> For the complete documentation index, see [llms.txt](https://mims.oascities.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mims.oascities.org/mims-specification-v9.0/citiverse/mim9-citiverse/mechanisms.md). # Mechanisms ### Mechanisms These mechanisms are under active development. Each mechanism lists the requirements it serves and the standards that can realise them; the status of every standard is listed in the next section. #### M1: Scene description and 3D content formats Serves R1.1 to R1.6, R4.2 and R4.5. The core principle is a split between authoring and delivery: compose and edit worlds in an authoring interchange format, and deliver them in a runtime format. Khronos positions glTF 2.1 explicitly as complementary to OpenUSD in this way, and an OpenUSD-to-3D Tiles conversion path already exists. | Role in the Virtual World | Preferred mechanism | Alternatives | | ------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------ | --------------------------------------------------------------- | | Authoring and composition (layers, references, non-destructive overrides) | OpenUSD (AOUSD Core Specification 1.0) | X3D (ISO/IEC 19775) | | Runtime delivery of assets and composed scenes | glTF 2.0 (ISO/IEC 12113); glTF 2.1 once final | MPEG-I Scene Description (ISO/IEC 23090-14), which extends glTF | | Massive geospatial streaming (terrain, city, photogrammetry) | OGC 3D Tiles 1.1; 3D Tiles 2.0 once final | OGC I3S | | City models | CityGML 3.0, CityJSON 2.0 (see MIM7) | GML | | Buildings and infrastructure (BIM) | IFC 4.3 (ISO 16739-1) | Native CAD via conversion, documented per R1.6 | | Reality capture | KHR\_gaussian\_splatting (glTF); UsdVolParticleField3DGaussianSplat (OpenUSD); point clouds as 3D Tiles or LAS/LAZ | SPZ compression extension | | Materials | glTF PBR material model | MaterialX / OpenPBR | #### M2: Binding scene objects to data Serves R2.1 to R2.5. The pattern is: every scene object carries a stable identifier; the identifier resolves to an entity in the data ecosystem; live values are fetched or subscribed at runtime. * Identifiers in the scene: glTF EXT\_mesh\_features and glTF 2.1 Unique IDs; OpenUSD prim paths and custom attributes; IFC GlobalId, which conversions SHALL preserve. * Per-object attributes: glTF EXT\_structural\_metadata (3D Metadata Specification); USD schemas; IFC property sets. * Live and historical data: NGSI-LD API with Smart Data Models (see MIM0, MIM2); OGC SensorThings API; OGC API Features; OGC API Processes for LDT model runs (see MIM8 M3). * Temporal navigation: timestamps per MIM2; time-sampled attributes in OpenUSD. #### M3: Collaboration and shared state Serves R3.1 to R3.7. * Non-destructive collaborative editing: OpenUSD layers and overrides, with each party writing to its own layer. * Pose of users, devices and objects: OGC GeoPose 1.0. * Identity and authorisation: OpenID Connect, W3C Verifiable Credentials and Decentralised Identifiers (see MIM6). * BIM issues and annotations: buildingSMART BCF (file and API). * State and event messaging: WebSocket or WebRTC data channels for session state; NGSI-LD subscriptions or MQTT for data events. * Spatial web (emerging): IEEE 2874-2025 with Spatial Web Identifiers (SWIDs), HSML for modelling and HSTP for transactions. * AI agents and avatars: MCP-style tool interfaces to query scene context (see MIM8 M3); OpenUSD scene graphs as structured context for language models, as proposed by the CU project. No open standard exists yet for real-time synchronisation of a shared scene between different engines. See the open issues. #### M4: Portable rendering Serves R4.1 to R4.6. * XR device access: Khronos OpenXR for native applications; W3C WebXR Device API for browsers. * Browser rendering: WebGPU, with WebGL 2 as fallback. * Level of detail and tiling: hierarchical level of detail and implicit tiling in 3D Tiles; bounding volume hierarchies and external assets in glTF 2.1. * Engine portability test: the same source content is loaded in at least two engines, for example one game engine (Unreal Engine, Unity, Omniverse) and one open web client (CesiumJS, three.js, Babylon.js). * Accessibility: EN 301 549 and WCAG 2.2 for all 2D interface elements. #### M5: Delivery and streaming modes Serves R5.1 to R5.6. A Virtual World service declares one or more of these delivery modes. | Delivery mode | Best fit | Mechanisms | Cost profile | | ------------------------------------------ | --------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------- | | Package download | Small scenes, offline use, kiosks | GLB, USDZ | One-off transfer | | Client-side progressive streaming | Public-facing worlds with many users | 3D Tiles and glTF over HTTP/2 or HTTP/3, cached on CDNs | Scales with the number of users at low marginal cost | | Server-side pixel streaming | Highest fidelity on thin devices, limited audiences | WebRTC video (H.264, HEVC, AV1) plus data channel for input | Per-user GPU; the main scalability barrier reported by CU and SENSE | | Split rendering over 5G | Mobile XR and AR glasses | 3GPP TS 26.565 (Rel-18, WebRTC, OpenXR-derived messages); TS 26.567 over IMS (Rel-19); device capabilities in TS 26.119 | Edge GPU, network-managed quality of service | | Scalable real-time distribution (emerging) | Live events and many subscribers to shared state | IETF Media over QUIC Transport (Internet-Draft) over QUIC and WebTransport | Relay-based fan-out | For 5G media streaming of scene-description content, 3GPP TS 26.501 and TS 26.512 apply. 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