transform from the user. The 3D version of the FHT that I recently posted
transform. It also works in 2D.
them until they are. (It should not actually be necessary. The FFT in
and use a mirroring process to fill in the outsides. This is intended to
Robert P. Dougherty, Ph.D.
> -----Original Message-----
> From: ImageJ Interest Group [mailto:
[hidden email]] On Behalf Of Dean
> Waldow
> Sent: Tuesday, July 19, 2005 5:56 PM
> To:
[hidden email]
> Subject: FFT questions...
>
> Hi Folks,
>
> I have a few FFT related questions. I have tried to look over the
> documentation but have not found answers to my questions as best as I
> can tell or I have missed them. Any help or suggestion of where to
> find answers would be useful.
>
> Here I refer to PROCESS->FFT->FFT...
>
> 1) When taking a FFT of an image that is not 2^Nx2^N, what portion of
> the image is used to do the FFT? For example in a 640x480 image, is
> only a 256x256 subsection used for the FFT? When I take the FFT of a
> 640x480 image, FFT doesn't seem to care that the image is not
> 2^Nx2^N. Or maybe it pads with zeros up to the next 2^Nx2^N?
>
> 2) After taking a FFT of an image that is calibrated, you can move
> the cursor over the image and it displays the radius in unit of
> distance / cycle, angle in degrees, and value of the pixel at that
> r, theta.
>
> a) How does the radius unit (distance/cycle) relate to a
> scattering wave vector unit (1/distance)? My thought would be:
> L(distance/cycle = 2*pi / [ wave vector (1/distance) ]
>
> b) How can I access that radius value from the ip.X which is
> displayed with the cursor is moved over the FFT image? The pixel
> value seems to be accessed in Java via ip.getPixelValue((int)i,(int)
> j) (if ip is an imageprocessor variable) but I have not found how the
> radius and theta are accessed for a given pixel. I have I have tried
> a bit to look at the source code but without much luck.
>
> 3) What is displayed is said to be the 8 bit power spectrum with log
> scaling after taking an FFT while the 32 bit FHT is attached to the 8
> bit image? Am I close to say that is
>
> powerspectrum = log ( complex conjugate 32 bit FHT (times)
> 32 bit FHT )?
>
> 4) Lastly, is there a plugin like radial profile plot (http://
> rsb.info.nih.gov/ij/plugins/radial-profile.html) which works with FFT
> images? The radial profile plot plugin appears to ignore the r and
> theta of an FFT powerspectrum image but really seems to have been
> written for real space images.
>
>
> I have looked at the following documents but likely have missed the
> info:
>
>
http://rsb.info.nih.gov/ij/docs/menus/process.html#fft>
http://rsb.info.nih.gov/ij/docs/examples/tem/>
http://rsb.info.nih.gov/ij/docs/ImageFFT/>
> Thanks,
>
> Dean
> --
> ------------------------------------------------------------------------
> Dean Waldow, Professor (253) 535-7533
> Department of Chemistry (253) 536-5055 (FAX)
> Pacific Lutheran University
[hidden email]
> Tacoma, WA 98447 USA
http://www.chem.plu.edu/waldow/> ------------------------------------------------------------------------
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