The ISNaS incompressible program computes the fluxes 
in the
midside points of the cells and the scalars in the centroids. For post-processing purposes these quantities are needed in the vertices of the cells. For
that reason it is necessary to interpolate (or at the boundary extrapolate)
the computed values to the vertex points. Numerical examples have shown
that a straighforward interpolation in the computational space is not
accurate enough. For that reason a weighted approach, taking into account
the distances in physical space, is necessary. In the following sections we
consider the interpolation and backtransformation applied both for scalar
quantities and for the fluxes.