Light-Cone Shell Queries for Scalable Time-Gated Rendering

Abstract

Monte Carlo time-gated rendering requires sampling light paths that not only connect a sensor to an emitter, but which also have a total travel time that falls within a narrow interval, a constraint that is difficult to importance sample. We show that this problem has an underlying geometric structure: in the joint space of position and accumulated travel time, the points yielding a time-valid connection to a given query point form a light-cone shell bounded by two cones. Prior methods sample this shell indirectly. Steady-state algorithms sample the full space and reject points outside it, giving high variance under tight gates. Ellipsoidal path connections target a single cone surface by intersecting an ellipsoid with scene geometry, coupling cost to scene complexity. Our key observation is that shell membership is cheap to test, needing only accumulated travel time and a Euclidean distance. We therefore store the vertices of traced light subpaths in a 4D spatiotemporal hierarchy and recast time-gated connection as a range query, using pruning and importance sampling over the shell to select time-valid vertices without intersecting scene geometry. This decouples the cost of time gating from scene complexity. Within a bidirectional path tracing framework, our method significantly reduces variance over existing approaches on scenes with up to 2.4M triangles.