Probabilistic Principal Components Analysis. [VAR, U, LAMBDA] = PPCA(X, PPCA_DIM) computes the principal

％　　component subspace U of dimension PPCA_DIM using a centred covariance

matrix X. The variable VAR contains the off-subspace variance (which

is assumed to be spherical), while the vector LAMBDA contains the

variances of each of the principal components. This is computed

using the eigenvalue and eigenvector decomposition of X.