OpenVDB bunny cloud volume available here.


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.

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.

Custom AOVs are not currently supported with volumes.


The default volume plugin is OpenVDB. In addition to the ray extents acceleration, auto bounding box and auto step size of the new volume API, it brings several production features such as velocity motion blur, scaling of velocity and step size and the ability to disable ray extents acceleration.


The grids to read from the OpenVDB data and provide as volume channels to shaders. You should specify at least one grid to read in. Note that KtoA will attempt to retrieve the bounding box of the volumetric data and display it in the viewer.

Auto Step Size

Auto step size, when turned on, will ignore the step size that is manually set. Instead, you can modify the step size from its natural value with step scale instead. When turned off, you can manually set the step size.

Step Size

Step size basically controls the rate at which the volume shader gets called. The value indicates the average distance between adjacent volume shader samples along a given ray, in world space units. Generally, you will want this setting to be as large as possible before visibly noticeable artifacts occur, and in the case of voxelized volumes no smaller than about half the size of your voxels (in world space). Low Step Sizes will increase render times. For example, when the step size is 0.1 and the volume is 10 units big in world space, there are approximately 100 primary samples and therefore the volume shader is called 100 times.

The step size has a fixed size, therefore, the larger the container the more steps will be taken and the longer it will take to render. Also, beware that if the step size is too small your render times will increase. If you cannot see the rendered Fluid, you may need to reduce the Step Size value.

More information about volume Step Size can be found here.

Step Scale

Bounds Slack

Additional space to put around the bounds of any volumetric data.  This is usually required when the volume shader adds positional noise to give the volume more apparent resolution; the positional displacement is not taken into account by the volume plugin, so this is how you indicate the distance you intend to move the volume data around in the shader.


Velocity Grids

Velocity Scale

Velocity FPS

Motion Start

Specifies the shutter open value at the current frame.

Motion End

Specifies the shutter close value at the current frame.


Set the transform (which can be interactively modified in the viewer with manipulators) for the volume.