  In light transport simulation, challenging situations are caused by the
variety of materials and the relative length of path segments. Path Tracing can
handle many situations and scales well to parallel hardware. However, it is not
able to produce paths which have a smooth surface in connection with a small
light source. Here, photon transports perform superior, which can be
ineffective if the smooth object is small compared to the scene size.
  We propose to use the last segment of a Path Tracer path as the first segment
of a photon path. As a result, the strengths of next event estimation are
inherited by the photon transport and photons are guided toward the regions
where they are most useful. To that end, we developed a lock-free sparse
octree, which we use for fast and robust density estimates. Summarizing, the
new method can outperform state of the art algorithms like Vertex Connection
and Merging in certain scenarios.
