Level 2 Help for FFTMAG

BY specifies the desired expansion factor of the output picture.  It should be
equal to 2^N where N is an integer.  If BY is not an integer power of two, it
will be rounded down as necessary. If BY is defaulted, it will be computed from
the SIZE specification; if BY and SIZE are specified, BY will take precedence.

BYL is the same as BY, except only in the line direction. The default if BY
wasn't specified is to calculate BYL from the SIZE specification.

BYS is the same as BY, except only in the sample direction. The default if BY
wasn't specified is to calculate BYS from the SIZE specification.