  A new, very fast method for 3D radiative transfer on fully threaded grids
with arbitrarily high angular resolution is presented. The method uses
completely cell-based discretization, and is ideally suited for problems with
diffuse background radiation, often encountered in cosmological and star
formation models. We find that for accurate statistical study of intergalactic
Ly-alpha absorption lines one needs of order of few hundred angular
discretization elements even for models without radiative feedback from star
forming galaxies.
