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This is Info file fftw.info, produced by Makeinfo version 1.68 from the input file fftw.texi.

This is the FFTW User's manual.

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File: fftw.info, Node: FFTW Reference, Next: Parallel FFTW, Prev: Tutorial, Up: Top

FFTW Reference **************

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 (*note Data Types::.). Then, in four sections, we explain the FFTW program interface for complex one-dimensional transforms (*note One-dimensional Transforms Reference::.), complex multi-dimensional transforms (*note Multi-dimensional Transforms Reference::.), and real one-dimensional transforms (*note Real One-dimensional Transforms Reference::.), real multi-dimensional transforms (*note Real Multi-dimensional Transforms Reference::.). *Note Wisdom Reference:: describes the `wisdom' mechanism for exporting and importing plans. Finally, *Note Memory Allocator Reference:: describes how to change FFTW's default memory allocator. For parallel transforms, *Note Parallel FFTW::.

* Menu:

* Data Types:: real, complex, and halfcomplex numbers * One-dimensional Transforms Reference:: * Multi-dimensional Transforms Reference:: * Real One-dimensional Transforms Reference:: * Real Multi-dimensional Transforms Reference:: * Wisdom Reference:: * Memory Allocator Reference:: * Thread safety::

File: fftw.info, Node: Data Types, Next: One-dimensional Transforms Reference, Prev: FFTW Reference, Up: FFTW Reference

Data Types ==========

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

By including `<fftw.h>' or `<rfftw.h>', you will have access to the following definitions:

typedef double fftw_real; typedef struct { fftw_real re, im; } fftw_complex; #define c_re(c) ((c).re) #define c_im(c) ((c).im)

All FFTW operations are performed on the `fftw_real' and `fftw_complex' data types. For `fftw_complex' numbers, the two macros `c_re' and `c_im' retrieve, respectively, the real and imaginary parts of the number.

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

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 "halfcomplex" arrays. A halfcomplex array of size n is a one-dimensional array of n `fftw_real' 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[i] = Re(X[i]). For all integers i such that 0 < i < n / 2, we have Y[n-i] = Im(X[i]).

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[0] = Re(X[0]), Y[1] = Re(X[1]), Y[2] = Re(X[2]), and Y[3] = Im(X[1]). Let now n = 5. In this case, we have Y[0] = Re(X[0]), Y[1] = Re(X[1]), Y[2] = Re(X[2]), Y[3] = Im(X[2]), and Y[4] = Im(X[1]).

By default, the type `fftw_real' equals the C type `double'. To work in single precision rather than double precision, `#define' the symbol `FFTW_ENABLE_FLOAT' in `fftw.h' and then recompile the library. On Unix systems, you can instead use `configure --enable-float' at installation time (*note Installation and Customization::.).

In version 1 of FFTW, the data types were called `FFTW_REAL' and `FFTW_COMPLEX'. We changed the capitalization for consistency with the rest of FFTW's conventions. The old names are still supported, but their use is deprecated.

File: fftw.info, Node: One-dimensional Transforms Reference, Next: Multi-dimensional Transforms Reference, Prev: Data Types, Up: FFTW Reference

One-dimensional Transforms Reference ====================================

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

* Menu:

* fftw_create_plan:: Plan Creation * Discussion on Specific Plans:: * fftw:: Plan Execution * fftw_destroy_plan:: Plan Destruction * What FFTW Really Computes:: Definition of the DFT.

File: fftw.info, Node: fftw_create_plan, Next: Discussion on Specific Plans, Prev: One-dimensional Transforms Reference, Up: One-dimensional Transforms Reference

Plan Creation for One-dimensional Transforms --------------------------------------------

#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);

The function `fftw_create_plan' creates a plan, which is a data structure containing all the information that `fftw' 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).

`fftw_create_plan' returns a valid plan, or `NULL' 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.

The `fftw_create_plan_specific' 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 `fftw'. 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 *destroyed* by the specific planner (the initial contents are ignored, so the arrays need not have been initialized).

Arguments .........

* `n' is the size of the transform. It can be any positive integer.

- FFTW is best at handling sizes of the form 2^a 3^b 5^c 7^d 11^e 13^f, 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. *Note Installation and Customization::, for more information.) Transforms whose sizes are powers of 2 are especially fast.

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

* `flags' is a boolean OR (`|') of zero or more of the following: - `FFTW_MEASURE': this flag tells FFTW to find the optimal plan by actually *computing* several FFTs and measuring their execution time. Depending on the installation, this can take some time. (1)

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

- `FFTW_OUT_OF_PLACE': produce a plan assuming that the input and output arrays will be distinct (this is the default).

- `FFTW_IN_PLACE': 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 `n'. 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. *Warning: This option changes the meaning of some parameters of `fftw'* (*note Computing the One-dimensional Transform: fftw.).

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 `n * n * n' 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 `n'. 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 `FFTW_OUT_OF_PLACE'.

- `FFTW_USE_WISDOM': use any `wisdom' that is available to help in the creation of the plan. (*Note Words of Wisdom::.) This can greatly speed the creation of plans, especially with the `FFTW_MEASURE' option. `FFTW_ESTIMATE' plans can also take advantage of `wisdom' to produce a more optimal plan (based on past measurements) than the estimation heuristic would normally generate. When the `FFTW_MEASURE' option is used, new `wisdom' will also be generated if the current transform size is not completely understood by existing `wisdom'.

* `in', `out', `istride', `ostride' (only for `fftw_create_plan_specific'): see corresponding arguments in the description of `fftw'. (*Note Computing the One-dimensional Transform: fftw.) In particular, the `out' and `ostride' parameters have the same special meaning for `FFTW_IN_PLACE' transforms as they have for `fftw'.

---------- Footnotes ----------

(1) The basic problem is the resolution of the clock: FFTW needs to run for a certain time for the clock to be reliable.

File: fftw.info, Node: Discussion on Specific Plans, Next: fftw, Prev: fftw_create_plan, Up: One-dimensional Transforms Reference

Discussion on Specific Plans ----------------------------

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:

* 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.)

* Less intermediate storage is required during the planning process. (The ordinary planner uses O(`N') temporary storage, where `N' is the maximum dimension, while it is creating the plan.)

* 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.)

On the other hand, note that *the specific planner destroys the contents of the `in' and `out' arrays*.

File: fftw.info, Node: fftw, Next: fftw_destroy_plan, Prev: Discussion on Specific Plans, Up: One-dimensional Transforms Reference

Computing the One-dimensional Transform ---------------------------------------

#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);

The function `fftw' computes the one-dimensional Fourier transform, using a plan created by `fftw_create_plan' (*Note Plan Creation for One-dimensional Transforms: fftw_create_plan.) The function `fftw_one' provides a simplified interface for the common case of single input array of stride 1.

Arguments .........

* `plan' is the plan created by `fftw_create_plan' (*note Plan Creation for One-dimensional Transforms: fftw_create_plan.).

* `howmany' is the number of transforms `fftw' will compute. It is faster to tell FFTW to compute many transforms, instead of simply calling `fftw' many times.

* `in', `istride' and `idist' describe the input array(s). There are `howmany' input arrays; the first one is pointed to by `in', the second one is pointed to by `in + idist', and so on, up to `in + (howmany - 1) * idist'. Each input array consists of complex numbers (*note Data Types::.), which are not necessarily contiguous in memory. Specifically, `in[0]' is the first element of the first array, `in[istride]' is the second element of the first array, and so on. In general, the `i'-th element of the `j'-th input array will be in position `in[i * istride + j * idist]'.

* `out', `ostride' and `odist' describe the output array(s). The format is the same as for the input array.

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

The function `fftw_one' transforms a single, contiguous input array to a contiguous output array. By definition, the call fftw_one(plan, in, out) is equivalent to fftw(plan, 1, in, 1, 1, out, 1, 1)

File: fftw.info, Node: fftw_destroy_plan, Next: What FFTW Really Computes, Prev: fftw, Up: One-dimensional Transforms Reference

Destroying a One-dimensional Plan ---------------------------------

#include <fftw.h> void fftw_destroy_plan(fftw_plan plan);

The function `fftw_destroy_plan' frees the plan `plan' and releases all the memory associated with it. After destruction, a plan is no longer valid.

File: fftw.info, Node: What FFTW Really Computes, Prev: fftw_destroy_plan, Up: One-dimensional Transforms Reference

What FFTW Really Computes -------------------------

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.

The forward transform of a complex array X of size n computes an array Y, where

Y[i] = sum for j = 0 to (n - 1) of X[j] * exp(-2 pi i j sqrt(-1)/n) .

The backward transform computes

Y[i] = sum for j = 0 to (n - 1) of X[j] * exp(2 pi i j sqrt(-1)/n) .

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.

An `FFTW_FORWARD' 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.)

File: fftw.info, Node: Multi-dimensional Transforms Reference, Next: Real One-dimensional Transforms Reference, Prev: One-dimensional Transforms Reference, Up: FFTW Reference

Multi-dimensional Transforms Reference ======================================

The multi-dimensional complex routines are generally prefixed with `fftwnd_'. Programs using FFTWND should be linked with `-lfftw -lm' on Unix systems, or with the FFTW and standard math libraries in general.

* Menu:

* fftwnd_create_plan:: Plan Creation * fftwnd:: Plan Execution * fftwnd_destroy_plan:: Plan Destruction * What FFTWND Really Computes::

File: fftw.info, Node: fftwnd_create_plan, Next: fftwnd, Prev: Multi-dimensional Transforms Reference, Up: Multi-dimensional Transforms Reference

Plan Creation for Multi-dimensional Transforms ----------------------------------------------

#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);

The function `fftwnd_create_plan' creates a plan, which is a data structure containing all the information that `fftwnd' 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 `fftw2d_create_plan' and `fftw3d_create_plan' are optional, alternative interfaces to `fftwnd_create_plan' for two and three dimensions, respectively.

`fftwnd_create_plan' returns a valid plan, or `NULL' 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 `rank' < 0).

The `create_plan_specific' 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 `fftwnd'. 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 *destroyed* by the specific planner (the initial contents are ignored, so the arrays need not have been initialized). *Note Discussion on Specific Plans::, for a discussion on specific plans.

Arguments .........

* `rank' is the dimensionality of the arrays to be transformed. It can be any non-negative integer.

* `n' is a pointer to an array of `rank' 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 row-major arrays--i.e. `n[0]' is the size of the dimension whose indices vary most slowly, and so on. (*Note Multi-dimensional Array Format::, for more information on row-major storage.) *Note Plan Creation for One-dimensional Transforms: fftw_create_plan, for more information regarding optimal array sizes.

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

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

- `FFTW_OUT_OF_PLACE': produce a plan assuming that the input and output arrays will be distinct (this is the default).

- `FFTW_IN_PLACE': produce a plan assuming that you want to perform the transform in-place. (Unlike the one-dimensional transform, this "really" (1) performs the transform in-place.) Note that, if you want to perform in-place transforms, you *must* use a plan created with this option.

The default mode of operation is `FFTW_OUT_OF_PLACE'.

* `in', `out', `istride', `ostride' (only for the `_create_plan_specific' variants): see corresponding arguments in the description of `fftwnd'. (*Note Computing the Multi-dimensional Transform: fftwnd.)

---------- Footnotes ----------

(1) `fftwnd' actually may use some temporary storage (hidden in the plan), but this storage space is only the size of the largest dimension of the array, rather than being as big as the entire array. (Unless you use `fftwnd' to perform one-dimensional transforms, in which case the temporary storage required for in-place transforms *is* as big as the entire array.)

File: fftw.info, Node: fftwnd, Next: fftwnd_destroy_plan, Prev: fftwnd_create_plan, Up: Multi-dimensional Transforms Reference

Computing the Multi-dimensional Transform -----------------------------------------

#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);

The function `fftwnd' computes one or more multi-dimensional Fourier Transforms, using a plan created by `fftwnd_create_plan' (*note Plan Creation for Multi-dimensional Transforms: fftwnd_create_plan.). (Note that the plan determines the rank and dimensions of the array to be transformed.) The function `fftwnd_one' provides a simplified interface for the common case of single input array of stride 1.

Arguments .........

* `plan' is the plan created by `fftwnd_create_plan'. (*note Plan Creation for Multi-dimensional Transforms: fftwnd_create_plan.). In the case of two and three-dimensional transforms, it could also have been created by `fftw2d_create_plan' or `fftw3d_create_plan', respectively.

* `howmany' is the number of multi-dimensional transforms `fftwnd' will compute.

* `in', `istride' and `idist' describe the input array(s). There are `howmany' multi-dimensional input arrays; the first one is pointed to by `in', the second one is pointed to by `in + idist', and so on, up to `in + (howmany - 1) * idist'. Each multi-dimensional input array consists of complex numbers (*note Data Types::.), stored in row-major format (*note Multi-dimensional Array Format::.), which are not necessarily contiguous in memory. Specifically, `in[0]' is the first element of the first array, `in[istride]' is the second element of the first array, and so on. In general, the `i'-th element of the `j'-th input array will be in position `in[i * istride + j * idist]'. Note that, here, `i' refers to an index into the row-major format for the multi-dimensional array, rather than an index in any particular dimension.

- *In-place transforms*: For plans created with the `FFTW_IN_PLACE' option, the transform is computed in-place--the output is returned in the `in' array, using the same strides, etcetera, as were used in the input.

* `out', `ostride' and `odist' describe the output array(s). The format is the same as for the input array.

- *In-place transforms*: These parameters are ignored for plans created with the `FFTW_IN_PLACE' option.

The function `fftwnd_one' transforms a single, contiguous input array to a contiguous output array. By definition, the call fftwnd_one(plan, in, out) is equivalent to fftwnd(plan, 1, in, 1, 1, out, 1, 1)

File: fftw.info, Node: fftwnd_destroy_plan, Next: What FFTWND Really Computes, Prev: fftwnd, Up: Multi-dimensional Transforms Reference

Destroying a Multi-dimensional Plan -----------------------------------

#include <fftw.h> void fftwnd_destroy_plan(fftwnd_plan plan);

The function `fftwnd_destroy_plan' frees the plan `plan' and releases all the memory associated with it. After destruction, a plan is no longer valid.

File: fftw.info, Node: What FFTWND Really Computes, Prev: fftwnd_destroy_plan, Up: Multi-dimensional Transforms Reference

What FFTWND Really Computes ---------------------------

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 TeX version of this manual contains the exact definition of the n-dimensional transform FFTW uses. It is not possible to display the definition on a ASCII terminal properly.

File: fftw.info, Node: Real One-dimensional Transforms Reference, Next: Real Multi-dimensional Transforms Reference, Prev: Multi-dimensional Transforms Reference, Up: FFTW Reference

Real One-dimensional Transforms Reference =========================================

The one-dimensional real routines are generally prefixed with `rfftw_'. (1) Programs using RFFTW should be linked with `-lrfftw -lfftw -lm' on Unix systems, or with the RFFTW, the FFTW, and the standard math libraries in general.

* Menu:

* rfftw_create_plan:: Plan Creation * rfftw:: Plan Execution * rfftw_destroy_plan:: Plan Destruction * What RFFTW Really Computes::

---------- Footnotes ----------

(1) The etymologically-correct spelling would be `frftw_', but it is hard to remember.

File: fftw.info, Node: rfftw_create_plan, Next: rfftw, Prev: Real One-dimensional Transforms Reference, Up: Real One-dimensional Transforms Reference

Plan Creation for Real One-dimensional Transforms -------------------------------------------------

#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);

The function `rfftw_create_plan' creates a plan, which is a data structure containing all the information that `rfftw' 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).

`rfftw_create_plan' returns a valid plan, or `NULL' 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.

The `rfftw_create_plan_specific' 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 `rfftw'. 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 *destroyed* by the specific planner (the initial contents are ignored, so the arrays need not have been initialized). *Note Discussion on Specific Plans::, for a discussion on specific plans.

Arguments .........

* `n' is the size of the transform. It can be any positive integer.

- RFFTW is best at handling sizes of the form 2^a 3^b 5^c 7^d 11^e 13^f, 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^2) performance for prime sizes). (It is possible to customize RFFTW for different array sizes. *Note Installation and Customization::, for more information.) Transforms whose sizes are powers of 2 are especially fast.

* `dir' is the direction of the desired transform, either `FFTW_REAL_TO_COMPLEX' or `FFTW_COMPLEX_TO_REAL', corresponding to `FFTW_FORWARD' or `FFTW_BACKWARD', respectively.

* `flags' is a boolean OR (`|') of zero or more of the following: - `FFTW_MEASURE': this flag tells RFFTW to find the optimal plan by actually *computing* several FFTs and measuring their execution time. Depending on the installation, this can take some time.

- `FFTW_OUT_OF_PLACE': produce a plan assuming that the input and output arrays will be distinct (this is the default).

- `FFTW_IN_PLACE': 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 `n'. 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. *Warning: This option changes the meaning of some parameters of `rfftw'* (*note Computing the Real One-dimensional Transform: rfftw.).

The default mode of operation is `FFTW_OUT_OF_PLACE'.

* `in', `out', `istride', `ostride' (only for `rfftw_create_plan_specific'): see corresponding arguments in the description of `rfftw'. (*Note Computing the Real One-dimensional Transform: rfftw.) In particular, the `out' and `ostride' parameters have the same special meaning for `FFTW_IN_PLACE' transforms as they have for `rfftw'.

File: fftw.info, Node: rfftw, Next: rfftw_destroy_plan, Prev: rfftw_create_plan, Up: Real One-dimensional Transforms Reference

Computing the Real One-dimensional Transform --------------------------------------------

#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);

The function `rfftw' computes the Real One-dimensional Fourier Transform, using a plan created by `rfftw_create_plan' (*note Plan Creation for Real One-dimensional Transforms: rfftw_create_plan.). The function `rfftw_one' provides a simplified interface for the common case of single input array of stride 1.

*Important:* When invoked for an out-of-place, `FFTW_COMPLEX_TO_REAL' transform, the input array is overwritten with scratch values by these routines. The input array is not modified for `FFTW_REAL_TO_COMPLEX' transforms.

Arguments .........

* `plan' is the plan created by `rfftw_create_plan' (*note Plan Creation for Real One-dimensional Transforms: rfftw_create_plan.).

* `howmany' is the number of transforms `rfftw' will compute. It is faster to tell RFFTW to compute many transforms, instead of simply calling `rfftw' many times.

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

* `out', `ostride' and `odist' describe the output array(s), and have the same meaning as the corresponding parameters for the input array.

The function `rfftw_one' transforms a single, contiguous input array to a contiguous output array. By definition, the call rfftw_one(plan, in, out) is equivalent to rfftw(plan, 1, in, 1, 1, out, 1, 1)

File: fftw.info, Node: rfftw_destroy_plan, Next: What RFFTW Really Computes, Prev: rfftw, Up: Real One-dimensional Transforms Reference

Destroying a Real One-dimensional Plan --------------------------------------

#include <rfftw.h> void rfftw_destroy_plan(rfftw_plan plan);

The function `rfftw_destroy_plan' frees the plan `plan' and releases all the memory associated with it. After destruction, a plan is no longer valid.

File: fftw.info, Node: What RFFTW Really Computes, Prev: rfftw_destroy_plan, Up: Real One-dimensional Transforms Reference

What RFFTW Really Computes --------------------------

In this section, we define precisely what RFFTW computes.

The real to complex (`FFTW_REAL_TO_COMPLEX') transform of a real array X of size n computes an hermitian array Y, where

Y[i] = sum for j = 0 to (n - 1) of X[j] * exp(-2 pi i j sqrt(-1)/n)

(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 (*note Data Types::.). Currently, RFFTW provides no way to compute a real to complex transform with a positive sign in the exponent.

The complex to real (`FFTW_COMPLEX_TO_REAL') transform of a hermitian array X of size n computes a real array Y, where

Y[i] = sum for j = 0 to (n - 1) of X[j] * exp(2 pi i j sqrt(-1)/n)

(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 (*note Data Types::.). Currently, RFFTW provides no way to compute a complex to real transform with a negative sign in the exponent.

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.

File: fftw.info, Node: Real Multi-dimensional Transforms Reference, Next: Wisdom Reference, Prev: Real One-dimensional Transforms Reference, Up: FFTW Reference

Real Multi-dimensional Transforms Reference ===========================================

The multi-dimensional real routines are generally prefixed with `rfftwnd_'. Programs using RFFTWND should be linked with `-lrfftw -lfftw -lm' on Unix systems, or with the FFTW, RFFTW, and standard math libraries in general.

* Menu:

* rfftwnd_create_plan:: Plan Creation * rfftwnd:: Plan Execution * Array Dimensions for Real Multi-dimensional Transforms:: * Strides in In-place RFFTWND:: * rfftwnd_destroy_plan:: Plan Destruction * What RFFTWND Really Computes::

File: fftw.info, Node: rfftwnd_create_plan, Next: rfftwnd, Prev: Real Multi-dimensional Transforms Reference, Up: Real Multi-dimensional Transforms Reference

Plan Creation for Real Multi-dimensional Transforms ---------------------------------------------------

#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);

The function `rfftwnd_create_plan' creates a plan, which is a data structure containing all the information that `rfftwnd' 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 `rfftw2d_create_plan' and `rfftw3d_create_plan' are optional, alternative interfaces to `rfftwnd_create_plan' for two and three dimensions, respectively.

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

Arguments .........

* `rank' is the dimensionality of the arrays to be transformed. It can be any non-negative integer.

* `n' is a pointer to an array of `rank' integers, giving the size of each dimension of the arrays to be transformed. Note that these are always the dimensions of the *real* arrays; the complex arrays have different dimensions (*note Array Dimensions for Real Multi-dimensional Transforms::.). These sizes, which must be positive integers, correspond to the dimensions of row-major arrays--i.e. `n[0]' is the size of the dimension whose indices vary most slowly, and so on. (*Note Multi-dimensional Array Format::, for more information.) - *Note Plan Creation for Real One-dimensional Transforms: rfftw_create_plan, for more information regarding optimal array sizes.

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

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

* `dir' is the direction of the desired transform, either `FFTW_REAL_TO_COMPLEX' or `FFTW_COMPLEX_TO_REAL', corresponding to `FFTW_FORWARD' or `FFTW_BACKWARD', respectively.

- `FFTW_OUT_OF_PLACE': produce a plan assuming that the input and output arrays will be distinct (this is the default).

- `FFTW_IN_PLACE': 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 *must* use a plan created with this option. The use of this option has important implications for the size of the input/output array (*note Computing the Real Multi-dimensional Transform: rfftwnd.).

The default mode of operation is `FFTW_OUT_OF_PLACE'.

File: fftw.info, Node: rfftwnd, Next: Array Dimensions for Real Multi-dimensional Transforms, Prev: rfftwnd_create_plan, Up: Real Multi-dimensional Transforms Reference

Computing the Real Multi-dimensional Transform ----------------------------------------------

#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);

These functions compute the real multi-dimensional Fourier Transform, using a plan created by `rfftwnd_create_plan' (*note Plan Creation for Real Multi-dimensional Transforms: rfftwnd_create_plan.). (Note that the plan determines the rank and dimensions of the array to be transformed.) The "rfftwnd_one_" 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.

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

Arguments .........

* `plan' is the plan created by `rfftwnd_create_plan'. (*note Plan Creation for Real Multi-dimensional Transforms: rfftwnd_create_plan.). In the case of two and three-dimensional transforms, it could also have been created by `rfftw2d_create_plan' or `rfftw3d_create_plan', respectively.

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

* `howmany' is the number of transforms to be computed.

* `in', `istride' and `idist' describe the input array(s). There are `howmany' input arrays; the first one is pointed to by `in', the second one is pointed to by `in + idist', and so on, up to `in + (howmany - 1) * idist'. Each input array is stored in row-major format (*note Multi-dimensional Array Format::.), and is not necessarily contiguous in memory. Specifically, `in[0]' is the first element of the first array, `in[istride]' is the second element of the first array, and so on. In general, the `i'-th element of the `j'-th input array will be in position `in[i * istride + j * idist]'. Note that, here, `i' 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 (*note Array Dimensions for Real Multi-dimensional Transforms::.).

- *In-place transforms*: For plans created with the `FFTW_IN_PLACE' option, the transform is computed in-place--the output is returned in the `in' array. The meaning of the `stride' and `dist' parameters in this case is subtle and is discussed below (*note Strides in In-place RFFTWND::.).

* `out', `ostride' and `odist' 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.

- *In-place transforms*: These parameters are ignored for plans created with the `FFTW_IN_PLACE' option.

The function `rfftwnd_one' transforms a single, contiguous input array to a contiguous output array. By definition, the call rfftwnd_one_...(plan, in, out) is equivalent to rfftwnd_...(plan, 1, in, 1, 1, out, 1, 1)

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