1. Global Illumination: Radiosity, Photon Mapping & Path Tracing
Rama Hoetzlein, 2009 Lecture Notes Cornell University

2. Global Illumination Consider all energy moving in a space,
not just the light that reaches the eye.

3. First used in the study of Heat Transfer (1950s),
how does energy move around a room or object.

4. Radiosity First used in Computer Graphics, 1984
Modeling the interaction of light between diffuse surfaces, C. Goral, K. E. Torrance, D. P. Greenberg and B. Battaile Computer Graphics, Vol. 18, No. 3.

5. Fij j i

6. Radiosity Solution
One eqn. for each patch

9. Radiosity - Overview Each patch contributes energy to other patches
Each patch i has a Radiosity equation:
Solve all equations simultaneously to get the energy at each patch
What is the hardest part of this eqn?
Ei = Energy emitted Ri = Energy reflected Fij = Energy on patch I from j

10. Radiosity - Form Factors
What things might contribute to the Form Factor?
Remember: Form Factor is amount of energy hitting patch i from patch j

11. Radiosity - Form Factors
What things might contribute to the Form Factor?
Remember: Form Factor is amount of energy hitting patch i from patch j
Size of the patch Bigger = more energy
Angle between patches Direct = more energy
Dist. between patches Father = less energy
Objects between patches Occlusion = less energy

13. How much does patch j block patch i ?

14. Raytracing

15. Raytracing /w Caustics

16. Radiosity

19. Radiosity Benefits Drawbacks
1. Very realistic (actually computes energy)
2. View independent.. Compute once, then view
3. Effects: Caustics, Color bleeding
Drawbacks
1. Even more expensive than raytracing
2. Cannot simulate mirror reflections ! (energy travels diffusely, not coherently)

20. Hybrid Rendering Combine Raytracing and Radiosity:
- Radiosity to give energy transfer: Color bleeding Light diffusion Caustics Soft shadows
Raytracing to give view-dependent terms: Reflections Refractions Specular highlights

21. Two Pass Rendering Wallace, Cohen, Greenberg, 1987
Lady and Gentleman at the Virginals Johannes Vermeer (Dutch),

22. Photon Mapping
Instead of computing all patches simultaneously, cast “photon rays” from light source.
Uses points instead of patches.
Photon Mapping - photons propogate
Radiosity – patches don’t move

23. Photon Mapping Two pass approach:
1. Shoot photons around scene 2. Collect photons to create image (nearby photons are smoothed)
- Much faster.. No form factors - Need lots of photons but easier to compute. - Realistic. Photons move the way light does.

26. Photon Mapping Made Easy, Yu, Lowther, Shene, SIGCSE 2005

27. Photon Mapping Made Easy, Yu, Lowther, Shene, SIGCSE 2005

28. Future Trends Recent Developments (past 5 yrs):
- Ambient Occlusion – Approximation to Photon Mapping
Real-Time Raytracing using GPUs
Hybrid Rasterization and Raytracing
Volumetric Raytracing & Radiosity