Introduction to Radiosity Geometry Group Discussion Session Jiajian (John) Chen 9/10/2007

Outline Global Illumination and Radiosity Overview of Radiosity Algorithms How to Solve Radiosity Equations Pros and Cons References

Global Illumination and Radiosity Global illumination – Add light intensity to 3D scene, including direct illumination and indirect illumination (reflect, refract) – Algorithms: ray tracing, radiosity, ambient occlusion, photon mapping, image based lighting, … – Radiosity was introduced into computer graphics as a rendering method in (Initially it’s used to solve radiative heat transfer problem)

Overview of Radiosity Algorithms Preprocess: Divide 3D scene into a group of surfaces. Assumptions: – Each surface can reflect light energy diffusely (Lambert’s Law). Colorful reflected light will lead to color bleeding on surfaces – Each surface emits and reflects energy uniformly over its whole area

The equation is applied to each surface Need to compute each color component (RGB) by this equation Fij-s are computed between each pair of surfaces. Purely decided by geometry between two surfaces. Expensive to calculate

How to Solve the Radiosity Equation “Full matrix” algorithm Given Fij, pj, Ei, Solve Bi. Bi is the radiosity (intensity for coloring) for each surface. Expensive to get all N*N Fij and then compute the inversion of N*N matrix.

How to Solve the Radiosity Equation “Progressive” algorithm Compute Fij between surface i and all other surfaces and update the radiosity values for all surfaces. It doesn't need to compute N*N Fij, hence It lowers the computation cost. Finally it gives the same result as the “full matrix” algorithm

Pros and Cons Pros: – Realistic lighting effects: shadow, reflection, … – Independent of viewpoint Cons: – Preprocess (divide environment into polygons) – Large storage and computation cost – Non-diffuse components not counted

References Radiosity and derived algorithms: /HyperGraph/radiosity/ /HyperGraph/radiosity/ Wiki: "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.Computer Graphics