Posted by
Michael Schmid on
Dec 29, 2008; 12:07pm
URL: http://imagej.273.s1.nabble.com/FFT-filter-conversion-to-real-space-tp3694163p3694173.html
Hi,
Eliminating a horizontal line with vertical frequency = 0 corresponds to
subtracting the average of each vertical line in the image. The wider the
rectangle that you block, the more you also remove slow variations along a
vertical line.
So, the kernel of the filter operation would be too large for a reasonably
fast convolution operation. It will be a n*1 kernel, with large 'n', a
value of 1 at the zero point and all other values negative, tapering off
to 0 at large radius. The sum of all kernel values must be zero.
A better alternative to the inverse of the Fourier domain operation is
subtracting a filtered image in real space. You can try it using the "Fast
Filters" plugin:
http://imagejdocu.tudor.lu/doku.php?id=plugin:filter:fast_filters:startSelect 'x radius = 0', a large value of 'y radius', and 'subtract filtered'.
The 'mean' or 'border-limited mean' will come close to your Fourier domain
filter. If your image has a dark (low-value) background you can also try
'background from minima' instead of 'mean'.
Some of these operations can be easily done in many other programs, e.g.
'mean' in one direction is the equivalent of 'motion blur' in Photoshop.
Running it twice will reduce 'hard edge' artifacts.
Michael
___________________________________________________________________________
On Wed, December 24, 2008 15:20, G. Esteban Fernandez wrote:
> Hello all,
>
> I have a slit-scanning confocal in my facility that sometimes introduces
> vertical line artifacts into images. I use ImageJ to correct these
> defects
> in frequency space; I make a thin horz. rectangle through the middle of
> the
> FFT map, except through the very center. How can I do the same filtering
> in
> real space? I'm looking for a kernel that I can use in other programs
> that
> don't have Fourier capabilities. (sample images and mask at
>
http://www.missouri.edu/~fernandezg/FFT)
>
> On a related note, can people suggest a resource/reference for converting
> Fourier filters to real space in general?
>
> Thanks and Happy Holidays!
>
> -Esteban
>
> --
> G. Esteban Fernandez, Ph.D.
> Associate Director
> Molecular Cytology Core Facility
> University of Missouri
> 120 Bond Life Sciences Center
> Columbia, MO 65211
>
>
http://www.biotech.missouri.edu/mcc/>
> 573-882-4895
> 573-884-9395 fax
>