Tutorial
********

This chapter describes the basic usage of FFTW, i.e., how to compute the Fourier transform of a single array.  This chapter tells the truth, but not the whole truth. Specifically, FFTW implements additional routines and flags that are not documented here, although in many cases we try to indicate where added capabilities exist.  For more complete information, see FFTW Reference.  (Note that you need to compile and install FFTW before you can use it in a program. For the details of the installation, see Installation and Customization.)

We recommend that you read this tutorial in order. (You can read the tutorial in bit-reversed order after computing your first transform.)  At the least, read the first section (see Complex One-Dimensional DFTs) before reading any of the others, even if your main interest lies in one of the other transform types.

Users of FFTW version 2 and earlier may also want to read Upgrading from FFTW version 2.

Complex One-Dimensional DFTs
============================

     Plan: To bother about the best method of accomplishing an accidental result. [Ambrose Bierce, The Enlarged Devil's Dictionary.]

The basic usage of FFTW to compute a one-dimensional DFT of size N is simple, and it typically looks something like this code:

     #include <fftw3.h>
     ...
     {
         fftw_complex *in, *out;
         fftw_plan p;
         ...
         in = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * N);
         out = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * N);
         p = fftw_plan_dft_1d(N, in, out, FFTW_FORWARD, FFTW_ESTIMATE);
         ...
         fftw_execute(p); /* repeat as needed */
         ...
         fftw_destroy_plan(p);
         fftw_free(in); fftw_free(out);
     }

You must link this code with the fftw3 library.  On Unix systems, link with -lfftw3 -lm.

The example code first allocates the input and output arrays.  You can allocate them in any way that you like, but we recommend using fftw_malloc, which behaves like malloc except that it properly aligns the array when SIMD instructions (such as SSE and Altivec) are available (see SIMD alignment and fftw_malloc). [Alternatively, we provide a convenient wrapper function fftw_alloc_complex(N) which has the same effect.]

The data is an array of type fftw_complex, which is by default a double[2] composed of the real (in[i][0]) and imaginary (in[i][1]) parts of a complex number.

The next step is to create a plan, which is an object that contains all the data that FFTW needs to compute the FFT. This function creates the plan:

     fftw_plan fftw_plan_dft_1d(int n, fftw_complex *in, fftw_complex *out,
                                int sign, unsigned flags);

The first argument, n, is the size of the transform you are trying to compute.  The size n can be any positive integer, but sizes that are products of small factors are transformed most efficiently (although prime sizes still use an Onlogn algorithm).

The next two arguments are pointers to the input and output arrays of the transform.  These pointers can be equal, indicating an in-place transform.

The fourth argument, sign, can be either FFTW_FORWARD (-1) or FFTW_BACKWARD (+1), and indicates the direction of the transform you are interested in; technically, it is the sign of the exponent in the transform.

The flags argument is usually either FFTW_MEASURE or FFTW_ESTIMATE.  FFTW_MEASURE instructs FFTW to run and measure the execution time of several FFTs in order to find the best way to compute the transform of size n.  This process takes some time (usually a few seconds), depending on your machine and on the size of the transform.  FFTW_ESTIMATE, on the contrary, does not run any computation and just builds a reasonable plan that is probably sub-optimal.  In short, if your program performs many transforms of the same size and initialization time is not important, use FFTW_MEASURE; otherwise use the estimate.

You must create the plan before initializing the input, because FFTW_MEASURE overwrites the in/out arrays. (Technically, FFTW_ESTIMATE does not touch your arrays, but you should always create plans first just to be sure.)

Once the plan has been created, you can use it as many times as you like for transforms on the specified in/out arrays, computing the actual transforms via fftw_execute(plan):

     void fftw_execute(const fftw_plan plan);

The DFT results are stored in-order in the array out, with the zero-frequency (DC) component in out[0]. If in != out, the transform is out-of-place and the input array in is not modified.  Otherwise, the input array is overwritten with the transform.

If you want to transform a different array of the same size, you can create a new plan with fftw_plan_dft_1d and FFTW automatically reuses the information from the previous plan, if possible.  Alternatively, with the ``guru'' interface you can apply a given plan to a different array, if you are careful. See FFTW Reference.

When you are done with the plan, you deallocate it by calling fftw_destroy_plan(plan):

     void fftw_destroy_plan(fftw_plan plan);

If you allocate an array with fftw_malloc() you must deallocate it with fftw_free().  Do not use free() or, heaven forbid, delete.

FFTW computes an unnormalized DFT.  Thus, computing a forward followed by a backward transform (or vice versa) results in the original array scaled by n.  For the definition of the DFT, see What FFTW Really Computes.

If you have a C compiler, such as gcc, that supports the C99 standard, and you #include <complex.h> before <fftw3.h>, then fftw_complex is the native double-precision complex type and you can manipulate it with ordinary arithmetic.  Otherwise, FFTW defines its own complex type, which is bit-compatible with the C99 complex type. See Complex numbers. (The C++ <complex> template class may also be usable via a typecast.)

To use single or long-double precision versions of FFTW, replace the fftw_ prefix by fftwf_ or fftwl_ and link with -lfftw3f or -lfftw3l, but use the same <fftw3.h> header file.

Many more flags exist besides FFTW_MEASURE and FFTW_ESTIMATE.  For example, use FFTW_PATIENT if you're willing to wait even longer for a possibly even faster plan (see FFTW Reference). You can also save plans for future use, as described by Words of Wisdom-Saving Plans.

Complex Multi-Dimensional DFTs
==============================

Multi-dimensional transforms work much the same way as one-dimensional transforms: you allocate arrays of fftw_complex (preferably using fftw_malloc), create an fftw_plan, execute it as many times as you want with fftw_execute(plan), and clean up with fftw_destroy_plan(plan) (and fftw_free).

FFTW provides two routines for creating plans for 2d and 3d transforms, and one routine for creating plans of arbitrary dimensionality. The 2d and 3d routines have the following signature:

     fftw_plan fftw_plan_dft_2d(int n0, int n1,
                                fftw_complex *in, fftw_complex *out,
                                int sign, unsigned flags);
     fftw_plan fftw_plan_dft_3d(int n0, int n1, int n2,
                                fftw_complex *in, fftw_complex *out,
                                int sign, unsigned flags);

These routines create plans for n0 by n1 two-dimensional (2d) transforms and n0 by n1 by n2 3d transforms, respectively.  All of these transforms operate on contiguous arrays in the C-standard row-major order, so that the last dimension has the fastest-varying index in the array.  This layout is described further in Multi-dimensional Array Format.

FFTW can also compute transforms of higher dimensionality.  In order to avoid confusion between the various meanings of the the word ``dimension'', we use the term rank to denote the number of independent indices in an array. (The term ``rank'' is commonly used in the APL, FORTRAN, and Common Lisp traditions, although it is not so common in the C world.)  For example, we say that a 2d transform has rank 2, a 3d transform has rank 3, and so on.  You can plan transforms of arbitrary rank by means of the following function:

     fftw_plan fftw_plan_dft(int rank, const int *n,
                             fftw_complex *in, fftw_complex *out,
                             int sign, unsigned flags);

Here, n is a pointer to an array n[rank] denoting an n[0] by n[1] by ... by n[rank-1] transform. Thus, for example, the call

     fftw_plan_dft_2d(n0, n1, in, out, sign, flags);

is equivalent to the following code fragment:

     int n[2];
     n[0] = n0;
     n[1] = n1;
     fftw_plan_dft(2, n, in, out, sign, flags);

fftw_plan_dft is not restricted to 2d and 3d transforms, however, but it can plan transforms of arbitrary rank.

You may have noticed that all the planner routines described so far have overlapping functionality.  For example, you can plan a 1d or 2d transform by using fftw_plan_dft with a rank of 1 or 2, or even by calling fftw_plan_dft_3d with n0 and/or n1 equal to 1 (with no loss in efficiency).  This pattern continues, and FFTW's planning routines in general form a ``partial order,'' sequences of interfaces with strictly increasing generality but correspondingly greater complexity.

fftw_plan_dft is the most general complex-DFT routine that we describe in this tutorial, but there are also the advanced and guru interfaces, which allow one to efficiently combine multiple/strided transforms into a single FFTW plan, transform a subset of a larger multi-dimensional array, and/or to handle more general complex-number formats.  For more information, see FFTW Reference.

One-Dimensional DFTs of Real Data
=================================

In many practical applications, the input data in[i] are purely real numbers, in which case the DFT output satisfies the ``Hermitian'' redundancy: out[i] is the conjugate of out[n-i].  It is possible to take advantage of these circumstances in order to achieve roughly a factor of two improvement in both speed and memory usage.

In exchange for these speed and space advantages, the user sacrifices some of the simplicity of FFTW's complex transforms. First of all, the input and output arrays are of different sizes and types: the input is n real numbers, while the output is n/2+1 complex numbers (the non-redundant outputs); this also requires slight ``padding'' of the input array for in-place transforms.  Second, the inverse transform (complex to real) has the side-effect of overwriting its input array, by default. Neither of these inconveniences should pose a serious problem for users, but it is important to be aware of them.

The routines to perform real-data transforms are almost the same as those for complex transforms: you allocate arrays of double and/or fftw_complex (preferably using fftw_malloc or fftw_alloc_complex), create an fftw_plan, execute it as many times as you want with fftw_execute(plan), and clean up with fftw_destroy_plan(plan) (and fftw_free).  The only differences are that the input (or output) is of type double and there are new routines to create the plan.  In one dimension:

     fftw_plan fftw_plan_dft_r2c_1d(int n, double *in, fftw_complex *out,
                                    unsigned flags);
     fftw_plan fftw_plan_dft_c2r_1d(int n, fftw_complex *in, double *out,
                                    unsigned flags);

for the real input to complex-Hermitian output (r2c) and complex-Hermitian input to real output (c2r) transforms. Unlike the complex DFT planner, there is no sign argument. Instead, r2c DFTs are always FFTW_FORWARD and c2r DFTs are always FFTW_BACKWARD. (For single/long-double precision fftwf and fftwl, double should be replaced by float and long double, respectively.)

Here, n is the ``logical'' size of the DFT, not necessarily the physical size of the array.  In particular, the real (double) array has n elements, while the complex (fftw_complex) array has n/2+1 elements (where the division is rounded down). For an in-place transform, in and out are aliased to the same array, which must be big enough to hold both; so, the real array would actually have 2*(n/2+1) elements, where the elements beyond the first n are unused padding.  (Note that this is very different from the concept of ``zero-padding'' a transform to a larger length, which changes the logical size of the DFT by actually adding new input data.)  The kth element of the complex array is exactly the same as the kth element of the corresponding complex DFT.  All positive n are supported; products of small factors are most efficient, but an algorithm is used even for prime sizes.

As noted above, the c2r transform destroys its input array even for out-of-place transforms.  This can be prevented, if necessary, by including FFTW_PRESERVE_INPUT in the flags, with unfortunately some sacrifice in performance. This flag is also not currently supported for multi-dimensional real DFTs (next section).

Readers familiar with DFTs of real data will recall that the 0th (the ``DC'') and n/2-th (the ``Nyquist'' frequency, when n is even) elements of the complex output are purely real.  Some implementations therefore store the Nyquist element where the DC imaginary part would go, in order to make the input and output arrays the same size.  Such packing, however, does not generalize well to multi-dimensional transforms, and the space savings are miniscule in any case; FFTW does not support it.

An alternative interface for one-dimensional r2c and c2r DFTs can be found in the r2r interface (see The Halfcomplex-format DFT), with ``halfcomplex''-format output that is the same size (and type) as the input array. That interface, although it is not very useful for multi-dimensional transforms, may sometimes yield better performance.

Multi-Dimensional DFTs of Real Data
===================================

Multi-dimensional DFTs of real data use the following planner routines:

     fftw_plan fftw_plan_dft_r2c_2d(int n0, int n1,
                                    double *in, fftw_complex *out,
                                    unsigned flags);
     fftw_plan fftw_plan_dft_r2c_3d(int n0, int n1, int n2,
                                    double *in, fftw_complex *out,
                                    unsigned flags);
     fftw_plan fftw_plan_dft_r2c(int rank, const int *n,
                                 double *in, fftw_complex *out,
                                 unsigned flags);

as well as the corresponding c2r routines with the input/output types swapped.  These routines work similarly to their complex analogues, except for the fact that here the complex output array is cut roughly in half and the real array requires padding for in-place transforms (as in 1d, above).

As before, n is the logical size of the array, and the consequences of this on the the format of the complex arrays deserve careful attention. Suppose that the real data has dimensions (in row-major order). Then, after an r2c transform, the output is an array of fftw_complex values in row-major order, corresponding to slightly over half of the output of the corresponding complex DFT.  (The division is rounded down.)  The ordering of the data is otherwise exactly the same as in the complex-DFT case.

For out-of-place transforms, this is the end of the story: the real data is stored as a row-major array of size and the complex data is stored as a row-major array of size ndimshalf.

For in-place transforms, however, extra padding of the real-data array is necessary because the complex array is larger than the real array, and the two arrays share the same memory locations.  Thus, for in-place transforms, the final dimension of the real-data array must be padded with extra values to accommodate the size of the complex data---two values 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[d-1]/2+1) double values (exactly enough to hold the complex data). This physical array size does not, however, change the logical array size---only n[d-1] values are actually stored in the last dimension, and n[d-1] is the last dimension passed to the plan-creation routine.

For example, consider the transform of a two-dimensional real array of size n0 by n1.  The output of the r2c transform is a two-dimensional complex array of size n0 by n1/2+1, where the y dimension has been cut nearly in half because of redundancies in the output.  Because fftw_complex is twice the size of double, the output array is slightly bigger than the input array.  Thus, if we want to compute the transform in place, we must pad the input array so that it is of size n0 by 2*(n1/2+1).  If n1 is even, then there are two padding elements at the end of each row (which need not be initialized, as they are only used for output).

These transforms are unnormalized, so an r2c followed by a c2r transform (or vice versa) will result in the original data scaled by the number of real data elements---that is, the product of the (logical) dimensions of the real data.

(Because the last dimension is treated specially, if it is equal to 1 the transform is not equivalent to a lower-dimensional r2c/c2r transform.  In that case, the last complex dimension also has size 1 (=1/2+1), and no advantage is gained over the complex transforms.)

More DFTs of Real Data
======================

FFTW supports several other transform types via a unified r2r (real-to-real) interface, so called because it takes a real (double) array and outputs a real array of the same size.  These r2r transforms currently fall into three categories: DFTs of real input and complex-Hermitian output in halfcomplex format, DFTs of real input with even/odd symmetry (a.k.a. discrete cosine/sine transforms, DCTs/DSTs), and discrete Hartley transforms (DHTs), all described in more detail by the following sections.

The r2r transforms follow the by now familiar interface of creating an fftw_plan, executing it with fftw_execute(plan), and destroying it with fftw_destroy_plan(plan).  Furthermore, all r2r transforms share the same planner interface:

     fftw_plan fftw_plan_r2r_1d(int n, double *in, double *out,
                                fftw_r2r_kind kind, unsigned flags);
     fftw_plan fftw_plan_r2r_2d(int n0, int n1, double *in, double *out,
                                fftw_r2r_kind kind0, fftw_r2r_kind kind1,
                                unsigned flags);
     fftw_plan fftw_plan_r2r_3d(int n0, int n1, int n2,
                                double *in, double *out,
                                fftw_r2r_kind kind0,
                                fftw_r2r_kind kind1,
                                fftw_r2r_kind kind2,
                                unsigned flags);
     fftw_plan fftw_plan_r2r(int rank, const int *n, double *in, double *out,
                             const fftw_r2r_kind *kind, unsigned flags);

Just as for the complex DFT, these plan 1d/2d/3d/multi-dimensional transforms for contiguous arrays in row-major order, transforming (real) input to output of the same size, where n specifies the physical dimensions of the arrays.  All positive n are supported (with the exception of n=1 for the FFTW_REDFT00 kind, noted in the real-even subsection below); products of small factors are most efficient (factorizing n-1 and n+1 for FFTW_REDFT00 and FFTW_RODFT00 kinds, described below), but an algorithm is used even for prime sizes.

Each dimension has a kind parameter, of type fftw_r2r_kind, specifying the kind of r2r transform to be used for that dimension. (In the case of fftw_plan_r2r, this is an array kind[rank] where kind[i] is the transform kind for the dimension n[i].)  The kind can be one of a set of predefined constants, defined in the following subsections.

In other words, FFTW computes the separable product of the specified r2r transforms over each dimension, which can be used e.g. for partial differential equations with mixed boundary conditions.  (For some r2r kinds, notably the halfcomplex DFT and the DHT, such a separable product is somewhat problematic in more than one dimension, however, as is described below.)

In the current version of FFTW, all r2r transforms except for the halfcomplex type are computed via pre- or post-processing of halfcomplex transforms, and they are therefore not as fast as they could be.  Since most other general DCT/DST codes employ a similar algorithm, however, FFTW's implementation should provide at least competitive performance.

The Halfcomplex-format DFT
--------------------------

An r2r kind of FFTW_R2HC (r2hc) corresponds to an r2c DFT (see One-Dimensional DFTs of Real Data) but with ``halfcomplex'' format output, and may sometimes be faster and/or more convenient than the latter. The inverse hc2r transform is of kind FFTW_HC2R. This consists of the non-redundant half of the complex output for a 1d real-input DFT of size n, stored as a sequence of n real numbers (double) in the format:

r0, r1, r2, r(n/2), i((n+1)/2-1), ..., i2, i1

Here, rk is the real part of the kth output, and ik is the imaginary part.  (Division by 2 is rounded down.) For a halfcomplex array hc[n], the kth component thus has its real part in hc[k] and its imaginary part in hc[n-k], with the exception of k == 0 or n/2 (the latter only if n is even)---in these two cases, the imaginary part is zero due to symmetries of the real-input DFT, and is not stored. Thus, the r2hc transform of n real values is a halfcomplex array of length n, and vice versa for hc2r.

Aside from the differing format, the output of FFTW_R2HC/FFTW_HC2R is otherwise exactly the same as for the corresponding 1d r2c/c2r transform (i.e. FFTW_FORWARD/FFTW_BACKWARD transforms, respectively). Recall that these transforms are unnormalized, so r2hc followed by hc2r will result in the original data multiplied by n.  Furthermore, like the c2r transform, an out-of-place hc2r transform will destroy its input array.

Although these halfcomplex transforms can be used with the multi-dimensional r2r interface, the interpretation of such a separable product of transforms along each dimension is problematic.  For example, consider a two-dimensional n0 by n1, r2hc by r2hc transform planned by fftw_plan_r2r_2d(n0, n1, in, out, FFTW_R2HC, FFTW_R2HC, FFTW_MEASURE).  Conceptually, FFTW first transforms the rows (of size n1) to produce halfcomplex rows, and then transforms the columns (of size n0).  Half of these column transforms, however, are of imaginary parts, and should therefore be multiplied by i and combined with the r2hc transforms of the real columns to produce the 2d DFT amplitudes; FFTW's r2r transform does not 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 Multi-Dimensional DFTs of Real Data).

Real even/odd DFTs (cosine/sine transforms)
-------------------------------------------

The Fourier transform of a real-even function f(-x) = f(x) is real-even, and i times the Fourier transform of a real-odd function f(-x) = -f(x) is real-odd.  Similar results hold for a discrete Fourier transform, and thus for these symmetries the need for complex inputs/outputs is entirely eliminated.  Moreover, one gains a factor of two in speed/space from the fact that the data are real, and an additional factor of two from the even/odd symmetry: only the non-redundant (first) half of the array need be stored.  The result is the real-even DFT (REDFT) and the real-odd DFT (RODFT), also known as the discrete cosine and sine transforms (DCT and DST), respectively.

(In this section, we describe the 1d transforms; multi-dimensional transforms are just a separable product of these transforms operating along each dimension.)

Because of the discrete sampling, one has an additional choice: is the data even/odd around a sampling point, or around the point halfway between two samples?  The latter corresponds to shifting the samples by half an interval, and gives rise to several transform variants denoted by REDFTab and RODFTab: a and b are 0 or 1, and indicate whether the input (a) and/or output (b) are shifted by half a sample (1 means it is shifted).  These are also known as types I-IV of the DCT and DST, and all four types are supported by FFTW's r2r interface. (There are also type V-VIII transforms, which correspond to a logical DFT of odd size N, independent of whether the physical size n is odd, but we do not support these variants.)

The r2r kinds for the various REDFT and RODFT types supported by FFTW, along with the boundary conditions at both ends of the input array (n real numbers in[j=0..n-1]), are:

  * FFTW_REDFT00 (DCT-I): even around j=0 and even around j=n-1.

  * FFTW_REDFT10 (DCT-II, ``the'' DCT): even around j=-0.5 and even around j=n-0.5.

  * FFTW_REDFT01 (DCT-III, ``the'' IDCT): even around j=0 and odd around j=n.

  * FFTW_REDFT11 (DCT-IV): even around j=-0.5 and odd around j=n-0.5.

  * FFTW_RODFT00 (DST-I): odd around j=-1 and odd around j=n.

  * FFTW_RODFT10 (DST-II): odd around j=-0.5 and odd around j=n-0.5.

  * FFTW_RODFT01 (DST-III): odd around j=-1 and even around j=n-1.

  * FFTW_RODFT11 (DST-IV): odd around j=-0.5 and even around j=n-0.5.

Note that these symmetries apply to the ``logical'' array being transformed; there are no constraints on your physical input data.  So, for example, if you specify a size-5 REDFT00 (DCT-I) of the data abcde, it corresponds to the DFT of the logical even array abcdedcb of size 8.  A size-4 REDFT10 (DCT-II) of the data abcd corresponds to the size-8 logical DFT of the even array abcddcba, shifted by half a sample.

All of these transforms are invertible.  The inverse of R*DFT00 is R*DFT00; of R*DFT10 is R*DFT01 and vice versa (these are often called simply ``the'' DCT and IDCT, respectively); and of R*DFT11 is R*DFT11. However, the transforms computed by FFTW are unnormalized, exactly like the corresponding real and complex DFTs, so computing a transform followed by its inverse yields the original array scaled by N, where N is the logical DFT size.  For REDFT00, N=2(n-1); for RODFT00, N=2(n+1); otherwise, N=2n.

Note that the boundary conditions of the transform output array are given by the input boundary conditions of the inverse transform. Thus, the above transforms are all inequivalent in terms of input/output boundary conditions, even neglecting the 0.5 shift difference.

FFTW is most efficient when N is a product of small factors; note that this differs from the factorization of the physical size n for REDFT00 and RODFT00!  There is another oddity: n=1 REDFT00 transforms correspond to N=0, and so are not defined (the planner will return NULL).  Otherwise, any positive n is supported.

For the precise mathematical definitions of these transforms as used by FFTW, see What FFTW Really Computes.  (For people accustomed to the DCT/DST, FFTW's definitions have a coefficient of 2 in front of the cos/sin functions so that they correspond precisely to an even/odd DFT of size N.  Some authors also include additional multiplicative factors of sqrt(2) for selected inputs and outputs; this makes the transform orthogonal, but sacrifices the direct equivalence to a symmetric DFT.)

Which type do you need?
.......................

Since the required flavor of even/odd DFT depends upon your problem, you are the best judge of this choice, but we can make a few comments on relative efficiency to help you in your selection.  In particular, R*DFT01 and R*DFT10 tend to be slightly faster than R*DFT11 (especially for odd sizes), while the R*DFT00 transforms are sometimes significantly slower (especially for even sizes). (R*DFT00 is
sometimes slower in FFTW because we discovered that the standard
algorithm for computing this by a pre/post-processed real DFT---the
algorithm used in FFTPACK, Numerical Recipes, and other sources for
decades now---has serious numerical problems: it already loses several
decimal places of accuracy for 16k sizes.  There seem to be only two
alternatives in the literature that do not suffer similarly: a
recursive decomposition into smaller DCTs, which would require a large
set of codelets for efficiency and generality, or sacrificing a factor of) 2 in speed to use a real DFT of twice the size.  We currently employ the latter technique for general n, as well as a limited form of the former method: a split-radix decomposition when n is odd (N a multiple of 4).  For N containing many factors of 2, the split-radix method seems to recover most of the speed of the standard algorithm without the accuracy tradeoff.

Thus, if only the boundary conditions on the transform inputs are specified, we generally recommend R*DFT10 over R*DFT00 and R*DFT01 over R*DFT11 (unless the half-sample shift or the self-inverse property is significant for your problem).

If performance is important to you and you are using only small sizes (say n<200), e.g. for multi-dimensional transforms, then you might consider generating hard-coded transforms of those sizes and types that you are interested in (see Generating your own code).

We are interested in hearing what types of symmetric transforms you find most useful.

The Discrete Hartley Transform
------------------------------

If you are planning to use the DHT because you've heard that it is ``faster'' than the DFT (FFT), stop here.  The DHT is not faster than the DFT.  That story is an old but enduring misconception that was debunked in 1987.

The discrete Hartley transform (DHT) is an invertible linear transform closely related to the DFT.  In the DFT, one multiplies each input by cos - i * sin (a complex exponential), whereas in the DHT each input is multiplied by simply cos + sin.  Thus, the DHT transforms n real numbers to n real numbers, and has the convenient property of being its own inverse.  In FFTW, a DHT (of any positive n) can be specified by an r2r kind of FFTW_DHT.

Like the DFT, in FFTW the DHT is unnormalized, so computing a DHT of size n followed by another DHT of the same size will result in the original array multiplied by n.

The DHT was originally proposed as a more efficient alternative to the DFT for real data, but it was subsequently shown that a specialized DFT (such as FFTW's r2hc or r2c transforms) could be just as fast.  In FFTW, the DHT is actually computed by post-processing an r2hc transform, so there is ordinarily no reason to prefer it from a performance perspective. (We provide the DHT mainly as a byproduct of some internal algorithms. FFTW computes a real input/output DFT of prime size by re-expressing it as a DHT plus post/pre-processing and then using Rader's prime-DFT algorithm adapted to the DHT.) However, we have heard rumors that the DHT might be the most appropriate transform in its own right for certain applications, and we would be very interested to hear from anyone who finds it useful.

If FFTW_DHT is specified for multiple dimensions of a multi-dimensional transform, FFTW computes the separable product of 1d DHTs along each dimension.  Unfortunately, this is not quite the same thing as a true multi-dimensional DHT; you can compute the latter, if necessary, with at most rank-1 post-processing passes [see e.g. H. Hao and R. N. Bracewell, Proc. IEEE 75, 264--266 (1987)].

For the precise mathematical definition of the DHT as used by FFTW, see What FFTW Really Computes.