We have done computer simulations on the electrodynamics of rectangular supercon- ducting films. The critical sheet current is an isotropic quantity. By reducing the critical sheet current in a stripe pattern, we have produced approximately 10 % anisotropy in the macroscopic current. We have also studied the two-dimensional branching patterns that are produced by magnetic flux avalanches in our perturbed samples. While the traditional dendritic patterns constitute the core of the avalanche, we also see branches that are aligned with the borders between high and low critical sheet current. The latter branching pattern has been investigated in great detail in order to increase our under- standing of magnetic flux avalanches in superconducting films. We have also developed a computer algorithm for drawing Bean model streamlines. While it was intended for studying the supercurrent in our perturbed samples, it should also be a valuable tool for studies of the Bean model in various geometries.