NAME fft - fft 2d on an image SYNOPSIS fft [options] <in> <out> DESCRIPTION fft computes a Fast Fourier Transform on an input image. fft can also compute an inverse FFT. Results can have swapped quadrants, and can be expressed in polar or carte- sian coordinates. Take care about the formats : polar/cartesian and swapped/unswapped. The default procedure is: In input of a forward FFT, an image is required. It is taken as the real part (in cartesian coordinates) of a complex 2d signal which imaginary part is set to zeros. The output of a forward FFT is a complex 2d signal, i.e. 2 image planes. By default, the output is converted to polar coordinates (modulus, phase), and then quadrants are swapped in both modulus and phase to put low frequencies at the center of the images. To prevent this default behaviour, use the -n and -c options described below. In input of an inverse FFT, a cube containing 2 images is required. It is taken as a complex 2d signal which first plane contains the modulus and second plane contains the phase. Before the inverse FFT is computed, a swapping of quadrants occurs, and then a conversion to cartesian (real, imaginary) coordinates is performed. To prevent quadrant swapping or cartesian conversion, use the -n and -c options described below. The output of an inverse FFT is a cube containing 2 images in cartesian unswapped format, first one being the real part, second one being the imaginary part (meaningless imagewise). ALGORITHM fft uses the Danielson-Lanczos lemma, in a code based on one originally written by N. M. Brenner, described in Numerical Recipes in C. OPTIONS -c or --noconv Switches to cartesian mode the following data: output for a forward FFT (default is polar), or input for an inverse FFT. -i or --inverse Inverse FFT. The input is a cube containing two planes only. Default I/O format for these 2 planes is polar coordinates (modulus, phase). It will output a cube containing 1 plane only (imaginary part is meaningless imagewise). The input will be swapped before FFT, unless the -n option is used. -n or --noswap This option prevents fft from swapping the output of a forward FFT and the input of an inverse FFT. -s or --swaponly This option can be used to apply the swapping and quit. Swapping is done according to the following rule : 1 2 4 3 becomes then 3 4 2 1 which puts then the lowest frequencies at the center of the image. FILES Input files shall all comply with FITS format.