Principles of Radiosity

If you have a short scientific background, you probably have already heard of the 'wave-particle duality' principle for light. It means that light behave both as a particle and a wave. Classical rendering algorithms (raytracing,z-buffer(including its scanline variant) use the particle aspect of light. Radiosity is based on on the wave properties of lights

Historically speaking, radiosity algorithms are the adaptation of a model created by physicians for heat transfers. The main principle of radiosity is to simulate the energetic exchanges induced by light, whose color is determinated by a wavelength.

Let's consider a simple 3D scene modelized by a polygonal mesh : an empty room with a cube on the roof. The basic elements for radiosity calculation are patches (plane surface elements). Each face of the cube and the room is patch. Each patch will receive energy (light) from the other patches. It will absorb some energy (according to the patch properties ("the material")), and will reflect the rest of this energy. The amount of energy transmitted from patch A to patch B is a function who depends on the distance between the two patches, on their respective orientations, and on the possible presence of occluding patches.

Calculation of energetic exchanges between two patches	Occlusion phenomenon

Of course, in order to get energy exchanges, there must be light sources in the scene. Some patches are defined as light emittors. In the case of our simple scene, we can define the ceiling as an emittor patche, or we can add a sphere in order to get a light bulb effect.