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

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

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

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. 1984 Computer Graphics, Vol. 18, No. 3.

Fij j i

Radiosity Solution One eqn. for each patch

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

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

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

How much does patch j block patch i ?

Raytracing

Raytracing /w Caustics

Radiosity

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)

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

Two Pass Rendering Wallace, Cohen, Greenberg, 1987 Lady and Gentleman at the Virginals Johannes Vermeer (Dutch), 1662-65

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

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.

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

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

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