> 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
> >>>
> >>> --
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http://imagej.nih.gov/ij/list.**html<
>
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> >>>
> >>>
> >> --
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>
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> >>
> >
> > --
> > ImageJ mailing list:
http://imagej.nih.gov/ij/list.html> > <FFT RGB vs 8b vs R+G+B_3.jpg>
>
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