Light Transport Simulation with Vertex Connection and Merging

SIGGRAPH Asia 2012

A comparison of our new progressive vertex connection and merging (VCM) algorithm against bidirectional path tracing (BPT) and stochastic progressive photon mapping (PPM) after 30 minutes of rendering. BPT fails to reproduce the reflected caustics produced by the vase, while PPM has difficulties in handling the illumination coming from the room seen in the mirror. Our new VCM algorithm automatically computes a good mixture of sampling techniques from BPT and PPM to robustly capture the entire illumination in the scene. The rightmost column shows the relative contributions of the BPT and PPM techniques to the VCM image in false color.


Developing robust light transport simulation algorithms that are capable of dealing with arbitrary input scenes remains an elusive challenge. Although efficient global illumination algorithms exist, an acceptable approximation error in a reasonable amount of time is usually only achieved for specific types of input scenes. To address this problem, we present a reformulation of photon mapping as a bidirectional path sampling technique for Monte Carlo light transport simulation. The benefit of our new formulation is twofold. First, it makes it possible, for the first time, to explain in a formal manner the relative efficiency of photon mapping and bidirectional path tracing, which have so far been considered conceptually incompatible solutions to the light transport problem. Second, it allows for a seamless integration of the two methods into a more robust combined rendering algorithm via multiple importance sampling. A progressive version of this algorithm is consistent and efficiently handles a wide variety of lighting conditions, ranging from direct illumination, diffuse and glossy inter-reflections, to specular-diffuse-specular light transport. Our analysis shows that this algorithm inherits the high asymptotic performance from bidirectional path tracing for most light path types, while benefiting from the efficiency of photon mapping for specular-diffuse-specular lighting effects.

Downloads and links

  • paper (PDF, 38 MB)
  • supplemental - includes an extended set of results and additional derivations (PDF, 157 MB)
  • citation (BIB)
  • images - all images from the paper and the supplemental document in one archive (ZIP, 150 MB)
  • slides - from the conference presentation (PPTX, 23 MB)
  • image comparison - a cool interactive JavaScript comparison against PPM and BPT (19MB web page, may load slowly)
  • code (SmallVCM) - a (not too) small open source physically based renderer that implements our algorithm
  • technical report - discusses the implementation of our algorithm, with a focus on efficient path MIS weight evaluation

BibTeX reference

  author = {Georgiev, Iliyan and K\v{r}iv\'{a}nek, Jaroslav and Davidovi\v{c}, Tom\'{a}\v{s} and Slusallek, Philipp},
  title = {Light Transport Simulation with Vertex Connection and Merging},
  journal = {ACM Trans. Graph.},
  issue_date = {November 2012},
  volume = {31},
  number = {6},
  month = nov,
  year = {2012},
  issn = {0730-0301},
  pages = {192:1--192:10},
  articleno = {192},
  numpages = {10},
  url = {},
  doi = {10.1145/2366145.2366211},
  acmid = {2366211},
  publisher = {ACM},
  address = {New York, NY, USA},
  keywords = {bidirectional path tracing, density estimation, global illumination, importance sampling, light transport, photon mapping},