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

Posted by Jack Hogan on Jul 27, 2013; 5:07pm
URL: http://imagej.273.s1.nabble.com/FFT-Power-Spectrum-Question-RGB-vs-8-bit-stack-tp5004177p5004182.html

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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>>
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