Global lighting

Global illumination ( English Global Illumination , abbreviated GI ) generally refers to the simulation of the light interactions of objects in 3D computer graphics . In particular, only those processes are meant that take into account all the possibilities for the propagation of light rays in a 3D scene . This fully complies with the laws of geometric optics and energy conservation and creates a relatively realistic image.

Procedure

Image rendered using a ray tracing algorithm with global lighting

Simulation of caustics . This light effect is part of global lighting.

To simulate global lighting, 3D programs are increasingly using special algorithms that can actually simulate global lighting. These are, for example, path tracing or photon mapping , and under certain conditions also radiosity . These are always methods that try to solve the rendering equation .

A distinction is made between the following approaches:

Inversion: ${\ displaystyle L = (1-T) ^ {- 1} L ^ {e} \,}$ $L = (1-T) ^ {- 1} L ^ e \,$
- is not used in practice
Expansion: ${\ displaystyle L = \ sum _ {i = 0} ^ {\ infty} T ^ {i} L ^ {e}}$ $L = \ sum_ {i = 0} ^ \ infty T ^ iL ^ e$
- gathering-type: path tracing
- shooting-type: light ray tracing
- bidirectional approach: photon mapping + diffuse ray tracing , bidirectional path tracing , metropolis light transport
Iteration: ${\ displaystyle L_ {n} = L ^ {e} + TL_ {n-1} \,}$ $L_n = L ^ e + TL_ {n-1} \,$
- Radiosity

In the light path notation , global lighting corresponds to the paths of the type L (D | S) * E.

Image-based lighting

Another way to simulate real global lighting is to use High Dynamic Range Images (HDRIs), also known as Environment Maps , that enclose and illuminate the scene. This process is known as image-based lighting .

Faked Global Illumination

The GI is simulated ( to fake ) within the 3D software by clever placement of various light sources and by using various tricks .

In reality, the light particles would be deflected, reflected, or even both by all objects.

With the “fake” GI, the lights are placed in such a way that they act as a substitute for deflected and / or reflected light particles. For example, a light to replace the primary light source (e.g. sun) and additional lights that brighten up all extremely dark areas that are not illuminated by the primary light source would be common.