Introduction

Formats

See 3D Tiles format for the full writeup of 3D Tiles 1.1 itself (implicit tiling, KHR_gaussian_splatting, REPLACE vs ADD refinement, QUADTREE vs OCTREE).

COPC (Cloud Optimized Point Cloud) — A LAZ 1.4 file with a specific VLR/EVLR layout that embeds an octree hierarchy. Each octree node is a contiguous chunk at a known offset, enabling HTTP range requests without a separate index. Internally organized in the same way as 3D Tiles implicit tiling — a direct 1:1 mapping. CRS is stored as WKT in a VLR (LASF_Projection record 2112). Not all files include it (e.g. the Autzen test file does not), in which case the output is in local dataset coordinates.

RAD (SparkJS Gaussian Splat LoD) — SparkJS binary streaming format for BhattLod Gaussian splat scenes. See structure in the rad-to-3dtiles section of CLI usage, and rad-format-notes.md for the full format writeup (Spark 2.0 LoD system, PackedSplats/ExtSplats byte layouts, geometric error tuning).

References

Local notes (in docs-references/)

The Design log (live-streaming harmonization — lazy hierarchy, the cache = none/hierarchy/full continuum, push/pull, per-format wiring + parallel hierarchy build, plus a running historical record of completed migrations) lives in the Guide, a companion narrative to "Architecture: the materialization continuum" rather than a per-format reference doc.

Upstream specs & reference implementations

glTF extensions (Khronos)

• KHR_gaussian_splatting — PR #2490 — the base extension; 555-comment thread documenting the design (attribute set, namespacing, shape/rendering hints, SH handling, the split from the original single extension). Merged Jan 2026.
• KHR_gaussian_splatting_compression_spz_2 — PR #2531 — the SPZ-compression child extension, pinned to SPZ v2.

CesiumJS implementation

• Issue #12837 — "Updating to latest 3D Gaussian Extensions and deprecating initial experimental version" (deprecation/removal timeline: deprecated Sep 2025, removed Nov 2025 / v1.135).
• PR #12843 — adds support for KHR_gaussian_splatting + KHR_gaussian_splatting_compression_spz_2, deprecates KHR_spz_gaussian_splats_compression. Confirms CesiumJS decodes via the Niantic spz library.

SPZ format & tooling

• nianticlabs/spz — reference C++ SPZ codec. The v2 vs v3 rotation split lives in src/cc/load-spz.cc (packQuaternionFirstThree = v2 / 3-byte, packQuaternionSmallestThree = v3 / 4-byte).
• @sparkjsdev/spark — SparkJS; writes SPZ v3 by default but version-dispatches on read (handles v2 and v3).
• javagl/JSplat — Java reference for writing splat glTFs that CesiumJS reads (GltfSpzSplatWriter); a useful cross-check when updating an SPZ writer.

PlayCanvas SOG / Streamed SOG (used by sog-to-3dtiles)

• playcanvas/splat-transform — SplatTransform: PlayCanvas's splat conversion library/CLI. sog-to-3dtiles uses its readSog (CPU webp.wasm decode) to turn each SOG chunk into a DataTable. Authors Streamed SOG datasets too.
• SOG format spec — local notes: sog.md.
• Streamed SOG (LOD) spec — local notes: streamed-sog.md.

3DTilesRendererJS Gaussian-splat ecosystem (WilliamLiu-1997) — closest reference implementations to this repo's RAD path; studied to improve tiling:

• 3DTilesRendererJS-3DGS-Plugin — the 3d-tiles-rendererjs-3dgs-plugin this viewer uses to render splats in 3DTRJS (parses KHR_gaussian_splatting + _spz_2, renders via SparkJS, one shared Spark renderer per scene/renderer, camera-relative rebasing, sparkRendererOptions, WebXR-aware). Its data/gaussianSplat1 & gaussianSplat2 are wired into this viewer's preset bar (◈ GSplat 1/2). Note its rendering tip: keep the globe in the opaque pass so it doesn't object-sort against transparent splats at the horizon.

• 3DGS-PLY-3DTiles-Converter — PLY→3D Tiles converter (kd-tree tiling, AABB bounds, SPZ v3 + quantized SH). Its src/tiling/geometric-error.js is the model our uniform-per-depth geometric error mirrors: GE(depth) = leafReference × (1/samplingRate)^(maxDepth−depth), anchored to a deepest-leaf "sparsity + extent" estimate, all siblings at a level sharing one error. Each tileset also emits a build_summary.json exposing the full GE-by-depth and sampling schedule — handy for cross-checking. (Issue #24: auto-depth can overload the root node on >8M-splat scenes — a tree-balancing concern worth noting for very large RAD inputs.)

• 3DTiles-Inspector — standalone web inspector for 3D Tiles / Gaussian-splat tilesets (root-transform edit, geometric-error & layer-multiplier scaling 1/16x–16x, splat crop regions, build_summary.json round-trip). A good complement to this repo's viewer.html 🪲 Debug-tiles overlay.

  The inspector is pre-installed in tools-inspector/ — run via npm scripts (no global install needed):

  cd tools-inspector
  npm run inspect:tastier    # tastier500k RAD tileset
  npm run inspect:coit       # coit RAD tileset (large, 778 tiles)
  npm run inspect:coit-s050  # coit σ×0.5 scale variant

  Or run directly from the repo root with npx:

  npx --prefix tools-inspector 3dtiles-inspector sample-data/output/rad/tastier500k-3dtiles-from-rad
  npx --prefix tools-inspector 3dtiles-inspector sample-data/output/rad/coit-3dtiles-from-rad

  Each command starts a localhost HTTP server and opens the inspector in your default browser. Its Geometric Error / Layer Multiplier sliders are the runtime analogue of this converter's uniform-per-depth error model — handy for confirming LoD behaviour without re-tiling.

Splatter (splatter.app, Jakub Červený) — a hosted Gaussian-splat viewer/host (tested against a 103M-splat scene). Its splatter-three library's actual source lives in splatter-app/splatter-three, a private repo — nothing beyond its published, intentionally-public consumer demo is reverse-engineered or reproduced here. From splatter-app/three-demo's main.js (the only public source), the observable public API surface:

• new Splatter(renderer, { splatId }) — content addressed by an opaque ID, loaded/streamed independent of the Three.js scene graph; renderer.render(scene, camera) and splatter.render(camera, controls.target) are called separately per frame.
• Progressive loading via 'update' (redraw on each streamed chunk) and 'loaded'(totalLoaded, numDisplayed) events — their own demo hides a loading spinner once numDisplayed > 1e6, well before totalLoaded finishes, i.e. partial-scene display while still streaming (conceptually close to this repo's own SOG octree LOD, though the wire format itself isn't public).
• splatter.setTransform(matrix) positions the splat cloud independent of the rest of the scene.
• splatter.addUniform(type, name) + splatter.setShaderEffect(glsl) — a small GLSL-injection point operating on per-splat position/scale (their demo uses it for a radial reveal-on-load animation) without needing to fork the splat renderer itself.
• splatter.hitTest(camera, [x, y], { alphaThreshold }) — screen-space picking weighted by actual rendered alpha, not just geometry — used for a recenter-on-double-click interaction.
• Requires an explicit WebGL2 context (canvas.getContext('webgl2', options)), passed into new THREE.WebGLRenderer({ canvas, context }).

Other specs (no dedicated converter row above)

Extension/format specs referenced by the tables above but not themselves a distinct converter target:

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