See Advanced Render Settings in the Arnold User Guide

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Lock Sampling Pattern

Locks the AA_seed so that the sampling noise won't change with the frame number (avoiding the film grain look).

Use Autobump in SSS (deprecated)

If enabled, autobump is visible in SSS. Enabling this option will take into account the effect that displacement_autobump has on the ray-traced BSSRDF's result. This helps capture the high-frequency details of the surface more accurately when using autobump.

Beware that enabling this option will triple shader evaluations with SSS, resulting in much longer render times.

Nested Dielectrics

Enables the nested dielectrics system for IOR tracking in scenes with adjacent dielectrics. Defaults to on.

Stochastic Volume Interpolation

The tricubic interpolation mode of AiVolumeSampleXXX() calls, such as those used by the standard_volume shader, now uses a stochastic method of interpolation that greatly reduces the amount of voxel data read in each sample call. This can speed up rendering by as much as 20% in assets such as the WDAS cloud data set. Note that this technique does not always produce a perfectly matching image as a result, and can be toggled via the stochastic_volume_interpolation render option (default is on).

Indirect Specular Blur

Sets indirect specular blurring to reduce caustic noise. Setting to zero gives the most accurate but also noisy renders, while higher values blur caustics to reduce noise.