039 · OPENUSD
— Universal Scene Description · stratum supra stratum —
Project Lavos · The Stack
PLATE XXXIX · STRATIGRAPHIC CROSS-SECTION

Stratum supra stratum.

— LAYER UPON LAYER —

the stage.

Pixar Animation Studios  ·  internal · 2012  ·  OpenUSD · 2016  ·  AOUSD · 2023

SCENE DESCRIPTION · COMPOSITION ENGINE · LIVRPS · HYDRA · LINGUA FRANCA

OpenUSD does not describe a scene. It describes a recipe for composing a scene — strata of opinions stacked in a strict precedence order, each layer a file, each file a contribution. Pixar's Toy Story 4 shipped on it. Karma is a Hydra delegate over it. Houdini, Maya, Cinema 4D, Cycles, Arnold, Mantra, Mitsuba, Unreal — all read the same .usda file. The strongest layer wins. The stage is what remains.

— Plate XXXIX · Stratigraphic cross-section · 8 layers, one stage — after Pixar Tech Memo TM-92 · 2012
— Fig. I · the composition · LIVRPS, top-down — STRENGTH ↑ STRONGEST L local I inherits V variants R references P payloads S specializes ↓ WEAKEST ▼ COMPOSED STAGE shot_010.usda — shot-level opinions, animation overrides — LOCAL lighting.usda — key, fill, kicker; HDRI dome; light linking — SUBLAYER fx_anim.usda — particle caches, simulation references, point clouds — SUBLAYER characters/alice.usda — variant set { coat: red | blue | none }; pose offset — └ active: coat=red VARIANT environment_set.usda — studio set assembly; references 12 props — REFERENCE characters/master_geo.usdc — 12 GB · subdivided meshes · loaded on demand — PAYLOAD base_class_character.usda — shared bind pose, rig structure, attribute schemas — INHERIT studio_template.usda — studio-wide defaults · render settings · color spaces — SPECIALIZE ARC KEY local inherit / sublayer variant reference / payload specialize FORMAT .usda — ASCII .usdc — Crate (binary) .usdz — package RUNTIME UsdStage — composed SdfLayer — opinions Hydra — render
— Layers — 8 · cross-section
— Strongest — shot_010.usda · local
— Weakest — studio_template · specialize
— Composes To — UsdStage · one scene
— LIVRPS · The Six Composition Arcs — strength order, top to bottom
L.
local
Direct opinions on this layer. The strongest. Whatever the artist just typed wins.
I.
inherits
Class-style inheritance. A prim borrows opinions from a base class — like inheriting from a template.
V.
variants
Switchable opinion sets. { coat: red | blue | none } — flip a flag, the variant active changes.
R.
references
Bring in another asset. The classic asset-assembly arc. Always loaded.
P.
payloads
References that are deferred. Only loaded when explicitly demanded — for heavy geometry.
S.
specializes
The weakest arc. A "default" — only wins if nothing stronger says otherwise. The bedrock.
— L · I · V · R · P · S — read it as a sentence: the strongest layer wins.

PRIM HIERARCHY — /World/Characters/Alice

/
├── World
│ ├── Characters
│ │ ├── Alicepayload
│ │ │ ├── Skeleton
│ │ │ ├── Mesh
│ │ │ └── UsdShade.Material
│ │ └── Bobreference
│ ├── Environmentreference
│ └── Cameras
│ └── shot_cam
└── Renderspecialize
    └── UsdRender.Settings

WHAT THE STAGE IS — composed, not stored

The hierarchy on the left does not exist as a file. It is the composed result of every layer in the stratigraphy contributing opinions and the engine resolving them by precedence. Save the stage to disk and you do not get one file — you get a tree of .usda files referencing each other.

Edit any weaker layer and the composition recomputes; every prim that depended on it changes shape. Edit the strongest layer and your opinion always wins. This is the lingua franca of modern animation production.

USD does not save a scene. It saves a recipe — eight layers of overlapping opinions, and the engine reads them in a strict order to produce one stage. The strongest layer wins. The weakest is the bedrock. The composition is what you see.

Every renderer in this stack ultimately reads the same .usda. The format is the lingua franca; the stage is the conversation.

Authorship & Provenance colophon — Pixar Tech Memo TM-92
Authorship
Pixar Animation Studios
internal pipeline tool · 2012
open-sourced as OpenUSD · 2016
Sebastian Grassia · F. Sebastian Grassia · al.
Architecture
C++ core · Python bindings
Sdf · Usd · UsdShade · UsdSkel · UsdGeom
Hydra · render abstraction
~2M LOC · Apache 2.0
Governance
Alliance for OpenUSD (AOUSD)
Linux Foundation · Aug 2023
Pixar · Adobe · Apple · Autodesk · NVIDIA
aousd.org

— Lineage · the format becomes the standard —

2003Pixar's Tidscene (predecessor) — original layered scene description for in-house production.
2012USD development begins internally at Pixar. Tech Memo TM-92 outlines the composition model.
2016OpenUSD released — Apache 2.0. Pixar's Toy Story 4 ships on it.
2017Hydra render abstraction matures. Storm (OpenGL) and Karma become Hydra delegates.
2019USDZ ships in iOS — Apple makes USD the AR file format. Universal in a different sense.
2021Maya, Houdini, Cinema 4D, Unreal, Omniverse all consume USD natively.
2023Alliance for OpenUSD (AOUSD) — Linux Foundation · Pixar, Adobe, Apple, Autodesk, NVIDIA. The standard graduates.
∞The lingua franca. Every other specimen in this stack ultimately reads the same .usda.

the strongest layer wins.

USD does not save a scene. It saves the recipe for one. The stage is what remains.

— § I · Authoring · Standards / Scene Description —
Plate XXXIX · MMXXVI
projectlavos-bauhaus · 039