  HERO (Hybrid Evaluator for Radiative Objects) is a 3D general relativistic
radiative transfer code which has been tailored to the problem of analyzing
radiation from simulations of relativistic accretion discs around black holes.
HERO is designed to be used as a postprocessor. Given some fixed fluid
structure for the disc (i.e. density and velocity as a function of position
from a hydrodynamics or magnetohydrodynamics simulation), the code obtains a
self-consistent solution for the radiation field and for the gas temperatures
using the condition of radiative equilibrium. The novel aspect of HERO is that
it combines two techniques: 1) a short characteristics (SC) solver that quickly
converges to a self consistent disc temperature and radiation field, with 2) a
long characteristics (LC) solver that provides a more accurate solution for the
radiation near the photosphere and in the optically thin regions. By combining
these two techniques, we gain both the computational speed of SC and the high
accuracy of LC. We present tests of HERO on a range of 1D, 2D and 3D problems
in flat space and show that the results agree well with both analytical and
benchmark solutions. We also test the ability of the code to handle
relativistic problems in curved space. Finally, we discuss the important topic
of ray-defects, a major limitation of the SC method, and describe our strategy
for minimizing the induced error.
