# gmm

## Purpose

Creates a Gaussian mixture model with specified architecture.

## Synopsis

    mix = gmm(dim, ncentres, covartype)
    mix = gmm(dim, ncentres, covartype, ppca_dim)

## Description

`mix = gmm(dim, ncentres, covartype)` takes the dimension of the space `dim`, the number of centres in the mixture model and the type of the mixture model, and returns a data structure `mix`. The mixture model type defines the covariance structure of each component Gaussian:

      'spherical' = single variance parameter for each component: stored as a vector
      'diag' = diagonal matrix for each component: stored as rows of a matrix
      'full' = full matrix for each component: stored as 3d array
      'ppca' = probabilistic PCA: stored as principal components (in a 3d array
        and associated variances and off-subspace noise

`mix = gmm(dim, ncentres, covartype, ppca_dim)` also sets the dimension of the PPCA sub-spaces: the default value is one.

The priors are initialised to equal values summing to one, and the covariances are all the identity matrix (or equivalent). The centres are initialised randomly from a zero mean unit variance Gaussian. This makes use of the MATLAB function `randn` and so the seed for the random weight initialisation can be set using `randn('state', s)` where `s` is the state value.

The fields in `mix` are

      type = 'gmm'
      nin = the dimension of the space
      ncentres = number of mixture components
      covartype = string for type of variance model
      priors = mixing coefficients
      centres = means of Gaussians: stored as rows of a matrix
      covars = covariances of Gaussians

The additional fields for mixtures of PPCA are

      U = principal component subspaces
      lambda = in-space covariances: stored as rows of a matrix

The off-subspace noise is stored in `covars`.

## Example

    mix = gmm(2, 4, 'spherical');

This creates a Gaussian mixture model with 4 components in 2 dimensions. The covariance structure is a spherical model.

## See Also

`gmmpak`, `gmmunpak`, `gmmsamp`, `gmminit`, `gmmem`, `gmmactiv`, `gmmpost`, `gmmprob`

Pages: Index

Copyright (c) Ian T Nabney (1996-9)