...
Rendering volumes in Katana is similar to rendering other geometry: you'll need a volume primitive and a volume shader. The RenderMan Volume primitive is created in Katana using the PrmanVolume node. For a simple homogeneous volume such as fog you'll just need to assign the PxrVolume shader to the PrmanVolume primitive. More complex shading can be built on top of a PxrPrimvar pattern wired to connect volume data into the volume shader. RenderMan's volume shader PxrVolume along the PxrPrimvar and other patterns, provide a complete volume workflow for either OpenVDB volume data or geometric volume primitives. For more details on RenderMan's volume primitives see the RenderMan Volume Documentation.
Currently PrmanVolume supports two types of volumes:
...
A VDB grid is a sparse tree representation of voxel data. For instance, a float grid stores one single-precision floating point value per voxel. After the OpenVDB file is loaded, a list of grid names become available in the pulldown menu. Pick the grid name you want to sample. All grids are sent to the renderer as primvars, however only those wired into the PxrVolume shader will be used.
Filter Width
If set to 0, this disables mipmapping. If greater than 0, values less than 1 bias towards finer levels of the mipmap and values larger than 1 bias towards coarser levels.
Density Multiplier
Scales the values in the density grid of the volume. This should be more efficient than scaling the density in the PxrVolume shader.
...
Values greater than 0 produce a logarithmic falloff in density values. Smaller rolloff values produce greater falloff.
Filter Width
If set to 0, this disables mipmapping. If greater than 0, values less than 1 bias towards finer levels of the mipmap and values larger than 1 bias towards coarser levels.
| Note |
|---|
Blobbies are currently not supported by the PrmanVolume node. |
...
RfK will internally create primvars for all grids found in the VDB file (float or vector3). There is no penalty if they are not used. PxrVolume can look up the density and velocity primvars by name. To access other primvars or to modify the existing density and velocity, primvars can also be accessed by the shading network via the PxrPrimvar pattern node:
Connect the result of PxrPrimvar directly to the PxrVolume shading node, or wire it through other shading nodes, as in the image below:
...