General Radially Symmetric (GNRL)

Use the General Radially Symmetric option to apply a user‑defined radially symmetric filter.

General Radially Symmetric Filter dialog options

Filter file name

Specifies the filter definition file. The default file extension is .flt.

The required file format is described below.

Application Notes

The general radially symmetric filter is used to apply a special‑purpose, custom filter to the data. Filter coefficients typically range from 0 to 1.

The filter coefficients must be defined at discrete wavenumber increments.

In the filter file, the first value specifies the wavenumber increment. All subsequent values are the filter coefficients.

You may only specify wavenumbers below the Nyquist wavenumber, defined as:

This limitation exists because the radial spectrum display can only represent wavelengths greater than the Nyquist sampling interval.

Assuming the increment is K, the filter file must follow this format:

GNRL K, I₀, I₁, I₂ ...

Where:

K – The wavenumber increment. It must be less than the Nyquist wavenumber.

I_i – The filter coefficients at successive multiples of K, starting at zero.

The final coefficient applies to all higher wavenumbers.

Coefficients may span multiple lines.

A slash (/) must follow the last coefficient.

Example

The following example defines a low-pass filter with a wavenumber increment of 0.001, which rolls off and removes all wavenumbers above 0.007:

L(0) = 1

L(0.001) = 1

L(0.002) = 1

L(0.003) = 1

L(0.004) = 0.8

L(0.005) = 0.5

L(0.006) = 0.2

L(0.007) = 0

Filter file syntax:

GNRL 0.001, 1, 1, 1, 1, .8, .5, .2, 0 /