Posted by
Herbie-3 on
Jul 27, 2013; 3:04pm
URL: http://imagej.273.s1.nabble.com/FFT-Power-Spectrum-Question-RGB-vs-8-bit-stack-tp5004177p5004178.html
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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