A collection of algorithms known as global illumination (GI), sometimes known as indirect illumination, is used in 3D computer graphics to bring more realistic lighting to 3D scenes. These methods account for both direct illumination—light that comes directly from a light source—and indirect illumination—light that originates from the same source but is reflected by other surfaces in the picture, whether or not they are reflective.
Theoretically, shadows, reflections, and refractions are all instances of global illumination because when they are simulated, one object impacts how another is shown, as opposed to an item being influenced exclusively by a direct source of light. However, in actuality, the term "global illumination" only refers to the modeling of diffuse inter-reflection or caustics.
Compared to renderings that exclusively use direct lighting algorithms, global illumination algorithms frequently produce more photorealistic-looking images. However, producing such pictures is more slower and more computationally costly. One typical method is to calculate a scene's global lighting and record that data with the geometry (for example, radiosity). In order to create walkthroughs of a scene, the recorded data may subsequently be utilized to create photos from various angles without having to repeatedly do expensive lighting calculations.
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 simulate diffuse inter-reflection, a crucial component of global illumination; however, all but one of them (radiosity) also simulate specular reflection, making them more precise algorithms for solving the lighting equation and producing a scene with more realistic illumination. In engineering design, finite-element simulations of heat transfer between surfaces are closely connected to the techniques used to compute the distribution of light energy across surfaces in a scene.
It is still challenging to compute global lighting accurately in real-time. The diffuse inter-reflection portion of the global illumination in real-time 3D graphics is occasionally approximated by a "ambient" term in the lighting equation, often known as "ambient lighting" or "ambient color" in 3D software programs. Though computationally simple, this approximation method—also characterized as a "cheat" because it isn't actually a global lighting method—doesn't provide an effect that is sufficiently realistic when used alone. In 3D environments, ambient lighting is known to "flatten" shadows, dulling the overall visual impact. However, ambient illumination may be an effective technique to compensate for a lack of processing power when applied appropriately.
The use of high-dynamic-range images (HDRIs), commonly referred to as environment maps, which surround and illuminate the scene, is another method for simulating true global illumination. This method is referred to as image-based lighting.