There are two kinds of VDBs that KtoA can handle: volume (density) and surface (levelset/implicit) VDBs. Once you have done your simulation or surfacing in Houdini and have exported your .vdb file, you should note which channels you have output into the file (such as field, density, velocity, etc).
Inside Katana/KtoA there are three steps you need to take:
- Create an ArnoldOpenVDBVolume or ArnoldOpenVDBSurface node in the node graph. Set the filename to your file (or use your asset system to point to it). Add the name of at least one grid from the file you will be using. In the case of the a surface, make sure the field channel is set to the main levelset field. Note (or change) the location in the scenegraph that will have the VDB.
- Create a material, either via the Material node or by creating a shader network through the NetworkMaterial node with an ArnoldSurfaceShader port. The main shader should be a standard_volume shader in the case of a volume, and another shader (such as standard_surface) in the case of the surface/implicit data. Note (or change) the material location.
- Assign the material to the vdb scenegraph location with a MaterialAssign node.
There are additional fields on the OpenVDB nodes that are of interest, for example volume_padding to give some slack if you use positional displacement (such as noise) in your volume shader network to add detail to the volume. You can control the time/quality tradeoff with the step_scale parameter, and you can control velocity in various ways. Usually Arnold will automatically detect the velocity grid in the file, but occasionally you may need to explicitly add the velocity channel(s) to get motion blur.
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