I come from Fluid Mechanics domain so pardon me for asking a naive and fundamental question.
I am trying to perform high pass filtering on a set of positive real numbers (~Energy) using FFT. However, the result I obtain has a lot of negative values. I tried to quantify how many of them are negative and it seems $\sim50\%$ of these values are negative.
I tried looking up for an answer and I saw a general approach used in Image processing domain, where people offset these negative values or use some threshold. But in my case, I can't use either of these techniques because I need to look at some statistical quantities after the filtering process.
So I want to ask why do these negative numbers appear for a purely positive input and if so, are there any solutions for it?