Global Illumination

Global Illumination is the simulation of all light inter-reflection effects in a scene, except Caustics. This includes effects such as color bleeding: if a red table is next to a white wall, the white wall gets a slightly pink tint. These global illumination effects are typically very subtle but add a lot of realism to a rendered image.

Simulation of global illumination has at least two distinct uses:

Physically accurate simulation of the illumination within an environment. For example, the light distribution inside an office building.
Visually pleasing lighting effects for applications in the entertainment industry. Here physical accuracy is not the most important aspect, but images have to look believable.

mental ray provides several techniques to compute global illumination.

Global Illumination (Photons)

One way is to use photons, similar to the computation of caustics. In fact, the same photon material shaders can be used. Since caustics are treated separately in mental ray, the global illumination simulation does not include caustics. If all light inter-reflections should be simulated, then both global illumination and caustics photons need to be enabled.

Photons are stored in an intermediate point cloud data structure called a photon map. The photons from the global illumination simulation are stored in a map independent of caustics photons. When a material shader calls mi_compute_irradiance, the irradiance from both the caustics photon map and the global illumination photon map are computed.

Global illumination with photons can be enabled with a scene option or on the command line of the standalone mental ray. Also, special properties need to be attached to scene elements like light sources and objects, as described in the following sections.

GI GPU 3.12

The rise of massively parallel compute capabilities in GPUs makes them very attractive for acceleration of typically expensive effects. Exploiting the GPU makes it affordable to always run the full and exact simulation rather than utilizing caching techniques and interpolation with its inherent deficiencies. The GI GPU mode is using brute-force ray tracing on a capable NVIDA GPU to calculate the indirect lighting interactions in a scene. Its result gets combined seamlessly and automatically with the primary rendering done on the CPU. This ensures full compatibility with existing custom shaders, which do not need to be touched in order to take benefit of the new GI engine.

In the current version, the GI GPU mode considers diffuse-diffuse bounces only, similar to what final gathering typically computes. In fact, if this mode is enabled without setting further parameters then finalgather settings are used to derive reasonable default parameters to render towards the same quality. If certain prominent ray tracing effects like mirror reflections or transparent windows are not used in a scene then the fastest diffuse mode is best suited. For current restrictions see known limitations.

The GI GPU mode can be enabled and controlled with scene options or on the command line of the standalone mental ray.

Global Illumination "Next" (Prototype) 3.13

The advancements of the computing power over the years make typically expensive brute-force algorithms more attractive to use and feasible on modern machines. mental ray provides a new global illumination engine that is based on such algorithms, which guarantee stable convergence and consistent results both in still images and animations. It is compatible with all the mental ray effects, like motion blur and depth-of-field, out of the box. At the same time, it is very easy to use because it only provides few parameters. Since no caching is involved, it does not require hand tuning of point cloud densities or similar parameters to achieve an artifact-free solution in an animation. Scene elements do not need to be touched to enable contribution to this GI solution. For current restrictions see known limitations.

The engine can be enabled and tuned with scene options or on the command line of the standalone mental ray.