I have some thermograph images that I took from an object every 3 secs. I used the thermography technique to detect some subsurface defetcs. I found a paper which followed the same approach I used in my research ( Attached below). The researchers of this paper had the same problem as I have in my research. The authors couldnot detect some of the defects from the original images but after a simple image processing the authors of this paper could observe the other defects. please look at Figure 3 in this paper. I want to know how to return the fft of an image on a specif frequency? Would you please help me write a code to do the samething for my images?