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<H1><A NAME="SEC16">FFTW Reference</A></H1>

<P>
This chapter provides a complete reference for all sequential (i.e.,
one-processor) FFTW functions. We first define the data types upon
which FFTW operates, that is, real, complex, and "halfcomplex" numbers
(see Section <A HREF="fftw_3.html#SEC17">Data Types</A>). Then, in four sections, we explain the FFTW
program interface for complex one-dimensional transforms
(see Section <A HREF="fftw_3.html#SEC18">One-dimensional Transforms Reference</A>), complex
multi-dimensional transforms (see Section <A HREF="fftw_3.html#SEC24">Multi-dimensional Transforms Reference</A>), and real one-dimensional transforms (see Section <A HREF="fftw_3.html#SEC29">Real One-dimensional Transforms Reference</A>), real multi-dimensional
transforms (see Section <A HREF="fftw_3.html#SEC34">Real Multi-dimensional Transforms Reference</A>).
Section <A HREF="fftw_3.html#SEC41">Wisdom Reference</A> describes the <CODE>wisdom</CODE> mechanism for
exporting and importing plans. Finally, Section <A HREF="fftw_3.html#SEC45">Memory Allocator Reference</A> describes how to change FFTW's default memory allocator.
For parallel transforms, See Section <A HREF="fftw_4.html#SEC47">Parallel FFTW</A>.

<H2><A NAME="SEC17">Data Types</A></H2>
<P>
<A NAME="IDX98"></A>
<A NAME="IDX99"></A>
<A NAME="IDX100"></A>

<P>
The routines in the FFTW package use three main kinds of data types.
<EM>Real</EM> and <EM>complex</EM> numbers should be already known to the
reader. We also use the term <EM>halfcomplex</EM> to describe complex
arrays in a special packed format used by the one-dimensional real
transforms (taking advantage of the <EM>hermitian</EM> symmetry that arises
in those cases).

<P>
By including <CODE><fftw.h></CODE> or <CODE><rfftw.h></CODE>, you will have access
to the following definitions:

<PRE>
typedef double fftw_real;

typedef struct {
fftw_real re, im;
} fftw_complex;

#define c_re(c) ((c).re)
#define c_im(c) ((c).im)
</PRE>

<P>
<A NAME="IDX101"></A>
<A NAME="IDX102"></A>

<P>
All FFTW operations are performed on the <CODE>fftw_real</CODE> and
<CODE>fftw_complex</CODE> data types. For <CODE>fftw_complex</CODE> numbers, the
two macros <CODE>c_re</CODE> and <CODE>c_im</CODE> retrieve, respectively, the real
and imaginary parts of the number.

<P>
A <EM>real array</EM> is an array of real numbers. A <EM>complex array</EM>
is an array of complex numbers. A one-dimensional array X of
n complex numbers is <EM>hermitian</EM> if the following property
holds:
for all 0 <= i < n, we have X<sub>i</sub> = conj(X<sub>n-i</sub>)}.
Hermitian arrays are relevant to FFTW because the Fourier transform of a
real array is hermitian.

<P>
Because of its symmetry, a hermitian array can be stored in half the
space of a complex array of the same size. FFTW's one-dimensional real
transforms store hermitian arrays as <EM>halfcomplex</EM> arrays. A
halfcomplex array of size n is
<A NAME="IDX103"></A>
a one-dimensional array of n <CODE>fftw_real</CODE> numbers. A
hermitian array X in stored into a halfcomplex array Y as
follows.
For all integers i such that 0 <= i <= n / 2, we have
Y<sub>i</sub> = Re(X<sub>i</sub>). For all integers i such that 0
< i < n / 2, we have Y<sub>n-i</sub> = Im(X<sub>i</sub>).

<P>
We now illustrate halfcomplex storage for n = 4 and n = 5,
since the scheme depends on the parity of n. Let n = 4.
In this case, we have
Y<sub>0</sub> = Re(X<sub>0</sub>), Y<sub>1</sub> = Re(X<sub>1</sub>),
Y<sub>2</sub> = Re(X<sub>2</sub>), and Y<sub>3</sub> = Im(X<sub>1</sub>).
Let now n = 5. In this case, we have
Y<sub>0</sub> = Re(X<sub>0</sub>), Y<sub>1</sub> = Re(X<sub>1</sub>),
Y<sub>2</sub> = Re(X<sub>2</sub>), Y<sub>3</sub> = Im(X<sub>2</sub>),
and Y<sub>4</sub> = Im(X<sub>1</sub>).

<P>
<A NAME="IDX104"></A>
By default, the type <CODE>fftw_real</CODE> equals the C type <CODE>double</CODE>.
To work in single precision rather than double precision, <CODE>#define</CODE>
the symbol <CODE>FFTW_ENABLE_FLOAT</CODE> in <CODE>fftw.h</CODE> and then recompile
the library. On Unix systems, you can instead use <CODE>configure
--enable-float</CODE> at installation time (see Section <A HREF="fftw_6.html#SEC66">Installation and Customization</A>).
<A NAME="IDX105"></A>
<A NAME="IDX106"></A>

<P>
In version 1 of FFTW, the data types were called <CODE>FFTW_REAL</CODE> and
<CODE>FFTW_COMPLEX</CODE>. We changed the capitalization for consistency with
the rest of FFTW's conventions. The old names are still supported, but
their use is deprecated.
<A NAME="IDX107"></A>
<A NAME="IDX108"></A>

<H2><A NAME="SEC18">One-dimensional Transforms Reference</A></H2>

<P>
The one-dimensional complex routines are generally prefixed with
<CODE>fftw_</CODE>. Programs using FFTW should be linked with <CODE>-lfftw
-lm</CODE> on Unix systems, or with the FFTW and standard math libraries in
general.

<H3><A NAME="SEC19">Plan Creation for One-dimensional Transforms</A></H3>

<PRE>
#include <fftw.h>

fftw_plan fftw_create_plan(int n, fftw_direction dir,
int flags);

fftw_plan fftw_create_plan_specific(int n, fftw_direction dir,
int flags,
fftw_complex *in, int istride,
fftw_complex *out, int ostride);
</PRE>

<P>
<A NAME="IDX109"></A>
<A NAME="IDX110"></A>
<A NAME="IDX111"></A>
<A NAME="IDX112"></A>

<P>
The function <CODE>fftw_create_plan</CODE> creates a plan, which is
a data structure containing all the information that <CODE>fftw</CODE>
needs in order to compute the 1D Fourier transform. You can
create as many plans as you need, but only one plan for a given
array size is required (a plan can be reused many times).

<P>
<CODE>fftw_create_plan</CODE> returns a valid plan, or <CODE>NULL</CODE>
if, for some reason, the plan can't be created. In the
default installation, this cannot happen, but it is possible
to configure FFTW in such a way that some input sizes are
forbidden, and FFTW cannot create a plan.

<P>
The <CODE>fftw_create_plan_specific</CODE> variant takes as additional
arguments specific input/output arrays and their strides. For the last
four arguments, you should pass the arrays and strides that you will
eventually be passing to <CODE>fftw</CODE>. The resulting plans will be
optimized for those arrays and strides, although they may be used on
other arrays as well. Note: the contents of the in and out arrays are
<EM>destroyed</EM> by the specific planner (the initial contents are
ignored, so the arrays need not have been initialized).

<H4>Arguments</H4>

<UL>
<LI>

<CODE>n</CODE> is the size of the transform. It can be
any positive integer.

<UL>
<LI>

FFTW is best at handling sizes of the form
2<SUP>a</SUP> 3<SUP>b</SUP> 5<SUP>c</SUP> 7<SUP>d</SUP>
11<SUP>e</SUP> 13<SUP>f</SUP>,
where e+f is either 0 or
1, and the other exponents are arbitrary. Other sizes are
computed by means of a slow, general-purpose routine (which nevertheless
retains
O(n lg n)
performance, even for prime sizes). (It is
possible to customize FFTW for different array sizes.
See Section <A HREF="fftw_6.html#SEC66">Installation and Customization</A> for more information.) Transforms
whose sizes are powers of 2 are especially fast.
</UL>

<LI>

<CODE>dir</CODE> is the sign of the exponent in the formula that
defines the Fourier transform. It can be -1 or +1.
The aliases <CODE>FFTW_FORWARD</CODE> and <CODE>FFTW_BACKWARD</CODE>
are provided, where <CODE>FFTW_FORWARD</CODE> stands for -1.

<LI>

<A NAME="IDX113"></A>
<CODE>flags</CODE> is a boolean OR (<SAMP>`|'</SAMP>) of zero or more of the following:

<UL>
<LI>

<CODE>FFTW_MEASURE</CODE>: this flag tells FFTW to find the optimal plan by
actually <EM>computing</EM> several FFTs and measuring their
execution time. Depending on the installation, this can take some
time. <A NAME="DOCF2" HREF="fftw_foot.html#FOOT2">(2)</A>

<LI>

<CODE>FFTW_ESTIMATE</CODE>: do not run any FFT and provide a "reasonable"
plan (for a RISC processor with many registers). If neither
<CODE>FFTW_ESTIMATE</CODE> nor <CODE>FFTW_MEASURE</CODE> is provided, the default is
<CODE>FFTW_ESTIMATE</CODE>.

<LI>

<CODE>FFTW_OUT_OF_PLACE</CODE>: produce a plan assuming that the input and
output arrays will be distinct (this is the default).
<A NAME="IDX114"></A>

<LI>

<A NAME="IDX115"></A>
<CODE>FFTW_IN_PLACE</CODE>: produce a plan assuming that you want the output
in the input array. The algorithm used is not necessarily in place:
FFTW is able to compute true in-place transforms only for small values
of <CODE>n</CODE>. If FFTW is not able to compute the transform in-place, it
will allocate a temporary array (unless you provide one yourself),
compute the transform out of place, and copy the result back.
<EM>Warning: This option changes the meaning of some parameters of
<CODE>fftw</CODE></EM> (see Section <A HREF="fftw_3.html#SEC21">Computing the One-dimensional Transform</A>).

The in-place option is mainly provided for people who want to write
their own in-place multi-dimensional Fourier transform, using FFTW as a
base. For example, consider a three-dimensional <CODE>n * n * n</CODE>
transform. An out-of-place algorithm will need another array (which may
be huge). However, FFTW can compute the in-place transform along
each dimension using only a temporary array of size <CODE>n</CODE>.
Moreover, if FFTW happens to be able to compute the transform truly
in-place, no temporary array and no copying are needed. As distributed,
FFTW `knows' how to compute in-place transforms of size 1, 2, 3, 4, 5, 6,
7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 32 and 64.

The default mode of operation is <CODE>FFTW_OUT_OF_PLACE</CODE>.

<LI>

<A NAME="IDX116"></A>
<CODE>FFTW_USE_WISDOM</CODE>: use any <CODE>wisdom</CODE> that is available to help
in the creation of the plan. (See Section <A HREF="fftw_2.html#SEC13">Words of Wisdom</A>.)
This can greatly speed the creation of plans, especially with the
<CODE>FFTW_MEASURE</CODE> option. <CODE>FFTW_ESTIMATE</CODE> plans can also take
advantage of <CODE>wisdom</CODE> to produce a more optimal plan (based on past
measurements) than the estimation heuristic would normally
generate. When the <CODE>FFTW_MEASURE</CODE> option is used, new <CODE>wisdom</CODE>
will also be generated if the current transform size is not completely
understood by existing <CODE>wisdom</CODE>.

</UL>

<LI>

<CODE>in</CODE>, <CODE>out</CODE>, <CODE>istride</CODE>, <CODE>ostride</CODE> (only for
<CODE>fftw_create_plan_specific</CODE>): see corresponding arguments in the
description of <CODE>fftw</CODE>. (See Section <A HREF="fftw_3.html#SEC21">Computing the One-dimensional Transform</A>.) In particular, the <CODE>out</CODE> and <CODE>ostride</CODE>
parameters have the same special meaning for <CODE>FFTW_IN_PLACE</CODE>
transforms as they have for <CODE>fftw</CODE>.

</UL>

<H3><A NAME="SEC20">Discussion on Specific Plans</A></H3>
<P>
<A NAME="IDX117"></A>
We recommend the use of the specific planners, even in cases where you
will be transforming arrays different from those passed to the specific
planners, as they confer the following advantages:

<UL>

<LI>

The resulting plans will be optimized for your specific arrays and
strides. This may or may not make a significant difference, but it
certainly doesn't hurt. (The ordinary planner does its planning based
upon a stride-one temporary array that it allocates.)

<LI>

Less intermediate storage is required during the planning process. (The
ordinary planner uses O(<CODE>N</CODE>) temporary storage, where <CODE>N</CODE> is
the maximum dimension, while it is creating the plan.)

<LI>

For multi-dimensional transforms, new parameters become accessible for
optimization by the planner. (Since multi-dimensional arrays can be
very large, we don't dare to allocate one in the ordinary planner for
experimentation. This prevents us from doing certain optimizations
that can yield dramatic improvements in some cases.)

</UL>

<P>
On the other hand, note that <EM>the specific planner destroys the
contents of the <CODE>in</CODE> and <CODE>out</CODE> arrays</EM>.

<H3><A NAME="SEC21">Computing the One-dimensional Transform</A></H3>

<PRE>
#include <fftw.h>

void fftw(fftw_plan plan, int howmany,
fftw_complex *in, int istride, int idist,
fftw_complex *out, int ostride, int odist);

void fftw_one(fftw_plan plan, fftw_complex *in,
fftw_complex *out);
</PRE>

<P>
<A NAME="IDX118"></A>
<A NAME="IDX119"></A>

<P>
The function <CODE>fftw</CODE> computes the one-dimensional Fourier transform,
using a plan created by <CODE>fftw_create_plan</CODE> (See Section <A HREF="fftw_3.html#SEC19">Plan Creation for One-dimensional Transforms</A>.) The function
<CODE>fftw_one</CODE> provides a simplified interface for the common case of
single input array of stride 1.
<A NAME="IDX120"></A>

<H4>Arguments</H4>

<UL>
<LI>

<CODE>plan</CODE> is the plan created by <CODE>fftw_create_plan</CODE>
(see Section <A HREF="fftw_3.html#SEC19">Plan Creation for One-dimensional Transforms</A>).

<LI>

<CODE>howmany</CODE> is the number of transforms <CODE>fftw</CODE> will compute.
It is faster to tell FFTW to compute many transforms, instead of
simply calling <CODE>fftw</CODE> many times.

<LI>

<CODE>in</CODE>, <CODE>istride</CODE> and <CODE>idist</CODE> describe the input array(s).
There are <CODE>howmany</CODE> input arrays; the first one is pointed to by
<CODE>in</CODE>, the second one is pointed to by <CODE>in + idist</CODE>, and so on,
up to <CODE>in + (howmany - 1) * idist</CODE>. Each input array consists of
complex numbers (see Section <A HREF="fftw_3.html#SEC17">Data Types</A>), which are not necessarily
contiguous in memory. Specifically, <CODE>in[0]</CODE> is the first element
of the first array, <CODE>in[istride]</CODE> is the second element of the
first array, and so on. In general, the <CODE>i</CODE>-th element of the
<CODE>j</CODE>-th input array will be in position <CODE>in[i * istride + j *
idist]</CODE>.

<LI>

<CODE>out</CODE>, <CODE>ostride</CODE> and <CODE>odist</CODE> describe the output
array(s). The format is the same as for the input array.

<UL>
<LI><EM>In-place transforms</EM>:

<A NAME="IDX121"></A>
If the <CODE>plan</CODE> specifies an in-place transform, <CODE>ostride</CODE> and
<CODE>odist</CODE> are always ignored. If <CODE>out</CODE> is <CODE>NULL</CODE>,
<CODE>out</CODE> is ignored, too. Otherwise, <CODE>out</CODE> is interpreted as a
pointer to an array of <CODE>n</CODE> complex numbers, that FFTW will use as
temporary space to perform the in-place computation. <CODE>out</CODE> is used
as scratch space and its contents destroyed. In this case, <CODE>out</CODE>
must be an ordinary array whose elements are contiguous in memory (no
striding).
</UL>

</UL>

<P>
The function <CODE>fftw_one</CODE> transforms a single, contiguous input array
to a contiguous output array. By definition, the call

<PRE>
fftw_one(plan, in, out)
</PRE>

<P>
is equivalent to

<PRE>
fftw(plan, 1, in, 1, 1, out, 1, 1)
</PRE>

<H3><A NAME="SEC22">Destroying a One-dimensional Plan</A></H3>

<PRE>
#include <fftw.h>

void fftw_destroy_plan(fftw_plan plan);
</PRE>

<P>
<A NAME="IDX122"></A>

<P>
The function <CODE>fftw_destroy_plan</CODE> frees the plan <CODE>plan</CODE> and
releases all the memory associated with it. After destruction, a plan
is no longer valid.

<H3><A NAME="SEC23">What FFTW Really Computes</A></H3>
<P>
<A NAME="IDX123"></A>
In this section, we define precisely what FFTW computes. Please be
warned that different authors and software packages might employ
different conventions than FFTW does.

<P>
The forward transform of a complex array X of size
n computes an array Y, where
<center><IMG SRC="equation-1.gif" ALIGN="top"></center>

<P>
The backward transform computes
<center><IMG SRC="equation-2.gif" ALIGN="top"></center>

<P>
<A NAME="IDX124"></A>
FFTW computes an unnormalized transform, that is, the equation
IFFT(FFT(X)) = n X holds. In other words, applying the forward
and then the backward transform will multiply the input by n.

<P>
<A NAME="IDX125"></A>
An <CODE>FFTW_FORWARD</CODE> transform corresponds to a sign of -1 in
the exponent of the DFT. Note also that we use the standard
"in-order" output ordering--the k-th output corresponds to the
frequency k/n (or k/T, where T is your total
sampling period). For those who like to think in terms of positive and
negative frequencies, this means that the positive frequencies are
stored in the first half of the output and the negative frequencies are
stored in backwards order in the second half of the output. (The
frequency -k/n is the same as the frequency (n-k)/n.)

<H2><A NAME="SEC24">Multi-dimensional Transforms Reference</A></H2>
<P>
<A NAME="IDX126"></A>
<A NAME="IDX127"></A>
The multi-dimensional complex routines are generally prefixed with
<CODE>fftwnd_</CODE>. Programs using FFTWND should be linked with <CODE>-lfftw
-lm</CODE> on Unix systems, or with the FFTW and standard math libraries in
general.
<A NAME="IDX128"></A>

<H3><A NAME="SEC25">Plan Creation for Multi-dimensional Transforms</A></H3>

<PRE>
#include <fftw.h>

fftwnd_plan fftwnd_create_plan(int rank, const int *n,
fftw_direction dir, int flags);

fftwnd_plan fftw2d_create_plan(int nx, int ny,
fftw_direction dir, int flags);

fftwnd_plan fftw3d_create_plan(int nx, int ny, int nz,
fftw_direction dir, int flags);

fftwnd_plan fftwnd_create_plan_specific(int rank, const int *n,
fftw_direction dir,
int flags,
fftw_complex *in, int istride,
fftw_complex *out, int ostride);

fftwnd_plan fftw2d_create_plan_specific(int nx, int ny,
fftw_direction dir,
int flags,
fftw_complex *in, int istride,
fftw_complex *out, int ostride);

fftwnd_plan fftw3d_create_plan_specific(int nx, int ny, int nz,
fftw_direction dir, int flags,
fftw_complex *in, int istride,
fftw_complex *out, int ostride);
</PRE>

<P>
<A NAME="IDX129"></A>
<A NAME="IDX130"></A>
<A NAME="IDX131"></A>
<A NAME="IDX132"></A>
<A NAME="IDX133"></A>
<A NAME="IDX134"></A>
<A NAME="IDX135"></A>
<A NAME="IDX136"></A>

<P>
The function <CODE>fftwnd_create_plan</CODE> creates a plan, which is a data
structure containing all the information that <CODE>fftwnd</CODE> needs in
order to compute a multi-dimensional Fourier transform. You can create
as many plans as you need, but only one plan for a given array size is
required (a plan can be reused many times). The functions
<CODE>fftw2d_create_plan</CODE> and <CODE>fftw3d_create_plan</CODE> are optional,
alternative interfaces to <CODE>fftwnd_create_plan</CODE> for two and three
dimensions, respectively.

<P>
<CODE>fftwnd_create_plan</CODE> returns a valid plan, or <CODE>NULL</CODE> if, for
some reason, the plan can't be created. This can happen if memory runs
out or if the arguments are invalid in some way (e.g. if <CODE>rank</CODE> <
0).

<P>
The <CODE>create_plan_specific</CODE> variants take as additional arguments
specific input/output arrays and their strides. For the last four
arguments, you should pass the arrays and strides that you will
eventually be passing to <CODE>fftwnd</CODE>. The resulting plans will be
optimized for those arrays and strides, although they may be used on
other arrays as well. Note: the contents of the in and out arrays are
<EM>destroyed</EM> by the specific planner (the initial contents are
ignored, so the arrays need not have been initialized).
See Section <A HREF="fftw_3.html#SEC20">Discussion on Specific Plans</A>, for a discussion on specific plans.

<H4>Arguments</H4>

<UL>
<LI>

<CODE>rank</CODE> is the dimensionality of the arrays to be transformed. It
can be any non-negative integer.

<LI>

<CODE>n</CODE> is a pointer to an array of <CODE>rank</CODE> integers, giving the
size of each dimension of the arrays to be transformed. These sizes,
which must be positive integers, correspond to the dimensions of
<A NAME="IDX137"></A>
row-major arrays--i.e. <CODE>n[0]</CODE> is the size of the dimension whose
indices vary most slowly, and so on. (See Section <A HREF="fftw_2.html#SEC7">Multi-dimensional Array Format</A>, for more information on row-major storage.)
See Section <A HREF="fftw_3.html#SEC19">Plan Creation for One-dimensional Transforms</A>,
for more information regarding optimal array sizes.

<LI>

<CODE>nx</CODE> and <CODE>ny</CODE> in <CODE>fftw2d_create_plan</CODE> are positive
integers specifying the dimensions of the rank 2 array to be
transformed. i.e. they specify that the transform will operate on
<CODE>nx x ny</CODE> arrays in row-major order, where <CODE>nx</CODE> is the number
of rows and <CODE>ny</CODE> is the number of columns.

<LI>

<CODE>nx</CODE>, <CODE>ny</CODE> and <CODE>nz</CODE> in <CODE>fftw3d_create_plan</CODE> are
positive integers specifying the dimensions of the rank 3 array to be
transformed. i.e. they specify that the transform will operate on
<CODE>nx x ny x nz</CODE> arrays in row-major order.

<LI>

<CODE>dir</CODE> is the sign of the exponent in the formula that defines the
Fourier transform. It can be -1 or +1. The aliases
<CODE>FFTW_FORWARD</CODE> and <CODE>FFTW_BACKWARD</CODE> are provided, where
<CODE>FFTW_FORWARD</CODE> stands for -1.

<LI>

<A NAME="IDX138"></A>
<CODE>flags</CODE> is a boolean OR (<SAMP>`|'</SAMP>) of zero or more of the following:

<UL>
<LI>

<CODE>FFTW_MEASURE</CODE>: this flag tells FFTW to find the optimal plan by
actually <EM>computing</EM> several FFTs and measuring their execution
time.

<LI>

<CODE>FFTW_ESTIMATE</CODE>: do not run any FFT and provide a "reasonable"
plan (for a RISC processor with many registers). If neither
<CODE>FFTW_ESTIMATE</CODE> nor <CODE>FFTW_MEASURE</CODE> is provided, the default is
<CODE>FFTW_ESTIMATE</CODE>.

<LI>

<CODE>FFTW_OUT_OF_PLACE</CODE>: produce a plan assuming that the input
and output arrays will be distinct (this is the default).

<LI>

<CODE>FFTW_IN_PLACE</CODE>: produce a plan assuming that you want to perform
the transform in-place. (Unlike the one-dimensional transform, this
"really" <A NAME="DOCF3" HREF="fftw_foot.html#FOOT3">(3)</A> performs the
transform in-place.) Note that, if you want to perform in-place
transforms, you <EM>must</EM> use a plan created with this option.

The default mode of operation is <CODE>FFTW_OUT_OF_PLACE</CODE>.

<LI>

<A NAME="IDX139"></A>
<CODE>FFTW_USE_WISDOM</CODE>: use any <CODE>wisdom</CODE> that is available to help
in the creation of the plan. (See Section <A HREF="fftw_2.html#SEC13">Words of Wisdom</A>.) This can greatly
speed the creation of plans, especially with the <CODE>FFTW_MEASURE</CODE>
option. <CODE>FFTW_ESTIMATE</CODE> plans can also take advantage of
<CODE>wisdom</CODE> to produce a more optimal plan (based on past
measurements) than the estimation heuristic would normally
generate. When the <CODE>FFTW_MEASURE</CODE> option is used, new <CODE>wisdom</CODE>
will also be generated if the current transform size is not completely
understood by existing <CODE>wisdom</CODE>. Note that the same <CODE>wisdom</CODE>
is shared between one-dimensional and multi-dimensional transforms.

</UL>

<LI>

<CODE>in</CODE>, <CODE>out</CODE>, <CODE>istride</CODE>, <CODE>ostride</CODE> (only for the
<CODE>_create_plan_specific</CODE> variants): see corresponding arguments in
the description of <CODE>fftwnd</CODE>. (See Section <A HREF="fftw_3.html#SEC26">Computing the Multi-dimensional Transform</A>.)

</UL>

<H3><A NAME="SEC26">Computing the Multi-dimensional Transform</A></H3>

<PRE>
#include <fftw.h>

void fftwnd(fftwnd_plan plan, int howmany,
fftw_complex *in, int istride, int idist,
fftw_complex *out, int ostride, int odist);

void fftwnd_one(fftwnd_plan p, fftw_complex *in,
fftw_complex *out);
</PRE>

<P>
<A NAME="IDX140"></A>
<A NAME="IDX141"></A>

<P>
The function <CODE>fftwnd</CODE> computes the multi-dimensional Fourier
Transform, using a plan created by <CODE>fftwnd_create_plan</CODE>
(see Section <A HREF="fftw_3.html#SEC25">Plan Creation for Multi-dimensional Transforms</A>). (Note that the plan determines the rank and dimensions of
the array to be transformed.) The function <CODE>fftwnd_one</CODE> provides a
simplified interface for the common case of single input array of stride
1.
<A NAME="IDX142"></A>

<H4>Arguments</H4>

<UL>
<LI>

<CODE>plan</CODE> is the plan created by <CODE>fftwnd_create_plan</CODE>.
(see Section <A HREF="fftw_3.html#SEC25">Plan Creation for Multi-dimensional Transforms</A>). In the case of two and three-dimensional transforms, it
could also have been created by <CODE>fftw2d_create_plan</CODE> or
<CODE>fftw3d_create_plan</CODE>, respectively.

<LI>

<CODE>howmany</CODE> is the number of transforms <CODE>fftwnd</CODE> will compute.

<LI>

<CODE>in</CODE>, <CODE>istride</CODE> and <CODE>idist</CODE> describe the input array(s).
There are <CODE>howmany</CODE> input arrays; the first one is pointed to by
<CODE>in</CODE>, the second one is pointed to by <CODE>in + idist</CODE>, and so on,
up to <CODE>in + (howmany - 1) * idist</CODE>. Each input array consists of
complex numbers (see Section <A HREF="fftw_3.html#SEC17">Data Types</A>), stored in row-major format
(see Section <A HREF="fftw_2.html#SEC7">Multi-dimensional Array Format</A>), which are not necessarily
contiguous in memory. Specifically, <CODE>in[0]</CODE> is the first element
of the first array, <CODE>in[istride]</CODE> is the second element of the
first array, and so on. In general, the <CODE>i</CODE>-th element of the
<CODE>j</CODE>-th input array will be in position <CODE>in[i * istride + j *
idist]</CODE>. Note that, here, <CODE>i</CODE> refers to an index into the row-major
format for the multi-dimensional array, rather than an index in any
particular dimension.

<UL>
<LI><EM>In-place transforms</EM>:

<A NAME="IDX143"></A>
For plans created with the <CODE>FFTW_IN_PLACE</CODE> option, the transform is
computed in-place--the output is returned in the <CODE>in</CODE> array, using
the same strides, etcetera, as were used in the input.
</UL>

<LI>

<CODE>out</CODE>, <CODE>ostride</CODE> and <CODE>odist</CODE> describe the output array(s).
The format is the same as for the input array.

<UL>
<LI><EM>In-place transforms</EM>:

These parameters are ignored for plans created with the
<CODE>FFTW_IN_PLACE</CODE> option.
</UL>

</UL>

<P>
The function <CODE>fftwnd_one</CODE> transforms a single, contiguous input
array to a contiguous output array. By definition, the call

<PRE>
fftwnd_one(plan, in, out)
</PRE>

<P>
is equivalent to

<PRE>
fftwnd(plan, 1, in, 1, 1, out, 1, 1)
</PRE>

<H3><A NAME="SEC27">Destroying a Multi-dimensional Plan</A></H3>

<PRE>
#include <fftw.h>

void fftwnd_destroy_plan(fftwnd_plan plan);
</PRE>

<P>
<A NAME="IDX144"></A>

<P>
The function <CODE>fftwnd_destroy_plan</CODE> frees the plan <CODE>plan</CODE>
and releases all the memory associated with it. After destruction,
a plan is no longer valid.

<H3><A NAME="SEC28">What FFTWND Really Computes</A></H3>
<P>
<A NAME="IDX145"></A>

<P>
The conventions that we follow for the multi-dimensional transform are
analogous to those for the one-dimensional transform. In particular, the
forward transform has a negative sign in the exponent and neither the
forward nor the backward transforms will perform any normalization.
Computing the backward transform of the forward transform will multiply
the array by the product of its dimensions. The output is in-order, and
the zeroth element of the output is the amplitude of the zero frequency
component.

The Gods forbade using HTML to display mathematical formulas. Please
see the TeX or Postscript version of this manual for the proper
definition of the n-dimensional Fourier transform that FFTW
uses. For completeness, we include a bitmap of the TeX output below:
<P><center><IMG SRC="equation-3.gif" ALIGN="top"></center>

<H2><A NAME="SEC29">Real One-dimensional Transforms Reference</A></H2>

<P>
The one-dimensional real routines are generally prefixed with
<CODE>rfftw_</CODE>. <A NAME="DOCF4" HREF="fftw_foot.html#FOOT4">(4)</A> Programs using RFFTW
should be linked with <CODE>-lrfftw -lfftw -lm</CODE> on Unix systems, or with
the RFFTW, the FFTW, and the standard math libraries in general.
<A NAME="IDX146"></A>
<A NAME="IDX147"></A>
<A NAME="IDX148"></A>

<H3><A NAME="SEC30">Plan Creation for Real One-dimensional Transforms</A></H3>

<PRE>
#include <rfftw.h>

rfftw_plan rfftw_create_plan(int n, fftw_direction dir, int flags);

rfftw_plan rfftw_create_plan_specific(int n, fftw_direction dir,
int flags, fftw_real *in, int istride,
fftw_real *out, int ostride);
</PRE>

<P>
<A NAME="IDX149"></A>
<A NAME="IDX150"></A>
<A NAME="IDX151"></A>

<P>
The function <CODE>rfftw_create_plan</CODE> creates a plan, which is a data
structure containing all the information that <CODE>rfftw</CODE> needs in
order to compute the 1D real Fourier transform. You can create as many
plans as you need, but only one plan for a given array size is required
(a plan can be reused many times).

<P>
<CODE>rfftw_create_plan</CODE> returns a valid plan, or <CODE>NULL</CODE> if, for
some reason, the plan can't be created. In the default installation,
this cannot happen, but it is possible to configure RFFTW in such a way
that some input sizes are forbidden, and RFFTW cannot create a plan.

<P>
The <CODE>rfftw_create_plan_specific</CODE> variant takes as additional
arguments specific input/output arrays and their strides. For the last
four arguments, you should pass the arrays and strides that you will
eventually be passing to <CODE>rfftw</CODE>. The resulting plans will be
optimized for those arrays and strides, although they may be used on
other arrays as well. Note: the contents of the in and out arrays are
<EM>destroyed</EM> by the specific planner (the initial contents are
ignored, so the arrays need not have been initialized).
See Section <A HREF="fftw_3.html#SEC20">Discussion on Specific Plans</A>, for a discussion on specific plans.

<H4>Arguments</H4>

<UL>
<LI>

<CODE>n</CODE> is the size of the transform. It can be
any positive integer.

<UL>
<LI>

RFFTW is best at handling sizes of the form
2<SUP>a</SUP> 3<SUP>b</SUP> 5<SUP>c</SUP> 7<SUP>d</SUP>
11<SUP>e</SUP> 13<SUP>f</SUP>,
where e+f is either 0 or
1, and the other exponents are arbitrary. Other sizes are
computed by means of a slow, general-purpose routine (reducing to
O(n<sup>2</sup>)
performance for prime sizes). (It is possible to customize RFFTW for
different array sizes. See Section <A HREF="fftw_6.html#SEC66">Installation and Customization</A>, for more
information.) Transforms whose sizes are powers of 2 are
especially fast.
</UL>

<LI>

<CODE>dir</CODE> is the direction of the desired transform, either
<CODE>FFTW_REAL_TO_COMPLEX</CODE> or <CODE>FFTW_COMPLEX_TO_REAL</CODE>,
corresponding to <CODE>FFTW_FORWARD</CODE> or <CODE>FFTW_BACKWARD</CODE>,
respectively.
<A NAME="IDX152"></A>
<A NAME="IDX153"></A>

<LI>

<A NAME="IDX154"></A>
<CODE>flags</CODE> is a boolean OR (<SAMP>`|'</SAMP>) of zero or more of the following:

<UL>
<LI>

<CODE>FFTW_MEASURE</CODE>: this flag tells RFFTW to find the optimal plan by
actually <EM>computing</EM> several FFTs and measuring their
execution time. Depending on the installation, this can take some
time.

<LI>

<CODE>FFTW_ESTIMATE</CODE>: do not run any FFT and provide a "reasonable"
plan (for a RISC processor with many registers). If neither
<CODE>FFTW_ESTIMATE</CODE> nor <CODE>FFTW_MEASURE</CODE> is provided, the default is
<CODE>FFTW_ESTIMATE</CODE>.

<LI>

<CODE>FFTW_OUT_OF_PLACE</CODE>: produce a plan assuming that the input
and output arrays will be distinct (this is the default).

<LI>

<CODE>FFTW_IN_PLACE</CODE>: produce a plan assuming that you want the output
in the input array. The algorithm used is not necessarily in place:
RFFTW is able to compute true in-place transforms only for small values
of <CODE>n</CODE>. If RFFTW is not able to compute the transform in-place, it
will allocate a temporary array (unless you provide one yourself),
compute the transform out of place, and copy the result back.
<EM>Warning: This option changes the meaning of some parameters of
<CODE>rfftw</CODE></EM> (see Section <A HREF="fftw_3.html#SEC31">Computing the Real One-dimensional Transform</A>).

The default mode of operation is <CODE>FFTW_OUT_OF_PLACE</CODE>.

<LI>

<CODE>FFTW_USE_WISDOM</CODE>: use any <CODE>wisdom</CODE> that is available to help
in the creation of the plan. (See Section <A HREF="fftw_2.html#SEC13">Words of Wisdom</A>.)
This can greatly speed the creation of plans, especially with the
<CODE>FFTW_MEASURE</CODE> option. <CODE>FFTW_ESTIMATE</CODE> plans can also take
advantage of <CODE>wisdom</CODE> to produce a more optimal plan (based on past
measurements) than the estimation heuristic would normally
generate. When the <CODE>FFTW_MEASURE</CODE> option is used, new <CODE>wisdom</CODE>
will also be generated if the current transform size is not completely
understood by existing <CODE>wisdom</CODE>.

</UL>

<LI>

<CODE>in</CODE>, <CODE>out</CODE>, <CODE>istride</CODE>, <CODE>ostride</CODE> (only for
<CODE>rfftw_create_plan_specific</CODE>): see corresponding arguments in the
description of <CODE>rfftw</CODE>. (See Section <A HREF="fftw_3.html#SEC31">Computing the Real One-dimensional Transform</A>.) In particular, the <CODE>out</CODE> and
<CODE>ostride</CODE> parameters have the same special meaning for
<CODE>FFTW_IN_PLACE</CODE> transforms as they have for <CODE>rfftw</CODE>.

</UL>

<H3><A NAME="SEC31">Computing the Real One-dimensional Transform</A></H3>

<PRE>
#include <rfftw.h>

void rfftw(rfftw_plan plan, int howmany,
fftw_real *in, int istride, int idist,
fftw_real *out, int ostride, int odist);

void rfftw_one(rfftw_plan plan, fftw_real *in, fftw_real *out);
</PRE>

<P>
<A NAME="IDX155"></A>
<A NAME="IDX156"></A>

<P>
The function <CODE>rfftw</CODE> computes the Real One-dimensional Fourier
Transform, using a plan created by <CODE>rfftw_create_plan</CODE>
(see Section <A HREF="fftw_3.html#SEC30">Plan Creation for Real One-dimensional Transforms</A>). The function <CODE>rfftw_one</CODE> provides a simplified
interface for the common case of single input array of stride 1.
<A NAME="IDX157"></A>

<P>
<EM>Important:</EM> When invoked for an out-of-place,
<CODE>FFTW_COMPLEX_TO_REAL</CODE> transform, the input array is overwritten
with scratch values by these routines. The input array is not modified
for <CODE>FFTW_REAL_TO_COMPLEX</CODE> transforms.

<H4>Arguments</H4>

<UL>
<LI>

<CODE>plan</CODE> is the plan created by <CODE>rfftw_create_plan</CODE>
(see Section <A HREF="fftw_3.html#SEC30">Plan Creation for Real One-dimensional Transforms</A>).

<LI>

<CODE>howmany</CODE> is the number of transforms <CODE>rfftw</CODE> will compute.
It is faster to tell RFFTW to compute many transforms, instead of
simply calling <CODE>rfftw</CODE> many times.

<LI>

<CODE>in</CODE>, <CODE>istride</CODE> and <CODE>idist</CODE> describe the input array(s).
There are two cases. If the <CODE>plan</CODE> defines a
<CODE>FFTW_REAL_TO_COMPLEX</CODE> transform, <CODE>in</CODE> is a real array.
Otherwise, for <CODE>FFTW_COMPLEX_TO_REAL</CODE> transforms, <CODE>in</CODE> is a
halfcomplex array <EM>whose contents will be destroyed</EM>.

<LI>

<CODE>out</CODE>, <CODE>ostride</CODE> and <CODE>odist</CODE> describe the output
array(s), and have the same meaning as the corresponding parameters for
the input array.

<UL>
<LI><EM>In-place transforms</EM>:

If the <CODE>plan</CODE> specifies an in-place transform, <CODE>ostride</CODE> and
<CODE>odist</CODE> are always ignored. If <CODE>out</CODE> is <CODE>NULL</CODE>,
<CODE>out</CODE> is ignored, too. Otherwise, <CODE>out</CODE> is interpreted as a
pointer to an array of <CODE>n</CODE> complex numbers, that FFTW will use as
temporary space to perform the in-place computation. <CODE>out</CODE> is used
as scratch space and its contents destroyed. In this case, <CODE>out</CODE>
must be an ordinary array whose elements are contiguous in memory (no
striding).
</UL>

</UL>

<P>
The function <CODE>rfftw_one</CODE> transforms a single, contiguous input array
to a contiguous output array. By definition, the call

<PRE>
rfftw_one(plan, in, out)
</PRE>

<P>
is equivalent to

<PRE>
rfftw(plan, 1, in, 1, 1, out, 1, 1)
</PRE>

<H3><A NAME="SEC32">Destroying a Real One-dimensional Plan</A></H3>

<PRE>
#include <rfftw.h>

void rfftw_destroy_plan(rfftw_plan plan);
</PRE>

<P>
<A NAME="IDX158"></A>

<P>
The function <CODE>rfftw_destroy_plan</CODE> frees the plan <CODE>plan</CODE> and
releases all the memory associated with it. After destruction, a plan
is no longer valid.

<H3><A NAME="SEC33">What RFFTW Really Computes</A></H3>
<P>
<A NAME="IDX159"></A>
In this section, we define precisely what RFFTW computes.

<P>
The real to complex (<CODE>FFTW_REAL_TO_COMPLEX</CODE>) transform of a real
array X of size n computes an hermitian array Y,
where
<center><IMG SRC="equation-1.gif" ALIGN="top"></center>
(That Y is a hermitian array is not intended to be obvious,
although the proof is easy.) The hermitian array Y is stored in
halfcomplex order (see Section <A HREF="fftw_3.html#SEC17">Data Types</A>). Currently, RFFTW provides no
way to compute a real to complex transform with a positive sign in the
exponent.

<P>
The complex to real (<CODE>FFTW_COMPLEX_TO_REAL</CODE>) transform of a hermitian
array X of size n computes a real array Y, where
<center><IMG SRC="equation-2.gif" ALIGN="top"></center>
(That Y is a real array is not intended to be obvious, although
the proof is easy.) The hermitian input array X is stored in
halfcomplex order (see Section <A HREF="fftw_3.html#SEC17">Data Types</A>). Currently, RFFTW provides no
way to compute a complex to real transform with a negative sign in the
exponent.

<P>
<A NAME="IDX160"></A>
Like FFTW, RFFTW computes an unnormalized transform. In other words,
applying the real to complex (forward) and then the complex to real
(backward) transform will multiply the input by n.

<H2><A NAME="SEC34">Real Multi-dimensional Transforms Reference</A></H2>
<P>
<A NAME="IDX161"></A>
<A NAME="IDX162"></A>

<P>
The multi-dimensional real routines are generally prefixed with
<CODE>rfftwnd_</CODE>. Programs using RFFTWND should be linked with
<CODE>-lrfftw -lfftw -lm</CODE> on Unix systems, or with the FFTW, RFFTW, and
standard math libraries in general.
<A NAME="IDX163"></A>

<H3><A NAME="SEC35">Plan Creation for Real Multi-dimensional Transforms</A></H3>

<PRE>
#include <rfftw.h>

rfftwnd_plan rfftwnd_create_plan(int rank, const int *n,
fftw_direction dir, int flags);

rfftwnd_plan rfftw2d_create_plan(int nx, int ny,
fftw_direction dir, int flags);

rfftwnd_plan rfftw3d_create_plan(int nx, int ny, int nz,
fftw_direction dir, int flags);
</PRE>

<P>
<A NAME="IDX164"></A>
<A NAME="IDX165"></A>
<A NAME="IDX166"></A>
<A NAME="IDX167"></A>
<A NAME="IDX168"></A>

<P>
The function <CODE>rfftwnd_create_plan</CODE> creates a plan, which is a data
structure containing all the information that <CODE>rfftwnd</CODE> needs in
order to compute a multi-dimensional real Fourier transform. You can
create as many plans as you need, but only one plan for a given array
size is required (a plan can be reused many times). The functions
<CODE>rfftw2d_create_plan</CODE> and <CODE>rfftw3d_create_plan</CODE> are optional,
alternative interfaces to <CODE>rfftwnd_create_plan</CODE> for two and three
dimensions, respectively.

<P>
<CODE>rfftwnd_create_plan</CODE> returns a valid plan, or <CODE>NULL</CODE> if, for
some reason, the plan can't be created. This can happen if the
arguments are invalid in some way (e.g. if <CODE>rank</CODE> < 0).

<H4>Arguments</H4>

<UL>
<LI>

<CODE>rank</CODE> is the dimensionality of the arrays to be transformed. It
can be any non-negative integer.

<LI>

<CODE>n</CODE> is a pointer to an array of <CODE>rank</CODE> integers, giving the
size of each dimension of the arrays to be transformed. Note that these
are always the dimensions of the <EM>real</EM> arrays; the complex arrays
have different dimensions (see Section <A HREF="fftw_3.html#SEC37">Array Dimensions for Real Multi-dimensional Transforms</A>). These sizes, which must be positive
integers, correspond to the dimensions of row-major
arrays--i.e. <CODE>n[0]</CODE> is the size of the dimension whose indices
vary most slowly, and so on. (See Section <A HREF="fftw_2.html#SEC7">Multi-dimensional Array Format</A>, for
more information.)

<UL>
<LI>

See Section <A HREF="fftw_3.html#SEC30">Plan Creation for Real One-dimensional Transforms</A>,
for more information regarding optimal array sizes.
</UL>

<LI>

<CODE>nx</CODE> and <CODE>ny</CODE> in <CODE>rfftw2d_create_plan</CODE> are positive
integers specifying the dimensions of the rank 2 array to be
transformed. i.e. they specify that the transform will operate on
<CODE>nx x ny</CODE> arrays in row-major order, where <CODE>nx</CODE> is the number
of rows and <CODE>ny</CODE> is the number of columns.

<LI>

<CODE>nx</CODE>, <CODE>ny</CODE> and <CODE>nz</CODE> in <CODE>rfftw3d_create_plan</CODE> are
positive integers specifying the dimensions of the rank 3 array to be
transformed. i.e. they specify that the transform will operate on
<CODE>nx x ny x nz</CODE> arrays in row-major order.

<LI>

<CODE>dir</CODE> is the direction of the desired transform, either
<CODE>FFTW_REAL_TO_COMPLEX</CODE> or <CODE>FFTW_COMPLEX_TO_REAL</CODE>,
corresponding to <CODE>FFTW_FORWARD</CODE> or <CODE>FFTW_BACKWARD</CODE>,
respectively.

<LI>

<A NAME="IDX169"></A>
<CODE>flags</CODE> is a boolean OR (<SAMP>`|'</SAMP>) of zero or more of the following:

<UL>
<LI>

<CODE>FFTW_MEASURE</CODE>: this flag tells FFTW to find the optimal plan by
actually <EM>computing</EM> several FFTs and measuring their execution
time.

<LI>

<CODE>FFTW_ESTIMATE</CODE>: do not run any FFT and provide a "reasonable"
plan (for a RISC processor with many registers). If neither
<CODE>FFTW_ESTIMATE</CODE> nor <CODE>FFTW_MEASURE</CODE> is provided, the default is
<CODE>FFTW_ESTIMATE</CODE>.

<LI>

<CODE>FFTW_OUT_OF_PLACE</CODE>: produce a plan assuming that the input
and output arrays will be distinct (this is the default).

<LI>

<A NAME="IDX170"></A>
<CODE>FFTW_IN_PLACE</CODE>: produce a plan assuming that you want to perform
the transform in-place. (Unlike the one-dimensional transform, this
"really" performs the transform in-place.) Note that, if you want to
perform in-place transforms, you <EM>must</EM> use a plan created with
this option. The use of this option has important implications for the
size of the input/output array (see Section <A HREF="fftw_3.html#SEC36">Computing the Real Multi-dimensional Transform</A>).

The default mode of operation is <CODE>FFTW_OUT_OF_PLACE</CODE>.

<LI>

<A NAME="IDX171"></A>
<CODE>FFTW_USE_WISDOM</CODE>: use any <CODE>wisdom</CODE> that is available to help
in the creation of the plan. (See Section <A HREF="fftw_2.html#SEC13">Words of Wisdom</A>.) This can greatly
speed the creation of plans, especially with the <CODE>FFTW_MEASURE</CODE>
option. <CODE>FFTW_ESTIMATE</CODE> plans can also take advantage of
<CODE>wisdom</CODE> to produce a more optimal plan (based on past
measurements) than the estimation heuristic would normally
generate. When the <CODE>FFTW_MEASURE</CODE> option is used, new <CODE>wisdom</CODE>
will also be generated if the current transform size is not completely
understood by existing <CODE>wisdom</CODE>. Note that the same <CODE>wisdom</CODE>
is shared between one-dimensional and multi-dimensional transforms.

</UL>

</UL>

<H3><A NAME="SEC36">Computing the Real Multi-dimensional Transform</A></H3>

<PRE>
#include <rfftw.h>

void rfftwnd_real_to_complex(rfftwnd_plan plan, int howmany,
fftw_real *in, int istride, int idist,
fftw_complex *out, int ostride, int odist);
void rfftwnd_complex_to_real(rfftwnd_plan plan, int howmany,
fftw_complex *in, int istride, int idist,
fftw_real *out, int ostride, int odist);

void rfftwnd_one_real_to_complex(rfftwnd_plan p, fftw_real *in,
fftw_complex *out);
void rfftwnd_one_complex_to_real(rfftwnd_plan p, fftw_complex *in,
fftw_real *out);
</PRE>

<P>
<A NAME="IDX172"></A>
<A NAME="IDX173"></A>
<A NAME="IDX174"></A>
<A NAME="IDX175"></A>

<P>
These functions compute the real multi-dimensional Fourier Transform,
using a plan created by <CODE>rfftwnd_create_plan</CODE>
(see Section <A HREF="fftw_3.html#SEC35">Plan Creation for Real Multi-dimensional Transforms</A>). (Note that the plan determines the rank and dimensions of
the array to be transformed.) The <SAMP>`<CODE>rfftwnd_one_</CODE>'</SAMP> functions
provide a simplified interface for the common case of single input array
of stride 1. Unlike other transform routines in FFTW, we here use
separate functions for the two directions of the transform in order to
correctly express the datatypes of the parameters.

<P>
<EM>Important:</EM> When invoked for an out-of-place,
<CODE>FFTW_COMPLEX_TO_REAL</CODE> transform with <CODE>rank > 1</CODE>, the input
array is overwritten with scratch values by these routines. The input
array is not modified for <CODE>FFTW_REAL_TO_COMPLEX</CODE> transforms or for
<CODE>FFTW_COMPLEX_TO_REAL</CODE> with <CODE>rank == 1</CODE>.

<H4>Arguments</H4>

<UL>
<LI>

<CODE>plan</CODE> is the plan created by <CODE>rfftwnd_create_plan</CODE>.
(see Section <A HREF="fftw_3.html#SEC35">Plan Creation for Real Multi-dimensional Transforms</A>). In the case of two and three-dimensional transforms, it
could also have been created by <CODE>rfftw2d_create_plan</CODE> or
<CODE>rfftw3d_create_plan</CODE>, respectively.

<CODE>FFTW_REAL_TO_COMPLEX</CODE> plans must be used with the
<SAMP>`<CODE>real_to_complex</CODE>'</SAMP> functions, and <CODE>FFTW_COMPLEX_TO_REAL</CODE>
plans must be used with the <SAMP>`<CODE>complex_to_real</CODE>'</SAMP> functions. It
is an error to mismatch the plan direction and the transform function.

<LI>

<CODE>howmany</CODE> is the number of transforms to be computed.

<LI>

<A NAME="IDX176"></A>
<CODE>in</CODE>, <CODE>istride</CODE> and <CODE>idist</CODE> describe the input array(s).
There are <CODE>howmany</CODE> input arrays; the first one is pointed to by
<CODE>in</CODE>, the second one is pointed to by <CODE>in + idist</CODE>, and so on,
up to <CODE>in + (howmany - 1) * idist</CODE>. Each input array is stored in
row-major format (see Section <A HREF="fftw_2.html#SEC7">Multi-dimensional Array Format</A>), and is not
necessarily contiguous in memory. Specifically, <CODE>in[0]</CODE> is the
first element of the first array, <CODE>in[istride]</CODE> is the second
element of the first array, and so on. In general, the <CODE>i</CODE>-th
element of the <CODE>j</CODE>-th input array will be in position <CODE>in[i *
istride + j * idist]</CODE>. Note that, here, <CODE>i</CODE> refers to an index into
the row-major format for the multi-dimensional array, rather than an
index in any particular dimension.

The dimensions of the arrays are different for real and complex data,
and are discussed in more detail below (see Section <A HREF="fftw_3.html#SEC37">Array Dimensions for Real Multi-dimensional Transforms</A>).

<UL>
<LI><EM>In-place transforms</EM>:

For plans created with the <CODE>FFTW_IN_PLACE</CODE> option, the transform is
computed in-place--the output is returned in the <CODE>in</CODE> array. The
meaning of the <CODE>stride</CODE> and <CODE>dist</CODE> parameters in this case is
subtle and is discussed below (see Section <A HREF="fftw_3.html#SEC38">Strides in In-place RFFTWND</A>).
</UL>

<LI>

<CODE>out</CODE>, <CODE>ostride</CODE> and <CODE>odist</CODE> describe the output
array(s). The format is the same as that for the input array. See
below for a discussion of the dimensions of the output array for real
and complex data.

<UL>
<LI><EM>In-place transforms</EM>:

These parameters are ignored for plans created with the
<CODE>FFTW_IN_PLACE</CODE> option.
</UL>

</UL>

<P>
The function <CODE>rfftwnd_one</CODE> transforms a single, contiguous input
array to a contiguous output array. By definition, the call

<PRE>
rfftwnd_one_...(plan, in, out)
</PRE>

<P>
is equivalent to

<PRE>
rfftwnd_...(plan, 1, in, 1, 1, out, 1, 1)
</PRE>

<H3><A NAME="SEC37">Array Dimensions for Real Multi-dimensional Transforms</A></H3>

<P>
<A NAME="IDX177"></A>
The output of a multi-dimensional transform of real data contains
symmetries that, in principle, make half of the outputs redundant
(see Section <A HREF="fftw_3.html#SEC40">What RFFTWND Really Computes</A>). In practice, it is not
possible to entirely realize these savings in an efficient and
understandable format. Instead, the output of the rfftwnd transforms is
<EM>slightly</EM> over half of the output of the corresponding complex
transform. We do not "pack" the data in any way, but store it as an
ordinary array of <CODE>fftw_complex</CODE> values. In fact, this data is
simply a subsection of what would be the array in the corresponding
complex transform.

<P>
Specifically, for a real transform of dimensions
n<sub>1</sub> x n<sub>2</sub> x ... x n<sub>d</sub>,
the complex data is an
n<sub>1</sub> x n<sub>2</sub> x ... x (n<sub>d</sub>/2+1)
array of <CODE>fftw_complex</CODE> values in row-major order (with the
division rounded down). That is, we only store the lower half (plus one
element) of the last dimension of the data from the ordinary complex
transform. (We could have instead taken half of any other dimension,
but implementation turns out to be simpler if the last, contiguous,
dimension is used.)

<P>
<A NAME="IDX178"></A>
<A NAME="IDX179"></A>
Since the complex data is slightly larger than the real data, some
complications arise for in-place transforms. In this case, the final
dimension of the real data must be padded with extra values to
accommodate the size of the complex data--two extra if the last
dimension is even and one if it is odd. That is, the last dimension of
the real data must physically contain
2 * (n<sub>d</sub>/2+1)
<CODE>fftw_real</CODE> values (exactly enough to hold the complex data).
This physical array size does not, however, change the <EM>logical</EM>
array size--only
n<sub>d</sub>
values are actually stored in the last dimension, and
n<sub>d</sub>
is the last dimension passed to <CODE>rfftwnd_create_plan</CODE>.

<H3><A NAME="SEC38">Strides in In-place RFFTWND</A></H3>

<P>
<A NAME="IDX180"></A>
<A NAME="IDX181"></A>
The fact that the input and output datatypes are different for rfftwnd
complicates the meaning of the <CODE>stride</CODE> and <CODE>dist</CODE> parameters
of in-place transforms--are they in units of <CODE>fftw_real</CODE> or
<CODE>fftw_complex</CODE> elements? When reading the input, they are
interpreted in units of the datatype of the input data. When writing
the output, the <CODE>istride</CODE> and <CODE>idist</CODE> are translated to the
output datatype's "units" in one of two ways, corresponding to the two
most common situations in which <CODE>stride</CODE> and <CODE>dist</CODE> parameters
are useful. Below, we refer to these "translated" parameters as
<CODE>ostride_t</CODE> and <CODE>odist_t</CODE>. (Note that these are computed
internally by rfftwnd; the actual <CODE>ostride</CODE> and <CODE>odist</CODE>
parameters are ignored for in-place transforms.)

<P>
First, there is the case where you are transforming a number of
contiguous arrays located one after another in memory. In this
situation, <CODE>istride</CODE> is <CODE>1</CODE> and <CODE>idist</CODE> is the product of
the physical dimensions of the array. <CODE>ostride_t</CODE> and
<CODE>odist_t</CODE> are then chosen so that the output arrays are contiguous
and lie on top of the input arrays. <CODE>ostride_t</CODE> is therefore
<CODE>1</CODE>. For a real-to-complex transform, <CODE>odist_t</CODE> is
<CODE>idist/2</CODE>; for a complex-to-real transform, <CODE>odist_t</CODE> is
<CODE>idist*2</CODE>.

<P>
The second case is when you have an array in which each element has
<CODE>nc</CODE> components (e.g. a structure with <CODE>nc</CODE> numeric fields),
and you want to transform all of the components at once. Here,
<CODE>istride</CODE> is <CODE>nc</CODE> and <CODE>idist</CODE> is <CODE>1</CODE>. For this
case, it is natural to want the output to also have <CODE>nc</CODE>
consecutive components, now of the output data type; this is exactly
what rfftwnd does. Specifically, it uses an <CODE>ostride_t</CODE> equal to
<CODE>istride</CODE>, and an <CODE>odist_t</CODE> of <CODE>1</CODE>. (Astute readers will
realize that some extra buffer space is required in order to perform
such a transform; this is handled automatically by rfftwnd.)

<P>
The general rule is as follows. <CODE>ostride_t</CODE> equals <CODE>istride</CODE>.
If <CODE>idist</CODE> is <CODE>1</CODE>, then <CODE>odist_t</CODE> is <CODE>1</CODE>.
Otherwise, for a real-to-complex transform <CODE>odist_t</CODE> is
<CODE>idist/2</CODE> and for a complex-to-real transform <CODE>odist_t</CODE> is
<CODE>idist*2</CODE>.

<H3><A NAME="SEC39">Destroying a Multi-dimensional Plan</A></H3>

<PRE>
#include <rfftw.h>

void rfftwnd_destroy_plan(rfftwnd_plan plan);
</PRE>

<P>
<A NAME="IDX182"></A>

<P>
The function <CODE>rfftwnd_destroy_plan</CODE> frees the plan <CODE>plan</CODE>
and releases all the memory associated with it. After destruction,
a plan is no longer valid.

<H3><A NAME="SEC40">What RFFTWND Really Computes</A></H3>
<P>
<A NAME="IDX183"></A>

<P>
The conventions that we follow for the real multi-dimensional transform
are analogous to those for the complex multi-dimensional transform. In
particular, the forward transform has a negative sign in the exponent
and neither the forward nor the backward transforms will perform any
normalization. Computing the backward transform of the forward
transform will multiply the array by the product of its dimensions (that
is, the logical dimensions of the real data). The forward transform is
real-to-complex and the backward transform is complex-to-real.

<P>
<A NAME="IDX184"></A>
<A NAME="IDX185"></A>
The Gods forbade using HTML to display mathematical formulas. Please
see the TeX or Postscript version of this manual for the proper
definition of the n-dimensional real Fourier transform that RFFTW
uses. For completeness, we include a bitmap of the TeX output below:
<P><center><IMG SRC="equation-4.gif" ALIGN="top"></center>

<H2><A NAME="SEC41">Wisdom Reference</A></H2>

<P>
<A NAME="IDX186"></A>

<H3><A NAME="SEC42">Exporting Wisdom</A></H3>

<PRE>
#include <fftw.h>

void fftw_export_wisdom(void (*emitter)(char c, void *), void *data);
void fftw_export_wisdom_to_file(FILE *output_file);
char *fftw_export_wisdom_to_string(void);
</PRE>

<P>
<A NAME="IDX187"></A>
<A NAME="IDX188"></A>
<A NAME="IDX189"></A>

<P>
These functions allow you to export all currently accumulated
<CODE>wisdom</CODE> in a form from which it can be later imported and
restored, even during a separate run of the program. (See Section <A HREF="fftw_2.html#SEC13">Words of Wisdom</A>.) The current store of <CODE>wisdom</CODE> is not
affected by calling any of these routines.

<P>
<CODE>fftw_export_wisdom</CODE> exports the <CODE>wisdom</CODE> to any output
medium, as specified by the callback function
<CODE>emitter</CODE>. <CODE>emitter</CODE> is a <CODE>putc</CODE>-like function that
writes the character <CODE>c</CODE> to some output; its second parameter is
the <CODE>data</CODE> pointer passed to <CODE>fftw_export_wisdom</CODE>. For
convenience, the following two "wrapper" routines are provided:

<P>
<CODE>fftw_export_wisdom_to_file</CODE> writes the <CODE>wisdom</CODE> to the
current position in <CODE>output_file</CODE>, which should be open with write
permission. Upon exit, the file remains open and is positioned at the
end of the <CODE>wisdom</CODE> data.

<P>
<CODE>fftw_export_wisdom_to_string</CODE> returns a pointer to a
<CODE>NULL</CODE>-terminated string holding the <CODE>wisdom</CODE> data. This
string is dynamically allocated, and it is the responsibility of the
caller to deallocate it with <CODE>fftw_free</CODE> when it is no longer
needed.

<P>
All of these routines export the wisdom in the same format, which we
will not document here except to say that it is LISP-like ASCII text
that is insensitive to white space.

<H3><A NAME="SEC43">Importing Wisdom</A></H3>

<PRE>
#include <fftw.h>

fftw_status fftw_import_wisdom(int (*get_input)(void *), void *data);
fftw_status fftw_import_wisdom_from_file(FILE *input_file);
fftw_status fftw_import_wisdom_from_string(const char *input_string);
</PRE>

<P>
<A NAME="IDX190"></A>
<A NAME="IDX191"></A>
<A NAME="IDX192"></A>

<P>
These functions import <CODE>wisdom</CODE> into a program from data stored by
the <CODE>fftw_export_wisdom</CODE> functions above. (See Section <A HREF="fftw_2.html#SEC13">Words of Wisdom</A>.)
The imported <CODE>wisdom</CODE> supplements rather than replaces any
<CODE>wisdom</CODE> already accumulated by the running program (except when
there is conflicting <CODE>wisdom</CODE>, in which case the existing wisdom is
replaced).

<P>
<CODE>fftw_import_wisdom</CODE> imports <CODE>wisdom</CODE> from any input medium,
as specified by the callback function <CODE>get_input</CODE>. <CODE>get_input</CODE>
is a <CODE>getc</CODE>-like function that returns the next character in the
input; its parameter is the <CODE>data</CODE> pointer passed to
<CODE>fftw_import_wisdom</CODE>. If the end of the input data is reached
(which should never happen for valid data), it may return either
<CODE>NULL</CODE> (ASCII 0) or <CODE>EOF</CODE> (as defined in <CODE><stdio.h></CODE>).
For convenience, the following two "wrapper" routines are provided:

<P>
<CODE>fftw_import_wisdom_from_file</CODE> reads <CODE>wisdom</CODE> from the
current position in <CODE>input_file</CODE>, which should be open with read
permission. Upon exit, the file remains open and is positioned at the
end of the <CODE>wisdom</CODE> data.

<P>
<CODE>fftw_import_wisdom_from_string</CODE> reads <CODE>wisdom</CODE> from the
<CODE>NULL</CODE>-terminated string <CODE>input_string</CODE>.

<P>
The return value of these routines is <CODE>FFTW_SUCCESS</CODE> if the wisdom
was read successfully, and <CODE>FFTW_FAILURE</CODE> otherwise. Note that, in
all of these functions, any data in the input stream past the end of the
<CODE>wisdom</CODE> data is simply ignored (it is not even read if the
<CODE>wisdom</CODE> data is well-formed).

<H3><A NAME="SEC44">Forgetting Wisdom</A></H3>

<PRE>
#include <fftw.h>

void fftw_forget_wisdom(void);
</PRE>

<P>
<A NAME="IDX193"></A>

<P>
Calling <CODE>fftw_forget_wisdom</CODE> causes all accumulated <CODE>wisdom</CODE>
to be discarded and its associated memory to be freed. (New
<CODE>wisdom</CODE> can still be gathered subsequently, however.)

<H2><A NAME="SEC45">Memory Allocator Reference</A></H2>

<PRE>
#include <fftw.h>

void *(*fftw_malloc_hook) (size_t n);
void (*fftw_free_hook) (void *p);
</PRE>

<P>
<A NAME="IDX194"></A>
<A NAME="IDX195"></A>
<A NAME="IDX196"></A>
<A NAME="IDX197"></A>

<P>
Whenever it has to allocate and release memory, FFTW ordinarily calls
<CODE>malloc</CODE> and <CODE>free</CODE>.
If <CODE>malloc</CODE> fails, FFTW prints an error message and exits. This
behavior may be undesirable in some applications. Also, special
memory-handling functions may be necessary in certain
environments. Consequently, FFTW provides means by which you can install
your own memory allocator and take whatever error-correcting action you
find appropriate. The variables <CODE>fftw_malloc_hook</CODE> and
<CODE>fftw_free_hook</CODE> are pointers to functions, and they are normally
<CODE>NULL</CODE>. If you set those variables to point to other functions,
then FFTW will use your routines instead of <CODE>malloc</CODE> and
<CODE>free</CODE>. <CODE>fftw_malloc_hook</CODE> must point to a <CODE>malloc</CODE>-like
function, and <CODE>fftw_free_hook</CODE> must point to a <CODE>free</CODE>-like
function.

<H2><A NAME="SEC46">Thread safety</A></H2>

<P>
<A NAME="IDX198"></A>
<A NAME="IDX199"></A>
Users writing multi-threaded programs must concern themselves with the
<EM>thread safety</EM> of the libraries they use--that is, whether it is
safe to call routines in parallel from multiple threads. FFTW can be
used in such an environment, but some care must be taken because certain
parts of FFTW use private global variables to share data between calls.
In particular, the plan-creation functions share trigonometric tables
and accumulated <CODE>wisdom</CODE>. (Users should note that these comments
only apply to programs using shared-memory threads. Parallelism using
MPI or forked processes involves a separate address-space and global
variables for each process, and is not susceptible to problems of this
sort.)

<P>
The central restriction of FFTW is that it is not safe to create
multiple plans in parallel. You must either create all of your plans
from a single thread, or instead use a semaphore, mutex, or other
mechanism to ensure that different threads don't attempt to create plans
at the same time. The same restriction also holds for destruction of
plans and importing/forgetting <CODE>wisdom</CODE>. Once created, a plan may
safely be used in any thread.

<P>
The actual transform routines in FFTW (<CODE>fftw_one</CODE>, etcetera) are
re-entrant and thread-safe, so it is fine to call them simultaneously
from multiple threads. Another question arises, however--is it safe to
use the <EM>same plan</EM> for multiple transforms in parallel? (It would
be unsafe if, for example, the plan were modified in some way by the
transform.) We address this question by defining an additional planner
flag, <CODE>FFTW_THREADSAFE</CODE>.
<A NAME="IDX200"></A>
When included in the flags for any of the plan-creation routines,
<CODE>FFTW_THREADSAFE</CODE> guarantees that the resulting plan will be
read-only and safe to use in parallel by multiple threads.

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