Global illumination is a general name for 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 which 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 (indirect illumination).
Theoretically reflections, refractions, and shadows are all examples of global illumination, because when simulating them, one object affects the rendering of another object (as opposed to an object being affected only by a direct 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 images rendered 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, i.e., radiosity. That 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, metropolis light transport, ambient occlusion, photon mapping, and image based lighting are examples of algorithms used in global illumination, some of which may be used together to yield results that are 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.
In real-time 3D graphics, the diffuse inter-reflection component of global illumination is sometimes approximated by an "ambient" term in the lighting equation, which is also called "ambient lighting" or "ambient color" in 3D software packages. Though this method of approximation (also known as a "cheat" because it's not really a global illumination method) is easy to perform computationally, when used alone it does not provide an adequately realistic effect. Ambient lighting is known to "flatten" shadows in 3D scenes, making the overall visual effect more bland. However, used properly, ambient lighting can be an efficient way to make up for a lack of processing power.
Video demonstrating global illumination and the ambient color effect
Rendering without Global Illumination. Note that we are looking at a fully-enclosed scene through a one-way-transparency scheme (see the chrome sphere's reflection of the otherwise invisible white and green walls). There is a lack of definition in areas that are outside the beam of direct light from the ceiling lamp. For example, the geometry of the ceiling lamp's housing is obscured within a solid grey area produced by an ambient color. Without the ambient color added into the rendering equation, this surface would be black
Global illumination rendering demonstrating how light is reflected by surfaces. Note how colors transfer (or "bleed") from one surface to another, an effect of diffuse inter-reflection. Notice how colors from the red and green walls are diffusely reflected by other surfaces in the scene (one-way transparency is used to allow us to see "through" two walls from the outside while preserving their effect inside the scene). Also notable is the caustic projected on the red wall as light passes through the glass sphere.
