radiosity

Radiosity is a global illumination method used in computer graphics to simulate the diffuse transfer of light between surfaces in a scene. It models light as energy that leaves surfaces and travels to other surfaces, undergoing multiple reflections, with surfaces treated as ideally diffuse reflectors. The result is realistic color bleeding and soft, indirect shadows in scenes where light bounces multiple times.

The method works by subdividing visible surfaces into small patches. For each patch i, a radiosity value B_i represents the outgoing energy per unit area. The energy leaving i equals its emission E_i plus the sum over all patches j of the energy received from j, weighted by the form factor F_ij, which encodes geometric visibility and relative form. The radiosity equation is B_i = E_i + sum_j F_ij B_j. Collecting all patches yields a linear system that can be solved by direct methods or iteratively by progressive refinement. Precomputed form factors require visibility testing (often with ray casting).

Radiosity assumes perfectly diffuse surfaces, which makes it unsuitable for sharp specular reflections or caustics unless combined with other techniques. It is computationally intensive for large scenes and produces precomputed lighting; it usually requires static geometry and lighting, though variants allow updates. The approach emphasizes energy conservation and can produce accurate renders of interiors and architectural spaces with natural light distributions.

Radiosity became widely studied in the 1980s and 1990s and influenced later global illumination methods. While widely supplanted by ray tracing and path tracing in modern production due to their ability to handle complex lighting and materials, it remains of historical importance and is still used in some architectural visualization and educational contexts, especially for precomputed lighting.