Volume file formats, such as OpenVDB, will allow storage of much larger datasets while being optimized for random access to volume data, which is more efficient for rendering. OpenVDB is an open source C++ library comprising a novel hierarchical data structure and a suite of tools for the efficient storage and manipulation of sparse volumetric data discretized on three-dimensional grids.

Tutorials about working with volumes can be found here.


A tutorial on how to write volumes to OpenVDB from Houdini can be found here.


Volumes can only receive indirect lighting from other emissive volumes when the Volume Ray Depth is above 0.


Arnold Volume (VDB).


Custom AOVs are not currently supported with volumes.


To reduce the amount of noise in a volume, you must ensure that the lights in your scene have enough 'volume samples'.

Parameters

VDB File Path

The location of the volume VDB file. 

Volume Shader

Volume shader that is assigned to the volume. Typically this is a Standard Volume shader.

Grids

A space-separated list of OpenVDB grids to read and make available as channels in the volume shading context. Examples of Grid names are density, fuel, heat, temperature or velocity.

Velocity Grids

Either one vector grid (e.g., v or vel) or three float grids (e.g., vel.x, vel.y, vel.z) representing the velocity field, to be used for motion blur. No motion blur will occur if an invalid combination of grids or if no grids are specified. Note that all velocity grids declared here are also available as channels in the shading context.

Ray Marching Step Size

Auto

Choose a step size relative to the voxel size.

Manual

Choose a step size (where an actual step size is given).

More information about volume Step Size can be found here.

Bounding Box

Padding

Motion Blur

Velocity Scale

Outlier Threshold

Animation Support

On

Enable reading a sequence of VDB files.

Sequence Start

The start frame of the VDB file sequence.
For example, if your sequence on disk goes from name1001.vdb to name 1100.vdb, you can pick the files from 1050 to 1080  by entering these values as the Sequence Start and the Sequence End respectively.

Sequence End

The end frame of the VDB file sequence.

Animation Start

The timeline's frame at which the VDB sequence starts to be used.

Animation End

The timeline's frame at which the VDB sequence ends to be used.
If there are not enough files in the sequence to cover the Animation Start-End range, the sequence can be repeated, either from its start or in ping-pong mode.


Replay Speed

Sets the playback speed of the volume's animation.

Loop Animation

When on, if the animation range exceeds the sequence range, the sequence is looped.

Extend Range

If on, the first and last VDB file of the sequence is used before and after the Animation start and end frame respectively. Else, for those frames, nothing gets rendered. 
 

Ping-Pong Replay

When on, if the animation range exceeds the sequence range, the sequence is read forward, then in reverse, then forward again and so on.

VDB Workflow

Select from the VDB Grids that the volume has been saved with. A default Standard Volume shader will be assigned to it.


If you need to further edit the result of the reading of a channel, for instance, to color correct it, there are the Volume Sample RGB and Volume Sample Float shaders.


Reducing Noise

To reduce the amount of noise in a volume, you must ensure that any lights in your scene have sufficient volume samples, or that you have enough AA samples.




Further examples

Smoke volume by Mads Droschler