Replaces the columns of a dense real matrix with their discrete
cosine transforms. The second argument, an integer between 1 and 4,
denotes the type of transform (DCT-I, DCT-II, DCT-III, DCT-IV).
The DCT-I transform requires that the row dimension of X is at
least 2.
These transforms are defined as follows
(for a matrix with n rows).
idct(
X[, type=2])
Replaces the columns of a dense real matrix with the inverses
of the discrete cosine transforms defined above.