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We present TREVR2 (Tree-based REVerse Ray Tracing 2), a fast, generalalgorithm for computing the radiation field, suitable for both particle andmesh codes. It is designed to self-consistently evolve chemistry for zoomed-inastrophysical simulations, such as cosmological galaxies with both internalsources and prescribed background radiation, rather than large periodicvolumes. Light is propagated until absorbed, with no imposed speed limit otherthan those due to opacity changes (e.g. ionization fronts). TREVR2 searchesoutward from receiving gas in discrete directions set by the HEALPIX algorithm(unlike its slower predecessor TREVR), accumulating optical depth and addingthe flux due to sources combined into progressively larger tree cells withdistance. We demonstrate $N_\textrm{active}\log_2 N$ execution time withabsorption and many sources. This allows multi-band RT costs comparable totree-based gravity and hydrodynamics, and the usual speed-up when activeparticles evolve on individual timesteps. Sources embedded in non-homogeneousabsorbing material introduce systematic errors. We introduce transmissionaveraging instead of absorption averaging which dramatically reduces thesesystematic effects. We outline other ways to address systematics including anexplicit complex source model. We demonstrate the overall performance of themethod via a set of astrophysical test problems.

TREVR2: Illuminating fast $N\log_2\,N$ radiative transfer