Design Nomenclature / Light & Optics
Global illumination
Also called GI, indirect illumination
Any solution to light transport accounting for light that arrives after reflecting off other surfaces, not only direct from emitters. It is a category, not an algorithm — radiosity, photon mapping, path tracing, and irradiance caching all compute it, with different biases and different failure modes.
In practice
GI on a render toolbar means one specific engine setting. The term itself promises nothing about method, accuracy, or which light paths are actually included.
Not to be confused with
- RadiosityStrictly, a finite-element method that divides a scene into patches, computes form factors between them, and solves the resulting linear system for diffuse interreflection — view-independent, no speculars, and effectively unused since the 1990s. Loosely, and confusingly, the same word gets thrown at global illumination in general.
- Ambient lightIn a renderer, a constant added to every shaded point regardless of direction, occlusion, or distance — a flat lift with no physical referent, invented to fake indirect light before global illumination was affordable. In a room, the same phrase means the real diffuse light arriving from every direction at once.
- Ambient occlusionThe fraction of the hemisphere above a point blocked by nearby geometry — a purely geometric visibility term, no light involved. As a material channel it attenuates ambient diffuse in creases. Physically it is a coarse stand-in for the interreflection a GI solver computes properly, so under real GI it double-darkens.
- Path tracingMonte Carlo integration of the rendering equation: follow a random walk from the camera, sampling one scattering direction per bounce, and average many such paths per pixel. Unbiased — noise falls as the square root of the sample count and the estimate converges on the correct answer.