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- The simplest is a box subdivided in three dimensions into a regular lattice of voxels, with values provided for arbitrary variables at every corner of the lattice. A lookup of a variable within the box performs trilinear or tricubic interpolation of the nearest lattice points. This representation, while simple, may lead to unwieldy RIB files for dense volumes.
This approach is used by RenderMan for Maya to render Maya fluids. In the Maya user interface shown below, the bounds of the box are directly manipulated in the viewer on the left. In the attribute editor, the resolution describes the number of subdivisions of the box, which ultimately affect the accuracy of the volume simulation and rendering; in this case, the box is divided into 10x10x10 voxels. The variables associated with the volume are described in the Contents Method pulldown; here, the variables "Density" and "Velocity" are enabled, while "Temperature" and "Fuel" are disabled.
- In situations where the volume is already described via an external resource such as an OpenVDB file on disk, the volume primitive supports arbitrary C++ volume field plugins which can interface directly with the external resource in order to compute the value of an arbitrary variable within the extents of the box. RenderMan provides a plugin called impl_openvdb which can use VDB files directly. In this case, the volume description simply involves the extents of the box, the OpenVDB file, and the grids from the OpenVDB file which correspond to useful primitive variables.
Below is an illustration of using the OpenVDB Viewer node supplied with RenderMan for Maya. An OpenVDB file has been loaded from disk and the contents of the density grid visualized in Maya. The Primitive Variables tab describes the mapping from the OpenVDB grids to the variables that will be made available to RenderMan.
Volume Shading
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