Fourier transform and filtering
Posted by Antonio on Oct 29, 2008; 12:24pm
URL: http://imagej.273.s1.nabble.com/Fourier-transform-and-filtering-tp3694627.html
Hi, I'm recently working with the options "FFT->custom filter" of ImageJ.
My goal is to extract from an image only a selected spatial frequency, then transform the image again to obtain the real space distribution of that frequency. As custom filters, I use rings of the radius of interest and 2-5 pixels large.
But if I execute the command "custom filter", I don't obtain back the same image I would by manually perform the filtering. That's to say: I transform an image, then overlay the filter (with "image processor->AND"), then transform again the result. The final image is totally different from what I get with built-in filtering.
Is not clear to me whence this difference should arise, as the "AND" operator between the filter and the transformed image is actually like a multiplication for a step-function (or square wave) in the frequency domain, that's to say a convolution in real space.
I guess there's some other operations or normalization ImgeJ performs while filtering, and I'd need to know some more details about that, to know how to interpret my result.
Thanks