Re: bandpass filter for 2D Data

[John Swensen]

On Aug 27, 2009, at 3:15 AM, Dr. Johannes Zellner wrote:

> Hi
>
> does anyone have an implementation of a 2D bandpass filter?
>
> Suppose I've 2d data in a matrix of 1000 x 1000 pixels which
> represents a spatial extension of 1 m (1 mm per pixel).
> Now I'd like to filter away all frequencies below a wavelength of 50
> mm and above a wavelength of 150 mm,
> so the resulting data should only have frequencies corresponding to
> wavelengths between 50 and 150 mm.
>
> Any octave examples for this?
>
> --  
> Johannes
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I don't know if there is an example of how to do this, but I think I  
remember doing the following pseudocode in my computer visions class:

1) perform the 2D FFT
2) figure out, based on your spatial sampling frequency, how much one  
"pixel" in the magnitude portion of the 2D FFT is
3) set everything but an annulus whose larger radius equates to 150mm  
wavelength and inner radius equates to 50mm to zero.
4) Perform the inverse 2D FFT

Someone please correct me if this doesn't work, but I think this ends  
up being a fairly quick way to do what you ask and easy to implement.

John Swensen