The Fourier operator is the kernel of the Fredholm integral of the first kind that defines the continuous Fourier transform. It may be thought of as a limiting case for when the size of the discrete Fourier transform increases without bound while its spatial resolution also increases without bound, so as to become both continuous and not necessarily periodic. As a teaching tool the Fourier operator is used widely and it has also been used as an art form, including the book cover of the book Advances in Machine Vision (ISBN 9810209762).