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  • Photon points and beams improves the convergence of illumination in homogeneous volumes by storing some light paths as volume photon points and beams. This is combined with the techniques below and is the basis for PxrUPBP's superior performance for volume rendering.
  • Bidirectional path tracing can converge faster than forward path tracing for interior scenes or scenes with significant indirect illumination. In particular, when forward path tracing can't easily find an indirect path to the dominant light source, bidirectional path tracing can help – especially if the dominant light source is "one bounce" away from illuminating the scene. It also offers the possibility of faster indirect lighting in general because bidirectional path tracing can make multiple connections between a light path and a eye path and reuse shader evaluations. (A forward path tracer has to run a shader at every bounce, and therefore cannot reuse shader evaluations.) Note that this benefit is more pronounced if your pattern evaluations are expensive; Bxdfs without pattern inputs will not benefit from this.
  • Progressive photon mapping converges faster for specular-diffuse-specular lighting (reflections or refractions of caustics), especially when dealing with relatively small light sources.


PxrUPBP fully supports multiple scattering of homogeneous volumes. Bidirectional path tracing, in particular, is well suited for multiple scattering in volumes. PxrUPBP improves performance by supporting a combination of volumetric photon points and beams, giving less noise in direct volume illumination and especially indirect volume illumination such as volume caustics.