Posted by
Joachim Wesner on
Dec 11, 2007; 11:26pm
URL: http://imagej.273.s1.nabble.com/FFT-Filtering-are-non-symmetric-masks-useful-tp3697755p3697756.html
Hi Michael,
as you mention it now, this is actually an issue I wanted to address quite
a while ago. I noted there is a slight "problem" in ImageJ when I added the
complex-to-complex routines to ImageJ.
In a true Real to Complex to Real Transform used for filtering, you would
*NEED* to do symmetrical filtering for a useful result, because otherwise
when transforming back, the filtered output is no longer real but complex
itself
However, due to the underlying inherently always real-to-real Hartley
transform (and how the blocking of the power spectrum is actually handled
in the current version of IJ) is *almost* works correctly even
if you slightly violate the symmetry, the output clearly remains real (as
it can only be), but if you look closer, strange "ghost/mirror artifacts"
appear at places different from the input data. I would need to go through
my stuff for an example the next days, I already had prepared an example
case with strong assymmetry to show this effect.
I would suggest that the handling of the filtering is changed so that the
symmetry of any black/white bars is automatically enforced.
Mit freundlichen Grüßen / Best regards
Joachim Wesner
Projektleiter Optik Technologiesysteme
Ernst Leitz Strasse 17-37 | 35578 Wetzlar (Germany)
Tel. +49 6441 29 2611 | Fax +49 6441 29 2700
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Michael Schmid
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11.12.2007 20:42
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Hi FFT experts,
for filtering images in the Fourier domain, one can paint white
or black on the power spectrum shown after Process>FFT (to pass
and remove the selected areas when transforming back).
The power spectrum has inversion symmetry.
Currently, one has to paint two areas for all off-center
positions, because they appear twice in the power spectrum.
For an example, see the two black bars in the bottom left
image in
http://rsb.info.nih.gov/ij/docs/examples/FFT/My question to the FFT experts: Is there any useful application
of filtering only on one side of the center?
Otherwise, with a small modification to the ImageJ code I could
avoid extra work by painting on one side only, having ImageJ do
the other side.
(Sometimes I have to filter or block many spatial frequencies,
then painting them twice makes a difference).
Michael
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