I saw a talk about four years ago on using one ray from the light source to a point (through a complicated scene) to efficiently find lots of nearby paths. The idea was to use 'fireflies' to find other rays and so light the whole area accurately. This allowed them to efficiently render scenes where the light source was very indirect, light the light in another room lights your dark room through the open doorway but most of the light from the source never makes it anywhere close to our scene. It was all a clever application of the inverse function theorem, in a sense.
I'm reminded of it mostly because they had a wonderful demonstration in 2D where you could drag a light path through a scene of a bunch of dielectric lenses, etc and have it find a nearby path matching how you dragged it. Was very fun to see the paths so clearly.