[var, U, lambda] = pca(x, ppca_dim)
[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.
eigdec, pcaCopyright (c) Ian T Nabney (1996-9)