We’re pleased to share the latest updates from the Alliance for OpenUSD (AOUSD) Core Specification Working Group. This Working Group’s focus is on formalizing the foundational data models and predictable behaviors of composition and population within OpenUSD. This is a crucial step towards establishing an international standard for interoperable and interchangeable datasets that describe virtual worlds.
Our journey began in early 2024 with initial outlines and has progressed to the development of preliminary draft specifications. We aim to finalize these specifications by late 2025, after which they will be submitted to the ISO for broader standardization.
Current Progress
Of the five outlined sections of our core specification, the Working Group has completed preliminary drafts of the first three areas:
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Foundational Data Types
The Working Group has broken down all supported attribute value types into a comprehensive table that any implementation can honor. This includes scalar types like floats, doubles, and asset paths (i.e., URIs for identifying content resources), as well as dimensioned types like matrix4d and other linear algebra representations.
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Document Data Model
This section specifies the fields for the USD document data model, ensuring that all USD-compliant documents can conform to this model and be readable as USD layers. This is where fields like “references” are specified in detail as sequences of asset paths, Prim paths, and layer offsets as inputs to the composition engine to combine into the composite scene.
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Core File Formats
The core file formats have been specified: USDA (human-readable), USDC (binary), and USDZ (packaged). The USDA format’s grammar is defined using the Parsing Expression Grammar Template Library (PEGTL), and for USDC, we have detailed the byte layout of all Crate Sections and fields.
Sample Implementations and Compliance Testing
The Working Group has also developed sample implementations and compliance tests to ensure consistency and reliability across different implementations. For instance:
- USDC Parser: Outputs human-readable dumps of the resulting parse tree and includes a harness for compliance testing.
- USDA Parser and Composition Engine: Similar approaches are being extended to these components, ensuring human-readable outputs for easier validation.
Keeping Up With OpenUSD Developments
The Core Specification Working Group is aligning their work with the latest developments in OpenUSD, such as the addition of the relocates arc for renaming and reparenting Prims originating in weaker layers. This ensures our specifications remain relevant and up-to-date.
Future Work and Member Engagement
Looking ahead, the Working Group is diving into stage population details, such as specifying how Prim definitions are populated by meta-schema inputs. They are also working on cross-section editorial activities, like providing a glossary for recurring conceptual terms. This glossary tightens up the coherence of key concepts like list ops across all sections.
The Working Group will be making the drafts available to the broader AOUSD membership for feedback. This ensures that the specifications are comprehensible and align with each member organization’s broader initiatives.
Looking Ahead
Having well-defined, unambiguous specifications for the foundations of OpenUSD will empower AOUSD members and the entire ecosystem to interoperate with confidence and clarity.
Members and liaison partners are welcome to provide feedback on the preliminary drafts and get hands-on with the sample implementations and compliance testing as we work towards completing Core Spec 1.0 in 2025.
If your company is interested in joining the Alliance for OpenUSD and contributing to the Core Specification Working Group, sign up to become a member. To read more about the AOUSD announcements unveiled at SIGGRAPH, see the press release.
Learn about all the latest OpenUSD advancements at SIGGRAPH by attending hands-on labs, sessions, the Alliance for OpenUSD and Academy Software Foundation USD Working Group and the OpenUSD Developer Meetup.
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