# GfsOutputSpectra

### From Gerris

**GfsOutputSpectra** is a GfsOutput object that allows you to obtain the Fast Fourier Transform of a given variable. The syntax is as follows:

[ GfsOutput ] NAME { x = x_c y = y_c z = z_c Lx = L_x Ly = L_y Lz = L_z } LEVEL

where

- NAME
- is the variable used to apply the fft.
- (x_c, y_c, z_c)
- is the center of the line (in 1D), square (in 2D) or cube (in 3D) where the fft is applied
- (L_x, L_y, L_z)
- is the length of the line (in 1D fft), square (in 2D fft) or cube (in 3D).
- LEVEL
- is an optional argument that specifies the number of points in the domain where the fft is applied (2^LEVEL points in L). By default is set to the maximum level of refinement in the domain.

Note that by default the lengths is set to zero. This feature allows to perform any type of fft in 2D and 3D simulations.