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Re: FFT Power Spectrum Question: RGB vs 8-bit stack

Posted by Jack Hogan on Jul 28, 2013; 8:33am
URL: http://imagej.273.s1.nabble.com/FFT-Power-Spectrum-Question-RGB-vs-8-bit-stack-tp5004177p5004188.html

Hey Jim,

Thanks, your suggestion seemed promising.  I produced both the FFT of the
quadrature sum of the R,G and B values and the quadrature sum of the
individual channels' Power Spectrum.  The Power Spectrum obtained by IJ's
FFT function directly on the RGB image looks closest to the latter (see the
rightmost column in the FFT Moduli image), but alas nowhere near the same
as you can see in the two attachments.

Getting closer but not quite there yet.  Any other ideas?

Jack


On Sat, Jul 27, 2013 at 8:49 PM, James Ewing <[hidden email]>wrote:

> Jack - to form the modulus image of the FT sum of the RGB channels, try
> Sum = sqrt(R^2 + G^2 + B^2)
>  -  Jim Ewing
>
> On Jul 27, 2013, at 1:07 PM, Jack Hogan wrote:
>
> > Hi Herbie,
> >
> > Yeah, I thought that Imagej would turn the image from RGB to 8 bit gray
> > before running the FFT on it, but apparently not.  The attached image
> shows
> > running FFT on the RGB image, on the RGB image after converting to 8 bit
> > gray and on the sum of the R, G, B slices divided by three respectively.
> >
> > The Raw Power Spectrum is not very helpful (a single dot with scaling, or
> > mostly washed out without).  Image conversion options are at default,
> which
> > means that the weighted RGB box is unchecked.
> >
> > The FFT run directly on the RGB image still looks quite different from
> the
> > others.  Any other ideas?
> >
> > Thanks for your help!
> >
> > Jack
> >
> >
> > On Sat, Jul 27, 2013 at 5:04 PM, Herbie <[hidden email]> wrote:
> >
> >> Good day Jack,
> >>
> >> the differences may appear big in the display but please note that the
> >> Fourier-spectral values are scaled according to the logarithm which
> >> emphasizes the small values. If you don't want logarithmic spectra, then
> >> check the "Raw power spectrum" option.
> >>
> >> I guess that IJ is creating a gray-value image before
> Fourier-transforming
> >> the RGB-image and it does this according to the choice made under
> >> "Conversion Options". Most probably you have "Weighted RGB Conversion"
> >> checked.
> >>
> >> HTH
> >>
> >> Herbie
> >>
> >>
> >> On 27.07.13 16:10, Jack Hogan wrote:
> >>
> >>> Hello folks,
> >>>
> >>> Can any of you Imagej experts illuminate a noob as to why the FFT Power
> >>> Spectra of two image types of the exact same image look so different?
> >>>  Specifically, referring to the attached image, the top row is the 8bit
> >>> RGB
> >>> rendition of a raw file with minimal demosaicing and its FFT.  The
> bottom
> >>> row is the same RGB 8 bit rendition but after having been 'stacked'
> >>> through
> >>> Image/Color/MakeComposite and an FFT generated for each color slice.
> >>>
> >>> How does Imagej's FFT algorithm combine the separate RGB channels to
> come
> >>> up with its power spectrum? And why is it so different from the
> individual
> >>> channels'?
> >>>
> >>> Thank you for your help.
> >>>
> >>> Jack
> >>>
> >>> --
> >>> ImageJ mailing list: http://imagej.nih.gov/ij/list.**html<
> http://imagej.nih.gov/ij/list.html>
> >>>
> >>>
> >> --
> >> ImageJ mailing list: http://imagej.nih.gov/ij/list.**html<
> http://imagej.nih.gov/ij/list.html>
> >>
> >
> > --
> > ImageJ mailing list: http://imagej.nih.gov/ij/list.html
> > <FFT RGB vs 8b vs R+G+B_3.jpg>
>
> --
> ImageJ mailing list: http://imagej.nih.gov/ij/list.html
>
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Quadrature Sum of Slices.jpg (229K) Download Attachment
FFT Moduli Ops.jpg (104K) Download Attachment