I am reading a book about image processing. I have found mention of a method for processing images.
It is said that a special effect could be reached when we multiply two results of the Fast Fourier Transform. But in the book, it does not mention the function of the multiplication.
Then my question is what the function of the multiplication of two results of the Fast Fourier Transform? If this question is too abstract to answer, can someone give me some practical usages of FFT in image processing?