# getAlphaFilter

Create filter for `medium.alpha_filter`.

## Syntax

    alpha_filter = getAlphaFilter(kgrid, medium, filter_cutoff)
    alpha_filter = getAlphaFilter(kgrid, medium, filter_cutoff, taper_ratio)

## Description

`getAlphaFilter` uses `getWin` to create a Tukey window via rotation to pass to the `medium.alpha_filter` input field of the first order simulation functions (`kspaceFirstOrder1D`, `kspaceFirstOrder2D`, and `kspaceFirstOrder3D`). This parameter is used to regularise time reversal image reconstruction when absorption compensation is included.

## Inputs

`kgrid` | k-Wave grid object returned by kWaveGrid
`medium` | k-Wave medium structure
`filter_cutoff` | alpha_filter cutoff frequency [Hz], where `filter_cutoff = [f_all]` in 1-D `filter_cutoff = [f_all]` or `[f_x, f_y]` in 2-D `filter_cutoff = [f_all]` or `[f_x, f_y, f_z]` in 3-D Any of the filter_cutoff inputs may be set to `'max'` to set the cutoff frequency to the maximum frequency supported by the grid

## Optional Inputs

`taper_ratio` | taper ratio for Tukey Window (default = 0.5)

## Outputs

`alpha_filter` | filter

## Examples

- Image Reconstruction With Compensation For Acoustic Absorption

## See Also

`getWin`, `kspaceFirstOrder1D`, `kspaceFirstOrder2D`, `kspaceFirstOrder3D`