Waves in excitable media can be treated by a simple geometric theory. The propagation velocity is assumed known and evolution of wave fronts is determined by elementary physical principles (Fermat's principle, Huygens' principle). Based on this geometric theory a fast computational method is developed. By this method the distorting effect of the spatial grid is avoided. The method is applied to the cases when a circular obstacle is surrounded by a homogeneous and heterogeneous medium, respectively. The numerical simulations show that the method is convenient, fast and reliable.