A 2D-wavelet packet decomposition is defined by a quad-tree and a \verb+Fsignal+ (or
a pair of \verb+Fsignal+ in case of biorthogonal bases).  
At each leaf of the tree,  coefficients
mostly contain the information in a particular sub-band of
the Fourier transform of the image. (See \cite{mallat:wavelet} and
references therein and \cite{malgouyres:framework} where this is
illustrated.) 

However, the ordering of these frequency sub-bands does not correspond
to most natural way to order a wavelet packet structure. We therefore
chose to implement the wavelet packet in a straightforward way, and to
makes the correspondence between wavelet packet indexes and frequency
bands only when necessary.

The correspondence is detailed in \cite{mallat:wavelet}.

One can find examples of the use of \verb+wp2dfreqorder+ in the
modules \verb+wp2mktree+, \verb+wp2deigenval+ and \verb+wp2dview+.
