The Volume material is dedicated to simulating volumetric objects such as atmosphere, fire, smoke and fog. This material is only meant to be used on volume geometries.
If you bring a volume material created before Clarisse 4.0, a compatibility attribute called Energy Conservation will appear in the material. This attribute enables the previous shading model which wasn't energy conservative. To match the look of the older volume material Optical Mode should be set to Legacy and Energy Conservation must be off.
The Optimal Mode defines which set of attribute is used to control the optical properties of the volume. While each attribute of each mode drive the attributes of the other ones, this documentation will cover the Artistic set of parameters because of their simplicity.
|Artistic||Control the material using Attenuation and Albedo set of attributes.|
|Physicist||Control the material using Absorption and Scattering set of attributes.|
|Legacy||Control the material using the Scattering and Attenuation set of attributes. Note that in this mode the volume no longer obeys the physical law of energy conservation (ie. the particles will scatter more light than they receive) whenever the value of the scattering parameter is greater than the value of the attenuation parameter (hence yielding a negative absorption and an albedo greater than one). This mode is kept for compatibility purpose only and will disappear in the next major version.|
The Albedo attribute defines the color of the volume. Technically it defines the reflectivity of the attenuating particles of the medium.
The Volume material evaluate multiple scattering when the Volume Depth attribute of the path tracer of material is higher than 0. While it is possible to override Volume Depth at the material level, please remember that the Total Maximum Depth which limits Volume Depth is always set at the renderer level. When setting a Volume Depth of 20, make sure that Total Maximum Depth is set to at least 20 on the renderer.
The secret for rendering realistic clouds lies in setting Forward Scattering to around 70% and more importantly a lot of bounces! Here the Volume Depth is set to 64.
Multiple Scattering greatly slows down rendering. One way to keep render times reasonable is to use Russian Roulette.
The Attenuation attribute defines the rate at which light is absorbed by the medium. The higher the attenuation, the shorter distance light travels through the medium.
The Density attribute multiplies the density value of the volume.
The Emission attribute set the quantity of light emitted by the particle of the medium. The effect is purely additive on the volume.
The Forward Scattering attribute controls the anisotropy of the scattering. By default, Forward Scattering is set to 0% and light is scattered isotropically (by the same quantity in all directions). It is possible to bias the scattering forward in the direction of the light by increasing the value of Forward Scattering. It is also possible to bias the scattering backward toward the light by setting Forward Scattering to a negative value.
The Sampling Optimization attribute controls if volume samples should be shared on overlapping volumes.
Decreasing this value improves the accuracy of the rendering for overlapping volumes at the expense of render times.