1. A method, comprising: receiving a three-dimensional (3D) model; receiving a set of probes, wherein each probe includes a first vector that specifies a location within the 3D model and a second vector that specifies an orientation of the probe; performing, via a light transport simulation tool executed by a processor, a light transport simulation based on the 3D model, the set of probes, and one or more light path expressions, wherein performing the light transport simulation comprises, for each probe in the set of probes: generating a number of light transport paths that are associated with the probe, selecting a subset of light transport paths in the number of light transport paths based on a set of light path expressions in the one or more light path expressions, and calculating a contribution to at least one illumination value for the probe for each light transport path in the subset of light transport paths, wherein calculating the contribution to the at least one illumination value for the probe comprises applying a path space filtering technique that includes calculating a filtered contribution for the probe by blending a plurality of contributions from a plurality of probes located within a sphere centered at the location of the probe.