Global illumination (GI), or indirect illumination, is a group of algorithms used in 3D computer graphics that are meant to add more realistic lighting to 3D scenes. Such algorithms take into account not only the Light that comes directly from a light source (direct illumination), but also subsequent cases in which light rays from the same source are reflected by other surfaces in the scene, whether reflective or not (indirect illumination).
Theoretically, reflections, refractions, and shadows are all examples of global illumination, because when simulating them, one object affects the Rendering of another (as opposed to an object being affected only by a direct source of light). In practice, however, only the simulation of diffuse inter-reflection or caustics is called global illumination.
Images rendered using global illumination algorithms often appear more photorealistic than those using only direct illumination algorithms. However, such images are computationally more expensive and consequently much slower to generate. One common approach is to compute the global illumination of a scene and store that information with the geometry (e.g., radiosity). The stored data can then be used to generate images from different viewpoints for generating walkthroughs of a scene without having to go through expensive lighting calculations repeatedly.
Radiosity, ray tracing, beam tracing, cone tracing, path tracing, volumetric path tracing, Metropolis light transport, ambient occlusion, photon mapping, signed distance field and image-based lighting are all examples of algorithms used in global illumination, some of which may be used together to yield results that are not fast, but accurate.
These algorithms model diffuse inter-reflection which is a very important part of global illumination; however most of these (excluding radiosity) also model specular reflection, which makes them more accurate algorithms to solve the lighting equation and provide a more realistically illuminated scene. The algorithms used to calculate the distribution of Light energy between surfaces of a scene are closely related to heat transfer simulations performed using finite-element methods in engineering design.