This method was inspired by the Lax-Friedrichs method for hyperbolic
conservation laws Lax (1954) due to its total variation diminishing property.
We use the ``Lax-Friedrichs averaging'' and the wide 5-point
stencil in space. The scheme is given by
We impose the following boundary conditions
corresponding the straight boundary rays.
We set the initial conditions
corresponding to the initial conditions
for the image rays traced backward: