Arnold Volume - Arnold for Houdini
The Arnold volume object node will translate as an Arnold volume shape and is part of the new volume API. It supports plugins for volume file formats or procedurals. A full-blown OpenVDB volume plugin is shipped with HtoA and fully interfaced with the Arnold volume.
Info: More information about volume step_size can be found here.
> Velocity Grids: Using a single vector grid for velocity instead of 3 float grids can render up to 10x faster. It is strongly advised to convert the velocity to a vector field using a VDB Vector Merge SOP.
Type
Choose between 'Custom' or 'OpenVDB'. For custom, you will need a volume plugin that will create volumes or load volumes of a specific format. In Data, you will need to assign attributes to that specific volume plugin.
OpenVDB
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.
Display As
How the volume is displayed in the viewport. The default is bounding_box, which displays the union of all selected grids bounding boxes, and a visual cue of the padding. In volume mode, the first grid of the list is displayed in the viewport, with its bounding box and padding as well. The SOP network inside determines how the volume is displayed in the viewport. You might want to change it for you own digital assets.
Max Vis Res
This parameter allows you to make a lower res volume for faster previsualization, or to allow the volume to render with full resolution. It is possible to exceed the default volume restriction of 128 voxels on each side.
 |
 |
 |
| 10 | 200 | 20 |
Volume Step
The default automatic will compute the minimum voxel size and use this as the ray marching step size. It is then useful to modulate the step size with the step_scale parameter (see below). In custom mode, the step_size is set manually with the step_size parameter.
Step Size
The volume ray marching step_size when volume_step is custom. When set to automatic, although the widget is disabled, its value displays the automatic value, ie. the minimum voxel size of all grids.
Custom
When the volume type is set to custom, you can point the volume to an arbitrary volume plugin DSO and pass it parameters.
Info: More information on how to implement your own volume plugin can be found here: Implementing a Volume DSO.
Please refer to the OpenVDB documentation above for the common parameters.
Custom Type
...
Data String
An arbitrary user data string is available to pass information to volume plugins. Another way to pass data is to create spare parameters after the user_parms_separator, those parameters will be translated a constant user data for the plugin to fetch.
To reduce the amount of noise in a volume, you must ensure that the lights in your scene have enough volume_samples.