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I am try to grasp the idea of frequency domain for images. I think get the basics, but now I've stuck with a question that I can't find appropriate answer anywhere. How are frequency domain and complex numbers in relationship? I've read this article from DSPguide and I understand that after applying DFT to an image in spatial domain, we get two planes: real and imaginary plane. By doing some calculations, we can get amplitude and phase planes. For what is the phase plane used for?