  An efficient algorithm for calculating radiative transfer on massively
parallel computers using domain decomposition is presented. The integral
formulation of the transfer equation is used to divide the problem into a local
but compute-intensive part for calculating the intensity and optical depth
integrals, and a nonlocal part for communicating the intensity between adjacent
processors. The waiting time of idle processors during the nonlocal
communication part does not have a severe impact on the scaling. The wall clock
time thus scales nearly linearly with the inverse number of processors.
