I3S — Esri Indexed 3D Scene Layer

Status: 🟢 live /stream adapter (packages/tile-server/i3s-live.js, loaders.gl-based) + 🟡 offline draft converter i3s-to-3dtiles/. Validated on Buildings_NewYork_v18 (3DObject v1.8). Ref: https://github.com/esri/i3s-spec (OGC Community Standard), docs/1.8.

Live streaming (/stream?format=i3s · i3s-live.js)

A "v2" adapter built on the loaders.gl SDK (@loaders.gl/i3s SLPK archive + scene-layer/nodepage parsing) that reuses this repo's I3S geometry decoder + ENU placement:

• tileset.json — read 3dSceneLayer.json + all nodepages (node OBBs + children + mesh refs). Each tile's box comes from the OBB's enclosing AABB in a shared ENU frame — no geometry decode. One ENU→ECEF root transform; content URIs route to /i3s/tile/<nodeIndex>.glb.
• tile/<i>.glb — fetch ONE node's uncompressed DefaultGeometrySchema buffer, decode it, place vertices in the ENU frame, build a glTF (TRIANGLES / POINTS) on demand.

Two source modes, auto-detected: REST / extracted-SLPK dir (…/3dSceneLayer.json[.gz], fetched file-by-file — best for multi-GB layers like NYC, never pulls the whole dataset) and SLPK archive (….slpk, fetched once and read via loaders.gl). Scope: uncompressed geometry only (Draco/LEPCC gated out), no textures yet — same subset as the offline draft.

Gist

I3S is Esri's streaming format for large 3D scene layers. Distributed as a REST service or a single-file SLPK (Scene Layer Package — a plain zip; resources inside are often gzip'd). It is the closest "other tiled 3D" analogue to OGC 3D Tiles — both are OGC standards solving the same streaming-LoD problem with different document layouts.

Layer types (what I3S can carry)

• IntegratedMesh — photogrammetry / reality mesh (one watertight textured mesh, tiled).
• 3DObject — discrete textured meshes with attributes (buildings, features).
• PointCloud (PCSL) — lidar; its own per-node binary encoding (not DefaultGeometrySchema).
• Point — places symbol/model instances (billboards or 3D models) at point features.
• Building (BSL) — a composite of sublayers (the "mix of types" case).

So: meshes ✓, point clouds ✓, instances ✓ (Point layers), and mixed ✓ (Building SL = multiple sublayers). A single scene layer is one type; heterogeneity comes from BSL or multiple layers.

Tiling & hierarchy — always explicit

• A node tree: internal nodes reference children by array index. I3S has no implicit/template tiling (nothing like 3D Tiles 3DTILES_implicit_tiling subtrees) — it is always an explicit tree.
• Modern I3S (≥1.7) uses node pages (nodepages/<id>.json, nodesPerPage entries each) for compact, range-friendly index access. Older layers used per-node 3dNodeIndexDocument.
• Refine: REPLACE-style — a node's children supersede it; selection driven by lodThreshold.
• HTTP: REST + range-friendly by design; SLPK packs the same layout into a zip.

Key on-disk structures (1.8)

• 3dSceneLayer.json — layerType, store (version, index.nodesPerPage, defaultGeometrySchema), spatialReference (wkid/latestWkid, optional vcsWkid), geometryDefinitions (where Draco shows up, under geometryBuffers[].compressedAttributes).
• nodePage entry: { index, children:[idx…], obb, mesh?, lodThreshold }.
  • obb = { center:[x,y,z], halfSize:[3], quaternion:[x,y,z,w] }. center is in the layer CRS (global ⇒ WGS84 lon°,lat°,h); the quaternion orients an ECEF-aligned box.
  • mesh = { geometry:{definition, resource, vertexCount, featureCount}, material:{…}, attribute:{…} }.
• DefaultGeometrySchema (uncompressed): topology: PerAttributeArray → sequential arrays, NOT interleaved. Header vertexCount u32, featureCount u32, then in ordering: position f32[3·V] · normal f32[3·V] · uv0 f32[2·V] · color u8[4·V], then per-feature id u64[F] · faceRange u32[2·F]. geometryType: triangles.

→ 3D Tiles mapping (what the draft does)

• node tree → tileset tree (children→children, REPLACE); lodThreshold/OBB diagonal → geometricError.
• OBB → 3D Tiles box: geodetic center → ECEF; rotate unit axes by quaternion, scale by halfSize. (Verified on synthetic input: ECEF(0,0,0)=(6378137,0,0); axes scale correctly.)
• mesh DefaultGeometrySchema → glTF TRIANGLES (POSITION[/NORMAL/COLOR_0]); PointCloud → POINTS.
• vertices placed via a tile transform translating to the OBB-centre ECEF.

Why still a draft (unvalidated / out of scope)

No SLPK sample to test against, and these are gated out with warnings: Draco geometry, textures/ materials, per-feature attributes (→ would need EXT_mesh_features/3D Tiles metadata), legacy 3dNodeIndexDocument index, Point/Building layer types, and non-WGS84 / vertical-CRS nuance. The exact resource path layout, the vertex-origin convention, and projected-CRS OBB handling all need confirmation against real data. Full-fidelity I3S overlaps mature tooling (loaders.gl, Cesium ion).