The one-sample Monte Carlo estimator of the radiative transfer equation
is
<L> = T(t) / p(t) * (L_e + σ_s * L_s + σ_n * L),
Which means we can also use another p(t) than majorant * exp(-majorant * t)
for sampling the distance. Thus, we use the baked σ_max for distance
sampling, but adjust the majorant when we encounter a density that is
larger than σ_max.
Note that this is not really unbiased because such scaling is not always
applied, but seems to work well in practice when the majorant is
reasonable.
Pull Request: https://projects.blender.org/blender/blender/pulls/146589
8d6b935466