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

Posted by Rasband, Wayne (NIH/NIMH) [E] on Aug 02, 2013; 2:53pm
URL: http://imagej.273.s1.nabble.com/FFT-Power-Spectrum-Question-RGB-vs-8-bit-stack-tp5004177p5004287.html

On Jul 27, 2013; 10:10am, 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.

Thanks to Michael Schmid, the ImageJ 1.48b daily build fixes a bug that caused the FFT power spectra of RGB images to not be correctly log scaled. ImageJ does FFTs on RGB images by splitting the image into hue, saturation and brightness channels and FFT transforming the brightness channel. The Inverse FFT command merges the hue, saturation and (inverse transformed) brightness channels to create and RGB image.

-wayne

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