A Simple and Practical Algorithm for Accurate Gravitational   Magnification Maps
  In this brief communication a new method is outlined for modelling magnification patterns on an observer's plane using a first order approximation to the null geodesic path equations for a point mass lens. For each ray emitted from a source, an explicit calculation is made for the change in position on the observer's plane due to each lens mass. By counting the number of points in each small area of the observer's plane, the magnification at that point can be determined. This allows for a very simple and transparent algorithm. A short Matlab code sample for creating simple magnification maps due to multiple point lenses is included in an appendix. 
S.J. Walters and L.K. Forbes
astro-ph.IM