FFT-bandpass filter: exact meaning of low- and high-pass thresshold

Posted by Jan-Peter Urbach on Sep 05, 2006; 1:32pm
URL: http://imagej.273.s1.nabble.com/FFT-bandpass-filter-exact-meaning-of-low-and-high-pass-thresshold-tp3701667.html

Hello,
I am just starting to use ImageJ for Image analysis and I am really
impressed by the various functionalities.
Particularily I want to convolute images with a defined convolution kernel.
This is where I have my question:
To my understanding a convolution with a gaussian kernel can be
accomplished by the Process -> FFT -> Bandpass Filter menu.
There you can input the thresholds for large and small structures to be
filtered out.
My question: What exactly do those number mean? Or more precisly the
function effectivly performs a convolution of the image in real space with
a gaussian kernel of a given size (well actually two kernels one for the
large and one for the small structures).
What is the relationship between the threshold you enter in the menu and
the size of the gaussian kernel in real space.
For example is it the FWHM (full width half max) or is it the standard
deviation of a normalized gaussian distribution?

Thanks in advance for any hints
Jan-Peter