  We introduce a non-exponential radiative framework that takes into account
the local spatial correlation of scattering particles in a medium. Most
previous works in graphics have ignored this, assuming uncorrelated media with
a uniform, random local distribution of particles. However, positive and
negative correlation lead to slower- and faster-than-exponential attenuation
respectively, which cannot be predicted by the Beer-Lambert law. As our results
show, this has a major effect on extinction, and thus appearance. From recent
advances in neutron transport, we first introduce our Extended Generalized
Boltzmann Equation, and develop a general framework for light transport in
correlated media. We lift the limitations of the original formulation,
including an analysis of the boundary conditions, and present a model suitable
for computer graphics, based on optical properties of the media and statistical
distributions of scatterers. In addition, we present an analytic expression for
transmittance in the case of positive correlation, and show how to incorporate
it efficiently into a Monte Carlo renderer. We show results with a wide range
of both positive and negative correlation, and demonstrate the differences
compared to classic light transport.
