The study of a real function of two real variables can be supported by visualisation using a Computer Algebra System (CAS). One type of constraint of the system is due to the algorithms implemented, yielding continuous approximations of the given function by interpolation. This often masks discontinuities of the given function and can provide very strange plots. The pixelisation of the visualisation adds to this strangeness. We present a study of such a "distortion of reality", using more than one system, and show how they can be used as a starting point to learn more Mathematics. This can be an obstacle, but we see it as a chance, as it is a motivating constraint, both for students and for educators. (Contains 10 figures.)