Final gathering is a technique to estimate global illumination in scenes where indirect lighting is varying rather slowly, like those containing mainly diffuse materials. For a given shading point, it collects illumination information by sampling the hemisphere over that point in a number of directions. The resulting irradiance data for a specific location may be stored in a cache as a so called finalgather point. This cache, the finalgather map, can then be used to accelerate final gathering dramatically by averaging a number of final gather points nearby instead of shooting a large number of new samples for every illuminated point. Finalgather maps can be stored to files on disk for later reuse, like in animations.
Final gathering can be combined with photon tracing to perform a complete and physically accurate lighting simulation that would include all possible light pathes, like caustics.
If coupled with global illumination simulation, final gathering can improve the quality of the final result noticeably. It eliminates photon map artifacts such as low frequency noise and dark corners. Fewer photons are needed in the respective photon map and lower accuracy is needed, since finalgather computation already averages over many values of indirect illumination. Without final gathering, the global illumination on a diffuse surface is computed by estimating the photon density (and energy) near that point. With final gathering, the hemisphere above the point is sampled to determine the incident illumination. If a diffuse surface is hit then the global illumination is computed by the material shader of that surface using information from the related photon map, if available, and other material properties. Hits on specular surfaces do not contribute to the final gather color since that type of light transport is a secondary caustic. Tracing many rays, each with a photon map lookup, is very time-consuming, so it is only done when necessary. In most cases, interpolation and extrapolation from previous nearby finalgather points is sufficient.
In film production work, final gathering is often preferred to photon mapping,
except for caustics. By default, final gathering supports a single bounce only
because multiple-bounce effects tend to have far less impact on the final
image. Secondary bounces are performed by photons by default, but may be
enabled for final gathering by adjusting the finalgather related
trace depth. Although
physical correctness is lost, this is often
good enough to achieve the
desired effect. Also, final gathering is easier to control than photons
emanating from distant light sources. However, for accurate indoor illumination
simulations and other CAD-related applications, photon mapping is recommended.
Final gathering can be enabled with a scene option or on the command line of the standalone mental ray. To change the number of rays shot in each final gather, together with the maximum distance at which a final gathering result can be used for interpolation and the minimum distance at which it must be used, the following setting can be used:
finalgather accuracy 500 1.5 0.25
By default, mental ray determines the maximum distance from the scene extent dynamically. Decreasing this value will reduce noise but increase render time. The default minimum distance is 10% of the maximum distance.
Note mental ray has improved the final gathering algorithm across versions. Today, it typically achieves better quality in less time with approximately half the number of rays compared to older versions. For example, the accuracy set in the preceding example would be comparable to around 1000 used in older mental ray to achieve similar quality but taking much more time to finish.
A progressive final gathering mode for faster pre-computation visualization is supported with the introduction of a novel multi-pass refinement algorithm. In this mode, the final gathering calculations will respect the tile order of regular rendering for a consistent display experience to the user. The number of final gathering pre-computation passes can be specified with a scene option or on the mental ray command line. This mode is used by default.
The exact final gathering mode allows to
bypass the finalgather map cache and execute the more expensive but superior
brute-force finalgather tracing. This is helpful to overcome interpolation
artifacts in critical areas of the scene where the resolution of the point
cache is not sufficient to capture fine detail, or leads to visible errors
in the illumination estimate known as
light leaks. This mode can be
activated for specific scene elements using the
force flag, while continuing to
use the cache for the rest of the scene. It may also be enabled globally using
the force option, for example to render high
quality reference images.
Final gathering can be combined with importance-based techniques like
Irradiance Particles, to achieve
better quality with less artifacts in shorter rendering time. In these
cases, importons are shot before final gathering, which are then used to control
the distribution of finalgather rays according to
mental ray offers convenient final gathering modes to switch general behavior easily towards specific use cases. The following modes are supported:
The mode can be selected by a
scene option or on the mental ray
The default is "3.4" compatibility mode.
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