alf of these column transforms, however,
are of imaginary parts, and should therefore be multiplied by <em>i</em>
and combined with the r2hc transforms of the real columns to produce the
2d DFT amplitudes; FFTW&rsquo;s r2r transform does <em>not</em> perform this
combination for you.  Thus, if a multi-dimensional real-input/output DFT
is required, we recommend using the ordinary r2c/c2r
interface (see <a href="Multi_002dDimensional-DFTs-of-Real-Data.html#Multi_002dDimensional-DFTs-of-Real-Data">Multi-Dimens