CCFFT(3S)CCFFT(3S)NAME
CCFFT, ZZFFT - Applies a complex-to-complex Fast Fourier Transform (FFT)
SYNOPSIS
Single precision complex -> Single precision complex
Fortran:
CALL CCFFT (isign, n, scale, x, y, table, work, isys)
C/C++:
#include <scsl_fft.h>
int ccfft (int isign, int n, float scale, scsl_complex *x,
scsl_complex *y, float *table, float *work, int *isys);
C++ STL:
#include <complex.h>
#include <scsl_fft.h>
int ccfft (int isign, int n, float scale, complex<float> *x,
complex<float> *y, float *table, float *work, int *isys);
Double precision complex -> Double precision complex
Fortran:
CALL ZZFFT (isign, n, scale, x, y, table, work, isys)
C/C++:
#include <scsl_fft.h>
int zzfft (int isign, int n, double scale, scsl_zomplex *x,
scsl_zomplex *y, double *table, double *work, int *isys);
C++ STL:
#include <complex.h>
#include <scsl_fft.h>
int zzfft (int isign, int n, double scale, complex<double> *x,
complex<double> *y, double *table, double *work, int *isys);
IMPLEMENTATION
These routines are part of the SCSL Scientific Library and can be loaded
using either the -lscs or the -lscs_mp option. The -lscs_mp option
directs the linker to use the multi-processor version of the library.
When linking to SCSL with -lscs or -lscs_mp, the default integer size is
4 bytes (32 bits). Another version of SCSL is available in which integers
are 8 bytes (64 bits). This version allows the user access to larger
memory sizes and helps when porting legacy Cray codes. It can be loaded
by using the -lscs_i8 option or the -lscs_i8_mp option. A program may use
only one of the two versions; 4-byte integer and 8-byte integer library
calls cannot be mixed.
The C and C++ prototypes shown above are appropriate for the 4-byte
integer version of SCSL. When using the 8-byte integer version, the
variables of type int become long long and the <scsl_fft_i8.h> header
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file should be included.
DESCRIPTION
These routines compute the Fast Fourier Transform (FFT) of the complex
vector x, and store the result in vector y.
In FFT applications, it is customary to use zero-based subscripts; the
formulas are simpler that way. Suppose that the arrays are declared as
follows:
Fortran:
COMPLEX X(0:N-1), Y(0:N-1)
C/C++:
scsl_complex x[n], y[n];
C++ STL:
complex<float> x[n], y[n];
The output array is the FFT of the input array, using the following
formula for the FFT:
n-1 isign * j * k
Y k = scale * Sum [X * w ]
k j=0 j
for k = 0 ..., n-1 where:
w = exp(2*pi*i/n),
i = + sqrt(-1),
pi = 3.14159...
isign = +1 or -1
Different authors use different conventions for which of the transforms,
isign = +1 or isign = -1, is the forward or inverse transform, and what
the scale factor should be in either case. You can make this routine
compute any of the various possible definitions, however, by choosing the
appropriate values for isign and scale.
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CCFFT(3S)CCFFT(3S)
The relevant fact from FFT theory is this: If you take the FFT with any
particular values of isign and scale, the mathematical inverse function
is computed by taking the FFT with -isign and n 1/(n*scale). In
particular, if you use isign = +1 and scale = 1.0 you can compute the
inverse FFT by using isign = -1 and scale = 1.0/n.
The output array may be the same as the input array.
See the NOTES section of this man page for information about the
interpretation of the data types described in the following arguments.
These routines have the following arguments:
isign Integer. (input)
Specifies whether to initialize the table array or to do the
forward or inverse Fourier transform, as follows:
If isign = 0, the routine initializes the table array and
returns. In this case, the only arguments used or checked are
isign, n, and table.
If isign = +1 or -1, the value of isign is the sign of the
exponent used in the FFT formula.
n Integer. (input)
Size of the transform (the number of values in the input
array). n >= 0.
scale Scale factor. (input).
CCFFT: Single precision.
ZZFFT: Double precision.
Each element of the output array is multiplied by scale after
taking the Fourier transform, as defined by the previous
formula.
x Array of size n. (input)
CCFFT: Single precision complex array.
ZZFFT: Double precision complex array.
Input array of values to be transformed.
y Array of size n. (output)
CCFFT: Single precision complex array.
ZZFFT: Double precision complex array.
Output array of transformed values. The output array may be
the same as the input array. In that case, the transform is
done in place and the input array is overwritten with the
transformed values.
table Array of size (2*n + NF) (input or output)
CCFFT: Single precision array.
ZZFFT: Double precision array.
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Table of factors and roots of unity. See the description of
the isys argument for the value of NF.
If isign = 0, the routine initializes table (table is output
only).
If isign = +1 or -1, the values in table are assumed to be
initialized already by a prior call with isign = 0 (table is
input only).
work Array of size (2 * n).
CCFFT: Single precision array.
ZZFFT: Double precision array.
Work array. This is a scratch array used for intermediate
calculations. Its address space must be different address
space from that of the input and output arrays.
isys Integer array dimensioned 0..isys(0)
An array that gives implementation-specific information. All
features and functions of the FFT routines specific to any
particular implementation are confined to this isys array.
In the Origin series implementation, isys(0)=0 and isys(0)=1
are supported. In SCSL versions prior to 1.3, only isys(0)=0
was allowed. For isys(0)=0, NF=30, and for isys(0)=1, NF=256.
The NF words of storage in the table array contain a
factorization of the length of the transform.
The smaller value of NF for isys(0)=0 is historical. It is too
small to store all the required factors for the highest
performing FFT, so when isys(0)=0, extra space is allocated
when the table array is initialized. To avoid memory leaks,
this extra space must be deallocated when the table array is no
longer needed. The CCFFTF routine is used to release this
memory. Due to the potential for memory leaks, the use of
isys(0)=0 should be avoided.
For isys(0)=1, the value of NF is large enough so that no extra
memory needs to be allocated, and there is no need to call
CCFFTF to release memory. If called, it does nothing.
NOTE: isys(0)=1 means that isys is an integer array with two
elements. The second element, isys(1), will not be accessed.
NOTES
The following data types are described in this documentation:
Term Used Data type
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Fortran:
Array dimensioned n x(n)
Array of dimensions (m,n) x(m,n)
Integer INTEGER (INTEGER*8 for -lscs_i8[_mp])
Single precision REAL
Double precision DOUBLE PRECISION
Single precision complex COMPLEX
Double precision complex DOUBLE COMPLEX
C/C++:
Array dimensioned n x[n]
Array of dimensions (m,n) x[m*n] or x[n][m]
Integer int (long long for -lscs_i8[_mp])
Single precision float
Double precision double
Single precision complex scsl_complex
Double precision complex scsl_zomplex
C++ STL:
Array dimensioned n x[n]
Array of dimensions (m,n) x[m*n] or x[n][m]
Integer int (long long for -lscs_i8[_mp])
Single precision float
Double precision double
Single precision complex complex<float>
Double precision complex complex<double>
CAUTIONS
Transform sizes with a prime factor exceeding 232-1 are not supported for
the 8-byte integer version of the library.
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In addition to the work array, the FFT routines also dynamically allocate
scratch space from the stack. The amount of space allocated can be
slightly bigger than the size of the largest processor cache. For single
processor runs, the default stack size is large enough that these
allocations generally cause no problems. But for parallel runs, you need
to ensure that the stack size of slave threads is big enough to hold this
scratch space. Failure to reserve sufficient stack space will cause
programs to dump core due to stack overflows. The stack size of MP
library slave threads is controlled via the MP_SLAVE_STACKSIZE
environment variable or the mp_set_slave_stacksize() library routine. See
the mp(3C), mp(3F) and pe_environ(5) reference pages for more information
on controlling the slave stack size. For pthreads applications, the
thread's stack size is specified as one of many creation attributes
provided in the pthread_attr_t argument to pthread_create(3P). The
stacksize attribute should be set explicitly to a non-default value using
the pthread_attr_setstacksize(3P) call, described in the
pthread_attr_init(3P) man page.
Care must be exercised if copies of the table array are used: even though
a copy exists, the original must persist. As an example, the following
code will not work:
#include <scsl_fft.h>
scsl_complex x[1024], y[1024];
float table[2048+256];
float work[2048];
int isys[2];
isys[0] = 1;
{
float table_orig[2048+256];
ccfft(0, 1024, 1.0f, x, y, table_orig, work, isys);
bcopy(table_orig, table, (2048+256)*sizeof(float));
}
ccfft(1, 1024, 1.0f, x, y, table, work, isys);
In this example, because table_orig is a stack variable that does not
persist outside of the code block delimited by the braces, the data in
the copy, table, are not guaranteed to be valid. However, the following
code will work because table_orig is persistent:
#include <scsl_fft.h>
scsl_complex x[1024], y[1024];
float table_orig[2048+256], table[2048+256];
float work[2048];
int isys[2];
isys[0] = 1;
ccfft(0, 1024, 1.0f, x, y, table_orig, work, isys);
bcopy(table_orig, table, (2048+256)*sizeof(float));
ccfft(1, 1024, 1.0f, x, y, table, work, isys);
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CCFFT(3S)CCFFT(3S)EXAMPLES
These examples use the table and workspace sizes appropriate to the
Origin series.
Example 1: Initialize the table array in preparation for doing an FFT of
size 1024. Only the isign, n, and table arguments are used in this case.
You can use dummy arguments or zeros for the other arguments in the
subroutine call.
Fortran:
REAL TABLE(2048+256)
INTEGER ISYS(0:1)
ISYS(0) = 1
CALL CCFFT(0, 1024, 0.0, DUMMY, DUMMY, TABLE, DUMMY, ISYS)
C/C++:
#include <scsl_fft.h>
float table[2048+256];
int isys[2];
isys[0] = 1;
ccfft(0, 1024, 0.0f, NULL, NULL, table, NULL, isys);
C++ STL:
#include <complex.h>
#include <scsl_fft.h>
float table[2048+256];
int isys[2];
isys[0] = 1;
ccfft(0, 1024, 0.0f, NULL, NULL, table, NULL, isys);
Example 2: x and y are complex arrays of dimension (0:1023). Take the
FFT of x and store the results in y. Before taking the FFT, initialize
the table array, as in example 1.
Fortran:
COMPLEX X(0:1023), Y(0:1023)
REAL TABLE(2048+256)
REAL WORK(2048)
INTEGER ISYS(0:1)
ISYS(0) = 1
CALL CCFFT(0, 1024, 1.0, X, Y, TABLE, WORK, ISYS)
CALL CCFFT(1, 1024, 1.0, X, Y, TABLE, WORK, ISYS)
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C/C++:
#include <scsl_fft.h>
scsl_complex x[1024], y[1024];
float table[2048+256];
float work[2048];
int isys[2];
isys[0] = 1;
ccfft(0, 1024, 1.0f, x, y, table, work, isys);
ccfft(1, 1024, 1.0f, x, y, table, work, isys);
C++ STL:
#include <complex.h>
#include <scsl_fft.h>
complex<float> x[1024], y[1024];
float table[2048+256];
float work[2048];
int isys[2];
isys[0] = 1;
ccfft(0, 1024, 1.0f, x, y, table, work, isys);
ccfft(1, 1024, 1.0f, x, y, table, work, isys);
Example 3: Using the same x and y as in example 2, take the inverse FFT
of y and store it back in x. The scale factor 1/1024 is used. Assume
that the table array is already initialized.
Fortran:
CALL CCFFT(-1, 1024, 1.0/1024.0, Y, X, TABLE, WORK, ISYS)
C/C++ and C++ STL:
ccfft(-1, 1024, 1.0f/1024.0f, y, x, table, work, isys);
Example 4: Perform the same computation as in example 2, but put the
output back in array x to save storage space. Use the 8-byte integer
version of SCSL.
Fortran:
COMPLEX X(0:1023)
REAL TABLE(2048+256)
REAL WORK(2048)
INTEGER*8 ISYS(0:1)
ISYS(0) = 1_8
CALL CCFFT(0_8, 1024_8, 1.0, X, X, TABLE, WORK, ISYS)
CALL CCFFT(1_8, 1024_8, 1.0, X, X, TABLE, WORK, ISYS)
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C/C++:
#include <scsl_fft_i8.h>
scsl_complex x[1024]
float table[2048+256];
float work[2048];
long long isys[2];
isys[0] = 1LL;
ccfft(0LL, 1024LL, 1.0f, x, x, table, work, isys);
ccfft(1LL, 1024LL, 1.0f, x, x, table, work, isys);
C++ STL:
#include <complex.h>
#include <scsl_fft_i8.h>
complex<float> x[1024]
float table[2048+256];
float work[2048];
long long isys[2];
isys[0] = 1LL;
ccfft(0LL, 1024LL, 1.0f, x, x, table, work, isys);
ccfft(1LL, 1024LL, 1.0f, x, x, table, work, isys);
Example 5: Perform the same computation as in example 2, but assume that
the lower bound of each Fortran array is 1, rather than 0. No change is
needed in the subroutine calls:
Fortran:
COMPLEX X(1024), Y(1024)
CALL CCFFT(0, 1024, 1.0, X, Y, TABLE, WORK, ISYS)
CALL CCFFT(1, 1024, 1.0, X, Y, TABLE, WORK, ISYS)
SEE ALSOINTRO_FFT(3S), INTRO_SCSL(3S), CCFFTM(3S), SCFFT(3S), SCFFTM(3S)
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