Erm... Photon mapping uses "forward" path tracing to trace photons from the lights, not the camera. In conventional (uni-directional) path tracing, rays are sh

Erm... Photon mapping uses "forward" path tracing to trace photons from the lights, not the camera. In conventional (uni-directional) path tracing, rays are shot from the camera only, and bounce around the scene (other than for light visibility testing). Bi-directional path tracing uses rays originating at both the camera and light sources going in opposite directions. VCM is a combination of Bi-directional path tracing and photon mapping.