Invert threshold on FFT

[olivier]

Dear All,

I would need some help to understand the result I am getting:
I have an image with structures mainly orientated in one direction. I would like to filter out structures going in other directions.

For this, I have performed an FFT, and tried to threshold it (so that the high frequency structures are selected). The automatic threshold gives a too broad selection, however, if I do Edit>Invert (before I apply the initial thresholding), I get an automatic selection of the frequencies I am interested in.

This is good, but I would like to understand what happened when I used the command "Invert" on the automatic threshold. This will help to assess is how valid/reliable this selection process would be.

Olivier

PS: I'd like to post images to illustrate my question but don't have a server.