Imagine I have a simple 1D height profile which is NOT symmetric. Now, what is truly important for me is to know what are the frequency content
of the top profile (i.e. a cut profile above the average line).
Target: I need the power spectral density of the top profile.
It shouldn't be this simple to just cut the profile from the mean line to have the top profile values and zero otherwise. Then when I apply FFT
in MATLAB, I believe those zero sections will be a problem, like the case for a step function that a wide range of sine waves will be added to the result while actually there are no such frequency content in my real top profile.
I have uploaded two images for you, to see what I mean; and both explanations in the images are from the same author. In the first image, the writer says that, simply cut the profile and calculate power spectrum for each top and bottom profile.
But the writer some years later, says something else that, for calculating top power spectrum, you need to cut the surface from the mean plane and then replace the bottom profile with some profile which has the same statistical properties as the profile above the average plane:
I hope I could have explained my point. I would really appreciate your help..