Global illumination

"Realistic rendering" redirects here. For physically-based realistic rendering, see Physically-based rendering.

Rendering without global illumination. Areas that lie outside of the ceiling lamp's direct light lack definition. For example, the lamp's housing appears completely uniform. Without the ambient light added into the render, it would appear uniformly black.
Rendering with global illumination. Light is reflected by surfaces, and colored light transfers from one surface to another. Notice how color from the red wall and green wall (not visible) reflects onto other surfaces in the scene. Also notable is the caustic projected onto the red wall from light passing through the glass sphere.

Global illumination (GI), or indirect illumination, refers to a group of algorithms used in 3D computer graphics 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 "bounces" where light rays are reflected by other surfaces in the scene (indirect illumination).

The term "global illumination" was first used by Turner Whitted in his paper "An improved illumination model for shaded display", to differentiate between illumination calculations at a local scale (using geometric information directly, such as in Phong shading), a microscopic scale (extending local geometry with microfacet detail), and a global scale, including not only the geometry itself but also the visibility of every other object in the scene. Theoretically, reflections, refractions, transparency, 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, especially in real-time settings.

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