usd-stage-population

name	usd-stage-population
description	Use this skill when implementing or verifying population of a stage hierarchy from a namespace source.
metadata	{"author":"NVIDIA"}

usd-stage-population

Use this skill when implementing or verifying population of a stage hierarchy from a namespace source.

Spec Sources

• specification/stage_population/README.md
• specification/document_data_model/README.md population fields
• specification/path_grammar/README.md path element ordering

Pinned tag / commit: v1.0.1

Provides

• Target-native Stage populated from a NamespaceSource
• Root scene object at /
• Prim traversal and path lookup for populated prims
• Parent and child prim queries
• Property enumeration for populated prims
• Population mask handling
• Active descendant pruning
• Default prim lookup
• Stage query flags for active, loaded, defined, abstract, and instanceable
• Portable model hierarchy traversal and query state for kind
• Bounded instance summaries and shared representation IDs for instanceable prims backed by supported composition-arc provenance
• Diagnostics passthrough from the namespace source

Contract

This skill owns contracts/handles/stage-population.handle.json and the coverage ledger in contracts/spec-coverage/stage-population.coverage.json. It also follows contracts/capabilities/semantic-runtime-types.json.

Stage population consumes the generic namespace-source capability rather than a particular layer or parser. The first provider is usd-single-layer-namespace-source, where every path has at most one authored opinion. Future composition nodes should provide the same namespace-source capability at a stronger capability level.

The populated stage is a target-native scene graph/query surface. It must not be a canonical JSON dump, a parser-local USDA tree, or the document-model layer itself. JSON appears only at conformance adapter boundaries. After population, the public Stage API is read-only: do not expose mutable maps, vectors, indexes, or caches that allow callers to replace populated prims/properties or reorder traversal.

Population starts at /, retrieves source child names, applies population-mask filtering when provided, applies active pruning, and creates populated prim nodes for reachable paths. Traversal exposed by this skill returns populated non-root prims in stage order.

For a single-layer source, child ordering is:

1. start with the source primChildren order
2. apply the source primOrder sparse order for the parent
3. leave names absent from primOrder in source order

For properties, apply path element ordering to the source property names, then apply the source propertyOrder sparse order. Future composed providers may expose merged children/properties with child_ordering=composed and property_ordering=composed; in that case stage population preserves provider order and remains responsible only for population-mask filtering, active pruning, and the query indexes built over populated nodes.

Query Policy

The first contract uses source-visible field opinions plus explicit fallbacks:

• active falls back to true
• loaded is true when the source declares payload loading absent
• specifier falls back to over
• defined is true only if the prim and all populated ancestors have specifier def or class
• abstract is true if the prim or any populated ancestor has specifier class
• instanceable falls back to false
• typeName, kind, and defaultPrim fall back to empty token/string

Model hierarchy uses only portable AOUSD kind values. group and assembly are included and continue traversal. component is included but terminates traversal, so descendants of a component are not part of the model hierarchy. subcomponent, empty, and unknown kind values are excluded and terminate continuity. Custom kind aliases are not portable and must not be enabled unless a separate contract defines that policy.

The instanceable flag remains the source-visible field opinion with fallback false. A populated prim can claim instance=true only when it is marked instanceable and the consumed composed namespace source supplies supported composition-arc provenance. Matching shared-representation IDs include both the supported arc provenance and the relative population-mask context, because the spec notes that masks can prevent otherwise matching instances from sharing. When a matching shared representation has already been populated, the later instance root remains populated but duplicate descendants are not added to stage traversal.

Attribute and relationship field maps are exposed as source-visible authored or schema-defined fields from the consumed namespace source. Do not resolve attribute values across opinions or time; that belongs to a value-resolution skill. Those field maps still preserve the typed inspection and checked-read semantics supplied by the namespace source and document model; stage population must not degrade them into adapter JSON.

Boundary Guards

Consume NamespaceSource. Do not read parser-local USDA structures or assume the source is a document-model layer. If an implementation bulk-enumerates a source for performance, the semantic boundary is still the namespace-source contract.

Consume usd-paths for path identity, parent/child construction, prefix checks for masks, and path element ordering. Do not implement a parallel string-only path model.

Consume usd-tokens or equivalent token identity for names, field keys, specifier/typeName tokens, and ordering fields.

Stage indexes must retain paths, property names, child names, typeName/kind tokens, and field keys as semantic handles. String paths and names may be materialized for adapter summaries, diagnostics, and reports, but they must not be the authoritative values used for stage lookup or traversal.

Use a private builder or equivalent construction-only path for internal population mutation. The generated public surface may expose const queries and const views, but it must not expose mutable ownership of path-to-prim maps, path-to-property maps, traversal arrays, property arrays, or ordering caches.

Do not perform composition arc evaluation, resource loading, asset resolution, schema fallback evaluation, value interpolation, or value clips. If a schema-aware namespace source supplies schema-defined properties, stage population may populate those properties, but it must not compute prim definitions itself.

For instancing, consume graph-owned composition provenance or instance-filtered descendant inputs supplied by the composed namespace source. Do not infer instance matching from OpenUSD behavior, parser-local syntax, or same-looking local child topology. Do not treat local descendant opinions under an instanceable prim as instance source opinions. Do not add implementation-specific prototype query APIs unless a later contract cites a pinned AOUSD requirement for them.

Do not reapply single-layer primOrder or propertyOrder rules to a source that declares composed child/property ordering. Those rules belong to the composition provider for composed sources.

Do not present the resulting Stage as full USD stage conformance when the consumed namespace source declares composition or value resolution absent.

Performance Model

contracts/performance/stage-population.performance.json defines the initial measurement model. The important shape is that construction tracks populated paths and exposed properties, lookup does not scan traversal results, ordering is cached or bounded per prim/property list, and the populated stage does not retain adapter JSON or duplicate full path strings unnecessarily.

Test Obligations

• hierarchy traversal from source child names
• primOrder and propertyOrder
• default prim lookup
• population masks include ancestors and exclude unrelated branches
• active=false prunes descendants
• path lookup and parent/child queries
• query flags for active, loaded, defined, abstract, and instanceable
• model hierarchy traversal and summaries for portable kind values
• instance summaries, shared representation IDs, population-mask separation, and descendant filtering for supported non-local composition arcs
• property summaries for attributes and relationships
• preservation of schema-defined properties when supplied by a schema-aware namespace source
• diagnostics passthrough from the source
• all cases in goldens/unit/usd-stage-population/stage-population.json
• all cases in goldens/integration/usd-instancing-model-stage-population/instancing-model-stage-population.json