  Sub-surface scattering is key to our perception of translucent materials.
Models based on diffusion theory are used to render such materials in a
realistic manner by evaluating an approximation of the material BSSRDF at any
two points of the surface. Under the assumption of perpendicular incidence,
this BSSRDF approximation can be tabulated over 2 dimensions to provide fast
evaluation and importance sampling. However, accounting for non-perpendicular
incidence with the same approach would require to tabulate over 4 dimensions,
making the model too large for practical applications. In this report, we
present a method to efficiently evaluate and importance sample the
multi-scattering component of diffusion based BSSRDFs for non-perpendicular
incidence. Our approach is based on tabulating a compressed angular model of
Photon Beam Diffusion. We explain how to generate, evaluate and sample our
model. We show that 1 MiB is enough to store a model of the multi-scattering
BSSRDF that is within $0.5\%$ relative error of Photon Beam Diffusion. Finally,
we present a method to use our model in a Monte Carlo particle tracer and show
results of our implementation in PBRT.
