fftw-internal: Low-level 'FFTW3' wrappers

In ravetools: Signal and Image Processing Toolbox for Analyzing Intracranial Electroencephalography Data

Low-level FFTW3 wrappers

Description

Thin R bindings around the FFTW3 library. These are low-level routines exposed primarily for advanced users and other packages that need maximum throughput. They perform minimal input checking and follow FFTW conventions (e.g. unnormalized inverse transforms, one-sided real-to-complex spectra). For most user code prefer fft, mvfft, or higher-level helpers in this package such as convolve, pwelch, multitaper, and the filtering utilities.

Warning: the API is intentionally close to FFTW's C interface and may change between releases. Outputs match the corresponding base R transforms up to floating-point round-off.

Arguments

data	Numeric (real) or complex input. For 2D/3D variants, a matrix or 3-dimensional array. For mvfftw_* functions, a matrix whose columns are transformed independently.
HermConj	Integer 0 or 1. When 1, return the full Hermitian-symmetric spectrum of length N (matching stats::fft); when 0, return only the non-redundant one-sided half of length floor(N/2) + 1.
inverse	Integer 0 (forward) or 1 (inverse). The inverse transform is unnormalized; divide by the number of elements to invert a forward transform, mirroring stats::fft(., inverse = TRUE).
fftwplanopt	Integer planner effort: 0 = FFTW_ESTIMATE (default, fast planning), 1 = FFTW_MEASURE, 2 = FFTW_PATIENT, 3 = FFTW_EXHAUSTIVE.
retrows	Integer; expected number of rows of the time-domain signal for mvfftw_c2r when reconstructing from a one-sided spectrum.
ret	Optional reusable output buffer of the correct type and length; pass NULL (default) to let the function allocate one. This input is for advanced users; leave NULL if you do not know what you are d

Details

All functions preserve their data argument: the input buffer is copied internally before planning when needed, so callers may safely reuse data after the call. For multi-dimensional variants, axis ordering follows R (column-major) conventions.

Value

A complex (or real, for *_c2r) vector / matrix / array matching the corresponding base R transform up to floating-point error.

Examples

set.seed(1)

## --- 1D real-to-complex --------------------------------------------------
x <- rnorm(16)
a <- ravetools::fftw_r2c(x, HermConj = 1)
b <- stats::fft(x)
all.equal(a, b)                                  # TRUE (within tol)

# one-sided spectrum (length floor(N/2)+1)
a_half <- ravetools::fftw_r2c(x, HermConj = 0)
all.equal(a_half, b[seq_len(length(x) %/% 2 + 1)])

## --- 1D complex-to-complex ----------------------------------------------
z <- complex(real = rnorm(16), imaginary = rnorm(16))
all.equal(ravetools::fftw_c2c(z, inverse = 0), stats::fft(z))
all.equal(ravetools::fftw_c2c(z, inverse = 1), stats::fft(z, inverse = TRUE))

## --- 1D complex-to-real (inverse of fftw_r2c) ---------------------------
# Using the full Hermitian spectrum:
xr <- ravetools::fftw_c2r(a, HermConj = 1) / length(x)
all.equal(xr, x)

## --- Multivariate (column-wise) ----------------------------------------
M <- matrix(rnorm(32), nrow = 8, ncol = 4)
all.equal(ravetools::mvfftw_r2c(M, HermConj = 1), stats::mvfft(M + 0i))

Mz <- matrix(complex(real = rnorm(32), imaginary = rnorm(32)),
             nrow = 8, ncol = 4)
all.equal(ravetools::mvfftw_c2c(Mz, inverse = 0), stats::mvfft(Mz))
all.equal(ravetools::mvfftw_c2c(Mz, inverse = 1),
          stats::mvfft(Mz, inverse = TRUE))

# one-sided -> back to real signal
Mh <- ravetools::mvfftw_r2c(M, HermConj = 0)
Mr <- ravetools::mvfftw_c2r(Mh, retrows = nrow(M)) / nrow(M)
all.equal(Mr, M)

## --- 2D ----------------------------------------------------------------
X2 <- matrix(rnorm(20), nrow = 5, ncol = 4)
all.equal(ravetools::fftw_r2c_2d(X2, HermConj = 1), stats::fft(X2 + 0i))

Z2 <- matrix(complex(real = rnorm(20), imaginary = rnorm(20)),
             nrow = 5, ncol = 4)
all.equal(ravetools::fftw_c2c_2d(Z2, inverse = 0), stats::fft(Z2))
all.equal(ravetools::fftw_c2c_2d(Z2, inverse = 1),
          stats::fft(Z2, inverse = TRUE))

## --- 3D ----------------------------------------------------------------
X3 <- array(rnorm(60), dim = c(5, 4, 3))
all.equal(ravetools::fftw_r2c_3d(X3, HermConj = 1), stats::fft(X3 + 0i))

Z3 <- array(complex(real = rnorm(60), imaginary = rnorm(60)),
            dim = c(5, 4, 3))
all.equal(ravetools::fftw_c2c_3d(Z3, inverse = 0), stats::fft(Z3))
all.equal(ravetools::fftw_c2c_3d(Z3, inverse = 1),
          stats::fft(Z3, inverse = TRUE))

ravetools documentation built on May 23, 2026, 9:10 a.m.