OpenUSD Terms & Concepts
Universal Scene Description (OpenUSD), like anything complex, mandates a lexicon capable of describing key ideas and concepts that may or may not exist similarly in other applications. This section is here to help you learn your way around some of the more confusing terms in OpenUSD and help clarify their intent and usage.
Definitions
- Abstract Prims
Primitives which have been declared with the “class” specifier and their descendsnts. Abstract prims will generally be skipped by stage traversals so they are relatively light weight but can provide metadata and opinions when composed onto other prims.
- Access Semantics
Any field in OpenUSD can be override-able. Prims or variant sets that are expected to be “internal” must use semantics (like _ prefixes) to communicate editability expectations
- Aggregate Model Structure
An
assemblymodel that exclusively contains references and high level asset prim interface edits- Anchored Asset Path
Asset identifiers may be “anchored” to their containing layer; empowers relocatability and deduplication of content
- Asset
Named, versioned, and structured container of resources (layers, textures, volumes, etc.) that is maintained and reused
- Asset Interface Layer
A root layer intended to be opened or referenced containing one or more entrypoint prims and lofted properties. Often uses the “reference-payload” pattern
- Asset Parameterization
Variant sets and primvars (usually on an entrypoint prim) that enable downstream variations of the same underlying asset
- Asset Path Valued Field
Property or metadata whose value is a path to an asset system managed resource (layer, texture, volume, etc.)
- Asset Prim Interface
A stable set of prims maintained for downstream overrides
- Atomic Model Structure
A
componentmodel whose dependencies are fully encapsulated- Branch
A parallel stream of an asset with its own set of versions
- Completeness (Model Hierarchy)
Aspect of model Hierarchy that encourages
componentandassemblyassets to work out of the box, generally with complete geometry and material descriptions- Component Model Boundary
The primary aspect of model hierarchy that separates “model hierarchy” (properly
kind-ed ancestors ofcomponentmodels from the non-model elements in the full “prim hierarchy”)- Composition
The process of combining various scene graph elements in OpenUSD to form a final scene.
- Composition Arcs
Mechanisms in OpenUSD that facilitate content reuse, such as references, payloads, inherits, and specializes.
- Composition Target
The prim (or layer) that is targeted by a composition arc. For example, a prim that is referenced onto another prim. Usually either a Layer identifier, an Sdf.Path or both.
- Computational Workstreams
Layer stacks backed by parallel computation that may not model a particular user workflow
- De-duplication
The process of automatically detecting and eliminating duplicate data to optimize storage and performance.
- Embedded Context
Asset interface prims that aren’t entrypoints but can be useful context for previewing and thumbnail generation
- Encapsulation
In OpenUSD, this refers to the containment of the scene hierarchy, ensuring that relationships and bindings in the branches of the hierarchy do not refer to external targets.
- Entrypoint Prim
A primary prim for inspection and referencing into downstream contexts
- Explicit Prototypes
Prototypes which have been created by the User as an explicit edit target for prototype contents
- External Resource
A resource whose versioning is external to an asset container. An external resource is usually a versioned as part of an another asset.
- Fork
A duplication of an asset to introduce breaking / incompatible changes
- Globally Refinable Instancing
A method of instancing where instances remain “refineable” in a downstream assembled scene.
- Implicit Prototypes
Ptototypes which are created by OpenUSD for instanceable prims.
- Inherits-instanceable Pattern
An
instanceableassets can be edited in context through aninheritsarc- Instanceable
A property in OpenUSD that, when set to true, allows a prim to be instanced.
- Instancing
The process of creating multiple instances of an object, sharing the same data for workflow or performance reasons.
- Instance Proxy Prim
Because an instance is essentially just a pointer to its prototype, local prim paths within the instance don’t really exist. OpenUSD provides “instance proxy prims” as placeholders which can be used to display the contents of instances as if they were there. Additionally, collections can include instance proxy prims.
- Internal Resource
A resource whose version is shared with other resources within an asset container. Generally represented in scene description with “anchored” identifiers
- Internally Refinable Instancing
A method of instancing where instances are not expected to be refined downstream.
- Lofting
Moving or replicating a sparse set of data out a payload and into an encapsulating reference to make important fields like variants, inherits, and other metadata visible without loading the payload
- Mirroring Resolver
A resolver which greedily localizes an entire resource (layer, texture, volume field) as part of resolution. Mirroring resolvers sometimes warrant additional usage of payloads or variant sets to avoid synchronization.
- Model Kind
Model Kinds provide a way to organize and categorize Prims within the Stage hierarchy or “model hierarchy”. Read more about them here.
- Native Instance
Instanceable prims (as opposed to point instancers)
- OpenUSD
OpenUSD (Universal Scene Description) is a framework for interchange of 3D computer graphics data, developed by Pixar.
- Package Model Structure
An
assemblymodel with one or internal primarycomponentthat may reference other externalcomponentmodels. Used as an alternative to nestingcomponentsassubcomponents.- Pipeline
The flow of assets through a production process
- Point Instancer
A specific type of instancer in OpenUSD that allows efficient representation and rendering of many instances of a scene element.
- Prims
Prims are fundamental building blocks or elements of a scene within the USD framework. Prim is short for “primitive” and it represents an individual object or entity within the scene hierarchy. Read more about Prims here.
- Primvars
Primitive variables in OpenUSD, used for storing data that varies per-instance, such as colors or positions.
- Prototype
In the context of OpenUSD, a prototype is a central copy of a scene graph or part of a scene graph that can be instanced.
- Pseudo-Root Prim
Every Stage has a pseudo-root Prim which ensures that the namespace hierarchy of a Stage is structured as a tree instead of a forest (i.e. one top-level prim). This simplifies Stage traversal algorithms. read more about Pseudo-Root Prims here.
- Reference-payload Pattern
A prim hierarchy split between lightweight important scene description that is “referenced” and its internal contents placed behind a “payload”
- Refinable Instancing
Composition arcs that provide edit targets for instance content refinements.
- Scene Graph
The hierarchical prim / namespace data structure used to organize data in OpenUSD.
- Scene Graph Instance
Instanceable prims (as opposed to point instancers)
- Selector Model Structure
A
componentorassemblywhose sole function is to act as a variant switch between a set of related assets- Session Artifact
Scene or data that does not persist beyond the length of a session
- Shallowness (Model Hierarchy)
Aspect of model hierarchy that discourages deep model hierarchies for performance / navigability reasons
- Stages
A Stage is an instance of the USD data model that holds the entire scene graph and associated data. It serves as the entry point for accessing and manipulating the contents of a USD file or composition. Read more about Stages here.
- UTF-8/ASCII Identifiers
Identifiers are whitespace free / alphanumeric character sets plus
_. ForASCII, this is usually the regular expression([A-Za-z_]+[A-Za-z_0-9]*). ForUTF-8, we generally mean the XID class of identifiers.- User Workstreams
User-facing units of work that may overlap or operate in parallel, typically modeled in layer stacks
- Variant Set
A Variant Set in OpenUSD represents a collection of alternative representations or configurations for a prim. For example, a Variant Set could include different material variations, different levels of detail (LODs), or different geometric representations of a model. Read more about variant sets here.