> F1 = numpy.array( [0.3833, 0.4516, 0.6173, 0.2277, 0.5755, 0.8044, 0.5301, 0.9861, 0.2751, 0.0300, 0.2486, 0.5357]).reshape((6,2))
   >>> F2 = numpy.array( [0.0871, 0.6838, 0.8021, 0.7837, 0.9891, 0.5341, 0.0669, 0.8854, 0.9394, 0.8990, 0.0182, 0.6259]).reshape((6,2))
   >>> F=[F1, F2]

   >>> N1 = numpy.array([0.1379, 0.1821, 0.2178, 0.0418]).reshape((2,2))
   >>> N2 = numpy.array([0.1069, 0.9397, 0.6164, 0.3545]).reshape((2,2))
   >>> N=[N1, N2]

   >>> gs11 = bob.learn.misc.GMMStats(2,3)
   >>> gs11.n = N1[:,0]
   >>> gs11.sum_px = F1[:,0].reshape(2,3)
   >>> gs12 = bob.learn.misc.GMMStats(2,3)
   >>> gs12.n = N1[:,1]
   >>> gs12.sum_px = F1[:,1].reshape(2,3)

   >>> gs21 = bob.learn.misc.GMMStats(2,3)
   >>> gs21.n = N2[:,0]
   >>> gs21.sum_px = F2[:,0].reshape(2,3)
   >>> gs22 = bob.learn.misc.GMMStats(2,3)
   >>> gs22.n = N2[:,1]
   >>> gs22.sum_px = F2[:,1].reshape(2,3)

   >>> TRAINING_STATS = [[gs11, gs12], [gs21, gs22]]

In the following, we will allocate a :py:class:`bob.learn.misc.JFABase` machine,
that will then be trained.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

    >>> jfa_base = bob.learn.misc.JFABase(gmm, 2, 2) # the dimensions of U and V are both equal to 2

Next, we initialize a trainer, which is an instance of
:py:class:`bob.learn.misc.JFATrainer`, as follows:

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> jfa_trainer = bob.learn.misc.JFATrainer(10) # 10 is the number of iterations

The training process is started by calling the
:py:meth:`bob.learn.misc.JFATrainer.train`.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> jfa_trainer.train(jfa_base, TRAINING_STATS)

Once the training is finished (i.e. the subspaces :math:`U`, :math:`V` and
:math:`D` are estimated), the JFA model can be shared and used by several
class-specific models.  As for the training samples, we first need to extract
GMM statistics from the samples.  These GMM statistics are manually defined in
the following.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> Ne = numpy.array([0.1579, 0.9245, 0.1323, 0.2458]).reshape((2,2))
   >>> Fe = numpy.array([0.1579, 0.1925, 0.3242, 0.1234, 0.2354, 0.2734, 0.2514, 0.5874, 0.3345, 0.2463, 0.4789, 0.5236]).reshape((6,2))
   >>> gse1 = bob.learn.misc.GMMStats(2,3)
   >>> gse1.n = Ne[:,0]
   >>> gse1.sum_px = Fe[:,0].reshape(2,3)
   >>> gse2 = bob.learn.misc.GMMStats(2,3)
   >>> gse2.n = Ne[:,1]
   >>> gse2.sum_px = Fe[:,1].reshape(2,3)
   >>> gse = [gse1, gse2]

Class-specific enrollment can then be perfomed as follows. This will estimate
the class-specific latent variables :math:`y` and :math:`z`:

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> m = bob.learn.misc.JFAMachine(jfa_base)
   >>> jfa_trainer.enrol(m, gse, 5) # where 5 is the number of enrollment iterations

More information about the training process can be found in [12]_ and [13]_.


Inter-Session Variability
=========================

The training of the subspace :math:`U` and :math:`D` of an Inter-Session
Variability model, is performed in two steps. As for JFA, GMM sufficient
statistics of the training samples should be computed against the UBM GMM. Once
done, we get a training set of GMM statistics.  Next, we will allocate an
:py:class:`bob.learn.misc.ISVBase` machine, that will then be trained.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

    >>> isv_base = bob.learn.misc.ISVBase(gmm, 2) # the dimensions of U is equal to 2

Next, we initialize a trainer, which is an instance of
:py:class:`bob.learn.misc.ISVTrainer`, as follows:

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> isv_trainer = bob.learn.misc.ISVTrainer(10, 4.) # 10 is the number of iterations, and 4 is the relevance factor

The training process is started by calling the
:py:meth:`bob.learn.misc.ISVTrainer.train`.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> isv_trainer.train(isv_base, TRAINING_STATS)

Once the training is finished (i.e. the subspaces :math:`V` and :math:`D` are
estimated), the ISV model can be shared and used by several class-specific
models.  As for the training samples, we first need to extract GMM statistics
from the samples.  Class-specific enrollment can then be perfomed, which will
estimate the class-specific latent variable :math:`z`:

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> m = bob.learn.misc.ISVMachine(isv_base)
   >>> isv_trainer.enrol(m, gse, 5) # where 5 is the number of iterations

More information about the training process can be found in [14]_ and [13]_.


Total Variability (i-vectors)
=============================

The training of the subspace :math:`T` and :math:`\Sigma` of a Total
Variability model, is performed in two steps. As for JFA and ISV, GMM
sufficient statistics of the training samples should be computed against the
UBM GMM. Once done, we get a training set of GMM statistics.  Next, we will
allocate an instance of :py:class:`bob.learn.misc.IVectorMachine`, that will
then be trained.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

    >>> m = bob.learn.misc.IVectorMachine(gmm, 2)
    >>> m.variance_threshold = 1e-5


Next, we initialize a trainer, which is an instance of
:py:class:`bob.learn.misc.IVectorTrainer`, as follows:

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> ivec_trainer = bob.learn.misc.IVectorTrainer(update_sigma=True, max_iterations=10)
   >>> TRAINING_STATS_flatten = [gs11, gs12, gs21, gs22]

The training process is started by calling the
:py:meth:`bob.learn.misc.IVectorTrainer.train`.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> ivec_trainer.train(m, TRAINING_STATS_flatten)

More information about the training process can be found in [15]_.

Probabilistic Linear Discriminant Analysis (PLDA)
=================================================

Probabilistic Linear Discriminant Analysis [16]_ is a probabilistic model that
incorporates components describing both between-class and within-class
variations. Given a mean :math:`\mu`, between-class and within-class subspaces
:math:`F` and :math:`G` and residual noise :math:`\epsilon` with zero mean and
diagonal covariance matrix :math:`\Sigma`, the model assumes that a sample
:math:`x_{i,j}` is generated by the following process:

.. math::

   x_{i,j} = \mu + F h_{i} + G w_{i,j} + \epsilon_{i,j}


An Expectaction-Maximization algorithm can be used to learn the parameters of
this model :math:`\mu`, :math:`F` :math:`G` and :math:`\Sigma`. As these
parameters can be shared between classes, there is a specific container class
for this purpose, which is :py:class:`bob.learn.misc.PLDABase`. The process is
described in detail in [17]_.

Let us consider a training set of two classes, each with 3 samples of
dimensionality 3.

.. doctest::
   :options: +NORMALIZE_WHITESPACE

   >>> data1 = numpy.array([[3,-3,100], [4,-4,50], [40,-40,150]], dtype=numpy.float64)
   >>> data2 = numpy.array([[3,6,-50], [4,8,-100], [40,79,-800]], dtype=numpy.float64)
   >>> data = [data1,data2]

Learning a PLDA model can be performed by instantiating the class
:py:class:`bob.learn.misc.PLDATrainer`, and calling the
:py:meth:`bob.learn.misc.PLDATrainer.train()` method.

.. doctest::

   >>> ### This creates a PLDABase container for input feature of dimensionality 3,
   >>> ### and with subspaces F and G of rank 1 and 2 respectively.
   >>> pldabase = bob.learn.misc.PLDABase(3,1,2)

   >>> trainer = bob.learn.misc.PLDATrainer()
   >>> trainer.train(pldabase, data)

Once trained, this PLDA model can be used to compute the log-likelihood of a
set of samples given some hypothesis. For this purpose, a
:py:class:`bob.learn.misc.PLDAMachine` should be instantiated. Then, the
log-likelihood that a set of samples share the same latent identity variable
:math:`h_{i}` (i.e. the samples are coming from the same identity/class) is
obtained by calling the
:py:meth:`bob.learn.misc.PLDAMachine.compute_log_likelihood()` method.

.. doctest::

   >>> plda = bob.learn.misc.PLDAMachine(pldabase)
   >>> samples = numpy.array([[3.5,-3.4,102], [4.5,-4.3,56]], dtype=numpy.float64)
   >>> loglike = plda.compute_log_likelihood(samples)

If separate models for different classes need to be enrolled, each of them with
a set of enrolment samples, then, several instances of
:py:class:`bob.learn.misc.PLDAMachine` need to be created and enroled using
the :py:meth:`bob.learn.misc.PLDATrainer.enrol()` method as follows.

.. doctest::

   >>> plda1 = bob.learn.misc.PLDAMachine(pldabase)
   >>> samples1 = numpy.array([[3.5,-3.4,102], [4.5,-4.3,56]], dtype=numpy.float64)
   >>> trainer.enrol(plda1, samples1)
   >>> plda2 = bob.learn.misc.PLDAMachine(pldabase)
   >>> samples2 = numpy.array([[3.5,7,-49], [4.5,8.9,-99]], dtype=numpy.float64)
   >>> trainer.enrol(plda2, samples2)

Afterwards, the joint log-likelihood of the enrollment samples and of one or
several test samples can be computed as previously described, and this
separately for each model.

.. doctest::

   >>> sample = numpy.array([3.2,-3.3,58], dtype=numpy.float64)
   >>> l1 = plda1.compute_log_likelihood(sample)
   >>> l2 = plda2.compute_log_likelihood(sample)

In a verification scenario, there are two possible hypotheses: 1.
:math:`x_{test}` and :math:`x_{enrol}` share the same class.  2.
:math:`x_{test}` and :math:`x_{enrol}` are from different classes.  Using the
methods :py:meth:`bob.learn.misc.PLDAMachine.forward` or
:py:meth:`bob.learn.misc.PLDAMachine.__call__` function, the corresponding
log-likelihood ratio will be computed, which is defined in more formal way by:
:math:`s = \ln(P(x_{test},x_{enrol})) - \ln(P(x_{test})P(x_{enrol}))`

.. doctest::

   >>> s1 = plda1(sample)
   >>> s2 = plda2(sample)

.. testcleanup:: *

  import shutil
  os.chdir(current_directory)
  shutil.rmtree(temp_dir)

.. Place here your external references
.. include:: links.rst
.. [1] http://dx.doi.org/10.1109/TASL.2006.881693
.. [2] http://publications.idiap.ch/index.php/publications/show/2606
.. [3] http://dx.doi.org/10.1016/j.csl.2007.05.003
.. [4] http://dx.doi.org/10.1109/TASL.2010.2064307
.. [5] http://dx.doi.org/10.1109/ICCV.2007.4409052
.. [6] http://doi.ieeecomputersociety.org/10.1109/TPAMI.2013.38

.. [7] http://en.wikipedia.org/wiki/K-means_clustering
.. [8] http://en.wikipedia.org/wiki/Expectation-maximization_algorithm
.. [9] http://en.wikipedia.org/wiki/Mixture_model
.. [10] http://en.wikipedia.org/wiki/Maximum_likelihood
.. [11] http://en.wikipedia.org/wiki/Maximum_a_posteriori_estimation
.. [12] http://dx.doi.org/10.1109/TASL.2006.881693
.. [13] http://publications.idiap.ch/index.php/publications/show/2606
.. [14] http://dx.doi.org/10.1016/j.csl.2007.05.003
.. [15] http://dx.doi.org/10.1109/TASL.2010.2064307
.. [16] http://dx.doi.org/10.1109/ICCV.2007.4409052
.. [17] http://doi.ieeecomputersociety.org/10.1109/TPAMI.2013.38

