Image processing by linear systems has some fascinating aspects due to the theorems of the Fourier transform (FT). Those, whose FT operator is a piece of glass are quite familiar to them, thinking in object space images and FT space images by reason of visual experience. But also in digital image processing the study of FT images is advantageous, e. g. when designing linear filters. The first step of the realization of a Laplacian operator is shown as an example.