Posted by
Herbie-4 on
Sep 24, 2014; 5:35pm
URL: http://imagej.273.s1.nabble.com/1D-Fourier-transform-in-macro-tp5009727p5009749.html
Ludovic,
you wrote that "What [you] get is a single colored pixel in the raw
spectrum window."
Does this mean that you are using color images?
Although ImageJ is able to Fourier-transform color images, it is not
what you should do.
Instead, if necessary split your color images and convert each of your
line images to 32bit float before you perform the transform. The dynamic
range of this number format should be sufficient for your purpose, i.e.
you get represented the high Fourier-spectral value at the origin (the
DC) and the low values at higher spatial frequencies. If you want to see
the latter, just adjust Brightness&Contrast as suggested by Michael
Schmid. Brightness&Contrast doesn't change the Fourier-spectral data,
only the way it is displayed.
I also suggest to remove the mean from the line image, as it was pointed
out by Michael Schmid. It doesn't really harm your signals. Windowing is
another important issue.
In any case you should know what Fourier transformation really means. If
not, you may end up with results that may pose more problems then they
solve..
Presently, I see no reason for another tool or FFT-approach.
You said that you want to get the MTF (of some optics I guess) but what
do you want to do with it?
Best
Herbie
::::::::::::::::::::::::::::::::::::::::
On 24.09.14 11:19, Ludovic Pinier wrote:
> Thanks for the tricks,
>
> but it is to perform double check of a device calibration, and I
> would avoid changing the brightness and contrast of the produced
> pictures.
>
> There is the Jtransforms library to perform the Fourier transform,
> written in pure Java. I expect it can be run within ImageJ; and also
> the "call" built-in function in the macro language to call it. Does
> anyone have experience with calling external libraries in a macro?
>
> Sincerely, Ludovic
>
> -- ImageJ mailing list:
http://imagej.nih.gov/ij/list.html>
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