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Since smoke is typically isotropic, the Anisotropy settings can be left at their default. The Equiangular weight can be left as is if it is expected that there are light sources near to, or within the volume, or set to 0.0 if no light sources are expected to be anywhere close to the volume.

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Clouds are very difficult volumes to render, because the aggregate behavior of water droplets and their interaction with light is a complicated phenomenon. Unlike smoke, the light that reaches the eye from a cloud has typically bounced inside the cloud tens or hundreds of times. On top of that, the scattering behavior (the way light bounces inside the cloud) is very complicated, as it involves both forward scattering and backwards scattering, with multiple peaks for both.



Note that scattering in the volume is controlled by the Max Path Length parameter in the chosen integrator and not the individual diffuse and specular trace depths. You can find more information on this under the section below called Multiple Scattering.



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Advanced Workflows

Crepuscular Beams


The emissive color of the volume. This is useful for modeling effects such as phosphorescent fog or fire. Default is black (0,0,0), i.e. the volume will not emit light. The following images demonstrate an emit color setting of (1, 1, 1) and (5, 2.5, 2.5), with no other light sources in the scene.

Light Source

When this option is selected, the emissive properties of the volume are sampled as a light source. This may brighten your scene through improved sampling. Note that the emit color cannot be black for this to operate. With a single scatter volume, even at maximum path length 1, it is equivalent to a multiscatter volume with the light source option turned off. Below is off versus on.

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Multiple Scattering

This parameter is used as a hint as to whether the volume should compute indirect illumination inside the volume (also known as multiple scattering, because light will scatter more than once inside the volume). If the multiple scattering parameter is set to 0, and the integrator respects this hint, PxrVolume will only perform single scattering: points inside the volume will only be lit directly by light sources. If set to 1, points inside the volume will be lit by indirect illumination as well. The first image below has multiple scattering set to 0, i.e. it is a single scatter volume. The middle image has multiple scattering turned on, with 2 bounces of light. The right image has multiple scattering turned on with 4 bounces of light.