Primitive Type - OpenVDB

The eyeball icon offers an OpenGL preview of the VDB cloud

Although there are three separate settings - for Emission, Scattering and Absorption - VDB files don't need to have all three active. Indeed, you don't have to use the same VDB file for each of the channels. You might have one for Emission, a separate one for Scattering and not use the Absorption channel at all.

Additionally, each channel can have different groups of data - density, fuel, heat, temperature, etc. - that represent the simulation that has been made. You can use one group for each channel.

Channels

• Density - Determines how dense or sparse the data from the VDB is
• Fuel - The source of the flames, explosion, etc. in your VDB file
• Heat - Units of energy measured in Joules supplied by the fuel in your VDB
• Temperature - The relative temperature of each voxel in the VDB file
• Velocity X, Y, Z - The speed at which each voxel is traveling

Settings

• Step Size - The resolution you wish to render your voxels at. The default of 100 mm should be good for most uses, but if you want faster results and are not bothered by a lack of definition this value can be made larger for a gain in rendering time. Likewise if you want more detail, the Step Size can be reduced for more detail but at an increased render time cost.
• Emission Channel - This channel is responsible for the color the VDB shines with, and is unaffected by the light shining on it.
• Scattering Channel - Scattering is the color of the VDB when light shines ON it, so gray or white for smoke etc. This is affected by lights etc. Scattering light can penetrate inside a volume, losing strength as it goes in.
• Absorption Channel - When light shines through the VDB, some of it is absorbed and doesn't come out again. You only see the color that's NOT absorbed, the color complementary to the one you chose.
• Scale Values - The Emission, Scattering and Absorption channels all have Scale settings. While Scale is initially set at 1.0 by default, a more dramatic effect can be achieved by increasing the multiplier.
• Sequence Options - becomes available once the channel is chosen and allows control over how many frames are used from the sequence.
• Asymmetry - Asymmetry allows lighting your voxels from a different direction.
• Interpolator - Applies a blur to the voxel, in Point mode you might see the cubes that are the basis for the voxels, in Bilinear and Quadratic they get smoothed out, but take longer to render.

Node Editor

Opening the Node Editor for VDB offers a Destination node with Color and Scale inputs for the different channels, an Anisotropy input for deciding whether the cloud should be front- or back-facing and an Extent input.