d'Eon, EugeneNovák, JanBousseau, Adrien and McGuire, Morgan2021-07-122021-07-122021978-3-03868-157-11727-3463https://doi.org/10.2312/sr.20211286https://diglib.eg.org:443/handle/10.2312/sr20211286We apply zero-variance theory to the Volterra integral formulation of volumetric transmittance.We solve for the guided sampling decisions in this framework that produce zero-variance ratio tracking and next-flight ratio tracking estimators. In both cases, a zero-variance estimate arises by colliding only with the null particles along the interval. For ratio tracking, this is equivalent to residual ratio tracking with a perfect control. The next-flight zero-variance estimator is of the collision type and can only produce zero-variance estimates if the random walk never terminates. In drawing these new connections, we enrich the theory of Monte Carlo transmittance estimation and provide a new rigorous path-stretching interpretation of residual ratio tracking.Computing methodologies --> Reflectance modeling10.2312/sr.2021128615-20