• physics-based illumination 
• origins: heat transfer and illumination engineering 
• diffuse reflection 
• results are view independent 
• object-space algorithm 
• infinite reflections: energy balance 
• soft shadows, colour bleeding, dark corners 

Definitions

flux 

energy per unit time (W)

 

radiosity, B

exiting flux density (W/m^2) 

 

E 

exiting flux density for lightt sources 

 

reflectivity, R

fraction of incoming light which is reflected (unitless)

 

form factor, Fij

fraction of energy leaving Ai and arriving at Aj,

determined by geometry of polygons i and j 

Energy balance 

Matrix form of radiosity system 

Problems

• matrix is O(n^2) 
• form-factor computation costly 
• computing visibility costly 
• reconstruction of continuous image from constant radiosity patches
• curved surfaces? 
• sharp shadows? 
• refraction, translucency, specularity? 

Partial Solutions

• use iterative methods 
• compute form-factors on-demand 
• progressive refinement: shoot from brightest emitters first 
• create and use hierarchy 
• use non-constant basis functions 

hybrid radiosity / ray-tracing techniques