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Basic Workflows

Constant fog

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If depth attenuation is the only effect that is required, the Diffuse Color can be set to black: light is not scattered by the volume, it is only absorbed, and Density is the only parameter that is important. Otherwise, the diffuse color needs to be set to the color of the fog. 

Smoke

For heterogeneous media like smoke, the typical approach is to bake the properties of the smoke into a file like an OpenVDB file. The OpenVDB file would contain a small number of properties including the density, the velocity (if using motion blur), and possibly the albedo. The OpenVDB should be attached to a RiVolume and read using the impl_openvdb plugin. The name of the density property of the OpenVDB file is then used directly with the PxrVolume Density Float PrimVar parameter.

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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.

Clouds

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.

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Advanced Workflows

Crepuscular Beams

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In order to efficiently render crepuscular beams, it is important to try to maximize the effect of direct lighting - i.e. maximize the volume that can directly trace a path to a light source. If the volume must trace a path to a light source indirectly by going through another surface or material, the convergence of the beams may be very slow and unacceptably noisy. In the image above, the stained glass window on the right is modelled as "thin glass": instead of relying on a secondary material bounce to refract through the glass, shadow rays from the volume that trace towards the light source are unimpeded (do not refract), and rely only on the shadow tinting behavior of the stained glass material. The "thin" parameter of PxrSurface (located under the glass parameters) allows one to achieve exactly this optimization.

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Performance Suggestions

Density is the most critical property of a volume. It is a measure of both how much light is absorbed and scattered by the volume. Volume integration may require many billions of density evaluations over the course of a single render, so it is important to ensure that the renderer can evaluate the density as quickly as possible. This is where baking a complicated signal to a volumetric file format such as OpenVDB can be useful, as it means the renderer can simply look up the value from the data. For best performance, we highly recommend using the PrimVar density parameters if at all possible, rather than using an input connection, even if that input connection is just a single PxrPrimVar node.

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The velocity vector value is expected to be relative to the entire frame. If you are working with velocity data measured in units per second, you may need to scale the data by the number of frames per second (i.e. 1/24.0) for a correct picture.

We supply a control as a Velocity Multiplier to aid in conversion should your data be in the wrong measurement or allow you to make artistic tweaks to the motion blur.

As noted above: unlike other parameters to PxrVolume, the accuracy of deformation motion blur is directly related to and controlled by the  dice attribute  micropolygonlength . If you find that the fine detail of a motion blurred volume is lost, you may need to decrease this attribute in order to regain the detail. This will have a direct effect on render times and memory. 

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