Re: FFT-Filtering - are non-symmetric masks useful?

Posted by jmutterer on Dec 12, 2007; 7:41am
URL: http://imagej.273.s1.nabble.com/FFT-Filtering-are-non-symmetric-masks-useful-tp3697755p3697762.html

Hi Michael, Joachim
a little while ago, I wrote the following simple macro that adds the current
ROI and a center-symmetrical copy of this ROI to the ROI Manager. It might
help when selecting symmetrical areas on the FFT image.
Jerome

macro "AddSym [F1]" {
roiManager("Add");
getSelectionCoordinates(xs,ys);
makeSelection ("polygon",xs,ys);
m=getWidth()/2;
for (i=0;i<xs.length;i++) {
xs[i]=m-(xs[i]-m);
ys[i]=m-(ys[i]-m);}
makeSelection ("polygon",xs,ys);
roiManager("Add");}
/// end macro




On Dec 12, 2007 12:26 AM, Joachim Wesner <
[hidden email]> wrote:

> 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
>
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>                                        masks useful?
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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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