The quasi random ray tracing of scenes with complex lighting setups can take a long time to render because it needs a massive amount of samples to compute a visually pleasing result without perceivable noise. If there is any knowledge available about emission or reflection properties then those can be taken into account to guide sampling towards the most contributing effects, also called importance sampling. This can lead to dramatically reduced render times for the same quality. mental ray employs such techniques in various places, for example when applying a BSDF to a material, or when using area lights with light importance sampling enabled.
These individual techniques can be combined to achieve even better efficiency, called multiple importance sampling. Once enabled with a scene option, it will be applied in cases where both a material BSDF and supported area lights are used together that are eligible for light importance sampling.
See also Known Limitations.
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