Kind : Extensible Categorization

The Kind library provides a runtime-extensible taxonomy known as "kinds".

Kinds are just TfToken symbols, but the KindRegistry allows for organizing kinds into taxonomies of related/refined concepts, and the KindRegistry::GetBaseKind() and KindRegistry::IsA() queries enable reasoning about the hierarchy and classifying objects by kind.

Kinds are useful for classifying scenegraph objects, such as by tagging objects representing the roots of models with the kind of model it represents; it is scenegraph taxonomy that motivates the "builtin" kind hierarchy.

The Core Kind Hierarchy

The KindRegistry begins its life pre-loaded with the following hierarchy of kinds that define our concept of "model hierarchy", which is so central to our organization and discovery of scene description. Of course, as described in the next section, unlimited new, entirely unrelated taxonomies can be created by extension.

  • model - Everything in the model hierarchy is a kind of model.
    • component - A component model is a terminal model in the model hierarchy - it can have no child models.
    • group - a model that is simply a container for other models.
      • assembly - An "important" group model - often because it is itself a published asset.
  • subcomponent - Within a component model, subcomponents identify important (generally articulable) sub-trees. Subcomponents are "stopping points" when dynamically unrolling/expanding a component.

Extending the KindRegistry

The kind registry can be extended using the facilities provided by PlugRegistry, by adding a 'Kinds' sub-dictionary to the plugInfo.json file of any module within your "pixar-base aware" build environment. The dictionary entries will look like the following:

"Info": {
"Kinds": {
"chargroup": {
"baseKind": "assembly",
"description": "A chargroup is an assembly comprised of a single character plus some associated models -- typically hair, garments, and charprops."
"charprop": {
"baseKind": "component"
"newRootKind": {
One cannot alter core kinds or their place in the hierarchy or any other data associated with them by attempting to override them in the extension file. Attempting to do so will result in a registration error.