2D Spatial Fast Fourier Transforms

The 2D Spatial Fast Fourier Transforms subsystem in DigImage provides access to spatial FFTs at a reduced resolution. Due to limitations in the available memory within MS-DOS, the routines accessile from this menu are restricted to a resolution of 256 pixels in the horizontal by 128 in the vertical. The FFT and inverse FFT algorithms operate at REAL*8 precision for intermediate results, but the FFT is only stored at REAL*4 precision.

These features of DigImage are designed for the analysis of length scales and not for filtering purposes. The reduced resolution required to store the entire frame in wave number space does not produce acceptable resolution for filtering. However, [F: Filter in wave number domain] does allow some filtering operations if so desired.

Note that some of the options may operate either directly from a buffer, or from a copy of the data already present in memory. The latter is achieved by specifying a buffer number of -1. Note that the contents of the memory will be saved to a temporary file iMemory.DIG on exiting this menu, and restored from that file on entering again. While this requires additional disk space and some time overhead, it enables the use of other DigImage features without concern as to the state of the data stored in memory.

A Autocorrelation function (FFT-->power-->inverse FFT).

This option computes the autocorrelation function of an image by evaluating the FFT, forming the power spectrum, and then evaluating the inverse FFT of the power spectrum.

B Multiply FFT by complex conjugate.

This option converts the FFT currently in memory into a power spectrum by multiplying each component by its complex conjugate.

F: Filter in wave number domain.

As mentioned earlier, these filter functions are of only limited value due to the reduced spatial resolution. The filter operations may act either directly on a buffer or the FFT data already in memory.

I Imaginary part to buffer.

Transfer the imaginary part of the current memory array (FFT or spatial domain) to a buffer.

K FFT.

Perform a Fast Fourier Transformation on a buffer (or memory copy) to transform the data to wave number space.

L Load FFT file.

Load to the memory copy of the FFT information from a file saved previously by option S.

O Phast to buffer.

Transfer the phase information of the current memory array to a buffer.

P Power spectrum to buffer.

Transfer the power information of the current memory array to a buffer.

R Real part to buffer.

Transfer the real part of the current memory array to a buffer.

S Save FFT file.

Save the data (assumed to be FFT data) currently in memory to a file. This file may subsequently be reloaded by [L Load FFT file].

X Transform a buffer (or memory copy) back to physical space.

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