Streamed SOG — PlayCanvas LOD format

Streamed SOG splits a Gaussian splat scene into spatial chunks at multiple levels of

Source: https://developer.playcanvas.com/user-manual/gaussian-splatting/formats/streamed-sog/ Version covered: 1

Streamed SOG splits a Gaussian splat scene into spatial chunks at multiple levels of detail (LOD). A viewer walks a binary spatial tree to decide which chunks/levels to load for the current camera, so very large scenes (tens of millions of Gaussians) stream progressively. Authored with SplatTransform (--generate-lod / "Generating Streamed SOG"). Each chunk is a standard unbundled SOG.

This is the structure sog-to-3dtiles consumes — it maps almost 1:1 onto an OGC 3D Tiles HLOD tree (each node already carries an AABB; leaves carry per-LOD splat ranges).

File set

scene/
├── lod-meta.json     # index: scene info + spatial tree (THE entry point)
├── 0_0/  meta.json + *.webp   # LOD 0 (finest), chunk 0 — unbundled SOG
├── 1_0/  …                    # LOD 1 (coarser), chunk 0
├── …                          # one dir per {lod}_{chunk}
└── env/  meta.json + *.webp   # optional environment (far-field, e.g. sky)

The index is always lod-meta.json — loaders identify the format by this filename.
Resolve chunk locations through the filenames array, not the {lod}_{chunk} naming.
All paths in lod-meta.json are relative to its directory.

lod-meta.json

interface LodMeta {
  version: 1;
  asset?:   { generator?: string };
  count:    number;     // total gaussians across all LODs (excludes environment)
  counts:   number[];   // gaussians per LOD level (index = level)
  lodLevels: number;
  environment?: string; // relative path to env SOG meta.json; omitted/null if none
  filenames: string[];  // chunk SOG meta.json paths, referenced by index
  tree: Node;
}
interface Node {
  bound: { min:[number,number,number]; max:[number,number,number] };  // AABB
  children?: [Node, Node];   // interior node — exactly two
  lods?: {                   // leaf node — map of LOD level → splat range
    [level: string]: { file: number; offset: number; count: number };
  };
}

Spatial tree

Binary subdivision. A node is interior (children, exactly 2) or leaf (lods) — never both.
Leaf bound encloses each Gaussian's rotated, scaled ellipsoid extent (not just centers).
Interior bound = union of children. Bounds are in the same frame as the chunk splat positions.

LOD levels

Level 0 = highest detail; higher levels are progressively coarser.
A leaf's lods is keyed by decimal strings "0".."lodLevels-1"; a missing key = no splats at that level.
All levels of a leaf cover the same region — a viewer picks exactly one per leaf (e.g. by distance).

Chunk references

file indexes filenames. offset/count select splats [offset, offset+count) in the chunk's storage order (splat indices, not bytes; pixel (i%W, floor(i/W))).
A chunk's contents = concatenation of the leaf runs that reference it (non-overlapping, complete). Within a run, splats are Morton-sorted. A chunk only ever holds splats of a single LOD level.

Environment

Optional environment → unbundled SOG of far-field splats outside the LOD scheme. A viewer should render it unconditionally.

Precision & versioning

Non-integer lod-meta.json numbers are quantized to 7 significant digits (~float32).
version: 1; reject greater major versions. Pre-versioned files omit version/asset/count/counts and may have "environment": null. Ignore unknown fields.

Mapping to 3D Tiles (this repo's `sog-to-3dtiles`)

Interior SOG node → empty 3D Tiles tile (boundingVolume from bound, children = its two subtrees).
Leaf SOG node → a vertical chain of tiles, coarsest LOD (highest level) as ancestor down to level 0 (finest), refine: REPLACE, each tile's content = the decoded [offset,count) run from its chunk SOG, re-encoded as an SPZ-v2 GLB (KHR_gaussian_splatting + _compression_spz_2).
Bounds are emitted in the Z-up tileset frame (SOG is Y-up) — same RX(+90°) convention as the RAD path. geometricError decreases with LOD level so a viewer refines the chain by distance.

References

Streamed SOG spec: https://developer.playcanvas.com/user-manual/gaussian-splatting/formats/streamed-sog/
SOG chunk format: sog.md
SplatTransform: https://github.com/playcanvas/splat-transform

Streamed SOG — PlayCanvas LOD format

On this page