The EM (Expectation Maximization) algorithm estimates the parameters of the multivariate probability density function in the form of a Gaussian mixture distribution with a specified number of mixtures.
.. highlight:: cpp
The Expectation Maximization(EM) algorithm estimates the parameters of the multivariate probability density function in the form of a Gaussian mixture distribution with a specified number of mixtures.
Consider the set of the N feature vectors
Consider the set of the N feature vectors
{ :math:`x_1, x_2,...,x_{N}` } from a d-dimensional Euclidean space drawn from a Gaussian mixture:
{ :math:`x_1, x_2,...,x_{N}` } from a d-dimensional Euclidean space drawn from a Gaussian mixture:
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@@ -59,7 +61,7 @@ At the second step (Maximization step or M-step), the mixture parameter estimate
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@@ -59,7 +61,7 @@ At the second step (Maximization step or M-step), the mixture parameter estimate
Alternatively, the algorithm may start with the M-step when the initial values for
Alternatively, the algorithm may start with the M-step when the initial values for
:math:`p_{i,k}` can be provided. Another alternative when
:math:`p_{i,k}` can be provided. Another alternative when
:math:`p_{i,k}` are unknown is to use a simpler clustering algorithm to pre-cluster the input samples and thus obtain initial
:math:`p_{i,k}` are unknown is to use a simpler clustering algorithm to pre-cluster the input samples and thus obtain initial
:math:`p_{i,k}` . Often (including ML) the
:math:`p_{i,k}` . Often (including macnine learning) the
:ref:`kmeans` algorithm is used for that purpose.
:ref:`kmeans` algorithm is used for that purpose.
One of the main problems of the EM algorithm is a large number
One of the main problems of the EM algorithm is a large number
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@@ -83,22 +85,16 @@ already a good enough approximation).
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@@ -83,22 +85,16 @@ already a good enough approximation).
*
*
Bilmes98 J. A. Bilmes. *A Gentle Tutorial of the EM Algorithm and its Application to Parameter Estimation for Gaussian Mixture and Hidden Markov Models*. Technical Report TR-97-021, International Computer Science Institute and Computer Science Division, University of California at Berkeley, April 1998.
Bilmes98 J. A. Bilmes. *A Gentle Tutorial of the EM Algorithm and its Application to Parameter Estimation for Gaussian Mixture and Hidden Markov Models*. Technical Report TR-97-021, International Computer Science Institute and Computer Science Division, University of California at Berkeley, April 1998.
.. index:: CvEMParams
.. _CvEMParams:
CvEMParams
CvEMParams
----------
----------
.. c:type:: CvEMParams
.. ocv:class:: CvEMParams
Parameters of the EM algorithm.
Parameters of the EM algorithm.
All parameters are public. You can initialize them by a constructor and then override some of them directly if you want.
All parameters are public. You can initialize them by a constructor and then override some of them directly if you want.
.. index:: CvEMParams::CvEMParams
.. _CvEMParams::CvEMParams:
CvEMParams::CvEMParams
CvEMParams::CvEMParams
----------------------
----------------------
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@@ -144,26 +140,16 @@ The default constructor represents a rough rule-of-the-thumb:
...
@@ -144,26 +140,16 @@ The default constructor represents a rough rule-of-the-thumb:
With another contstructor it is possible to override a variety of parameters from a single number of mixtures (the only essential problem-dependent parameter) to initial values for the mixture parameters.
With another contstructor it is possible to override a variety of parameters from a single number of mixtures (the only essential problem-dependent parameter) to initial values for the mixture parameters.
.. index:: CvEM
.. _CvEM:
CvEM
CvEM
----
----
.. c:type:: CvEM
.. ocv:class:: CvEM
The EM model.
The class implements the EM algorithm as described in the beginning of this section.
.. index:: CvEM::train
The class implements the EM algorithm as described in the beginning of this section.
:param labels: The optional output "class label" for each sample: :math:`\texttt{labels}_i=\texttt{arg max}_k(p_{i,k}), i=1..N` (indices of the most probable mixture component for each sample).
:param labels: The optional output "class label" for each sample: :math:`\texttt{labels}_i=\texttt{arg max}_k(p_{i,k}), i=1..N` (indices of the most probable mixture component for each sample).
Estimates the Gaussian mixture parameters from a sample set.
Unlike many of the ML models, EM is an unsupervised learning algorithm and it does not take responses (class labels or function values) as input. Instead, it computes the
Unlike many of the ML models, EM is an unsupervised learning algorithm and it does not take responses (class labels or function values) as input. Instead, it computes the
*Maximum Likelihood Estimate* of the Gaussian mixture parameters from an input sample set, stores all the parameters inside the structure:
*Maximum Likelihood Estimate* of the Gaussian mixture parameters from an input sample set, stores all the parameters inside the structure:
:math:`p_{i,k}` in ``probs``,
:math:`p_{i,k}` in ``probs``,
:math:`a_k` in ``means`` ,
:math:`a_k` in ``means`` ,
:math:`S_k` in ``covs[k]``,
:math:`S_k` in ``covs[k]``,
:math:`\pi_k` in ``weights`` ,
:math:`\pi_k` in ``weights`` , and optionally computes the output "class label" for each sample:
:math:`\texttt{labels}_i=\texttt{arg max}_k(p_{i,k}), i=1..N` (indices of the most probable mixture for each sample).
The trained model can be used further for prediction, just like any other classifier. The trained model is similar to the
The trained model can be used further for prediction, just like any other classifier. The trained model is similar to the
:ref:`Bayes classifier`.
:ref:`Bayes classifier`.
For an example of clustering random samples of the multi-Gaussian distribution using EM, see ``em.cpp`` sample in the OpenCV distribution.
.. index:: CvEM::predict
.. _CvEM::predict:
CvEM::predict
CvEM::predict
-------------
-------------
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@@ -205,10 +192,6 @@ CvEM::predict
...
@@ -205,10 +192,6 @@ CvEM::predict
:param probs: If it is not null then the method will write posterior probabilities of each component given the sample data to this parameter.
:param probs: If it is not null then the method will write posterior probabilities of each component given the sample data to this parameter.
.. index:: CvEM::getNClusters
.. _CvEM::getNClusters:
CvEM::getNClusters
CvEM::getNClusters
------------------
------------------
.. ocv:function:: int CvEM::getNClusters() const
.. ocv:function:: int CvEM::getNClusters() const
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@@ -218,10 +201,6 @@ CvEM::getNClusters
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@@ -218,10 +201,6 @@ CvEM::getNClusters
Returns the number of mixture components :math:`M` in the gaussian mixture model.
Returns the number of mixture components :math:`M` in the gaussian mixture model.
