20 - Fourier transform
The Fourier transform simply states that non-periodic signals whose area under the curve is finite can also be represented into integrals of the sines and cosines after being multiplied by a certain weight. Both Fourier series and Fourier transform are given by Fourier, but the difference between them is Fourier series is applied on periodic signals and Fourier transform is applied for non-periodic signals. Now the question is that which one is applied on the images, the Fourier series or the Fourier transform. Well, the answer to this question lies in the fact that what images are. Images are non–periodic. And since the images are non periodic, so Fourier transform is used to convert them into frequency domain. The Fourier transform is a complex concept. For a more intuitive way of learning, check the first video. Check the other videos for a more indepth understanding of the Fourier transform.
Theory
Demonstration