feat(loader): support sparse accessors

[RaananW]

Summary

Implements glTF 2.0 sparse accessor resolution in the Lite glTF loader. Fixes the SimpleSparseAccessor model (from cx20/gltf-test), which previously rendered flat instead of bent. (model #1 of 5 parallel glTF-feature PRs)

Spec

A sparse accessor stores a base set of values — either the referenced bufferView, or an all-zero array when bufferView is absent — plus a compact list of (index, value) overrides. Per spec, resolution starts from the base and then overwrites sparse.count elements: the element at position sparse.indices[i] is replaced by sparse.values[i].

• sparse.indices has its own bufferView + unsigned componentType (UBYTE / USHORT / UINT).
• sparse.values has its own bufferView and shares the accessor's componentType.

Previously resolveAccessor zero-filled bufferView-less accessors and never applied the substitution, so sparse geometry rendered wrong.

Implementation

Rather than teaching the synchronous resolveAccessor (and its many consumers — geometry, animation, morph, skeleton, instancing) about sparse, this adds a preParse feature (gltf-feature-sparse.ts) that materializes each sparse accessor before any accessor is read:

1. Allocate a tightly-packed region appended to the binary chunk.
2. Fill the base — copy the referenced bufferView (honoring byteStride), or leave zeros when absent.
3. Apply the sparse overrides (indices[i] → values[i]), preserving the accessor's componentType byte-for-byte (normalized accessors are denormalized later, downstream, by their consumer).
4. Rebind the accessor onto the new complete bufferView ({ buffer: 0, byteOffset, byteLength }) and delete a.sparse.

Downstream, the accessor looks like any other plain bufferView-backed accessor, so no other loader code changes. This mirrors the existing gltf-ext-quantization.ts preParse pattern. Feature order is meshopt → sparse → quantization, so a sparse base can read decompressed data and quantization still dequantizes the materialized result.

Malformed input is rejected with clear errors: unsupported accessor/index componentType, a missing required indices.componentType, and any out-of-range sparse index (target >= accessor.count) all throw instead of producing NaN offsets or silently corrupting an adjacent materialized accessor.

Tree-shaking

Per GUIDANCE.md §4c′ (always-use-extensions / lazy features):

• The sparse module is dynamic-imported only when an accessor actually carries a .sparse property — registered with an inline predicate in gltf-feature-registry.ts.
• The assetUsesGltfFeatures gate in load-gltf.ts is widened (/extras|sparse/) so sparse-only assets (like SimpleSparseAccessor, which uses no extensions) still load the registry.
• Non-sparse assets pay zero bytes and zero runtime cost; the common accessor hot path is unchanged.

Validation

• ✅ SimpleSparseAccessor renders pixel-perfect vs a Babylon.js oracle (Playwright, WebGPU/Metal). Throwaway probes deleted.
• ✅ Parity scenes: 214 passed / 0 regressions. The single local failure is scene116 — a pre-existing, documented shadow-depth RTT flake that is skipParityOnCI: true and does not use the glTF loader.
• ✅ Bundle-size: 207 passed / 0 failed, verified via exact-byte comparison (scene12 checked specifically, not rounded KB).
• ✅ tsc + ESLint clean.

Bundle-size ceilings — combined sparse+IBL values (user-approved) ✅

Registering a lazy feature adds ~0.1 KB to the shared, per-scene gltf-feature-registry detection chunk, which every glTF scene loads. To keep the PR4 (sparse) → PR5 (IBL) merge stack conflict-free and avoid a mid-stack CI failure, scene-config.json is aligned to the final combined ceilings — identical to PR5 — covering both this PR's sparse entry and PR5's IBL entry (no extra headroom beyond that):

Scene	Master	Ceiling (combined)
12 — PBR Shader Balls	~103.44 KB	103.6 KB
41 — Physics Shape Debug Viewer	~93.2 KB	93.4 KB
47 — Physics Heightfield	~92.5 KB	92.7 KB

On this branch alone all three already pass exact-byte comparison; the combined values leave room for PR5's registry entry so the shared scene-config.json never conflicts across the stack. The growth is purely the shared registry chunk (one predicate + dynamic-import thunk per feature); the sparse module itself is a separate lazy chunk that no committed scene loads. No golden reference changes.

Files

• new packages/babylon-lite/src/loader-gltf/gltf-feature-sparse.ts
• packages/babylon-lite/src/loader-gltf/gltf-feature-registry.ts — lazy registration
• packages/babylon-lite/src/loader-gltf/load-gltf.ts — widen feature-detection gate
• scene-config.json — combined sparse+IBL ceilings for scenes 12/41/47 (matches PR5)
• lab/public/bundle/manifest/*.json — regenerated (56 scenes)

[Copilot]

Pull request overview

Adds glTF 2.0 sparse accessor support to the Babylon Lite glTF loader by materializing sparse accessors during the loader’s preParse phase, so downstream accessor consumers can remain unchanged.

Changes:

• Introduce a new lazy-loaded glTF feature module that resolves sparse accessors by appending tightly-packed materialized data to the binary chunk.
• Register the sparse feature in the glTF feature registry (ordered between meshopt decompression and quantization).
• Expand the “features needed” gate and regenerate per-scene bundle manifests to reflect the new registry chunk hash/size.

Reviewed changes

Copilot reviewed 59 out of 59 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
packages/babylon-lite/src/loader-gltf/load-gltf.ts	Broadens the feature-registry import gate to include sparse-only assets.
packages/babylon-lite/src/loader-gltf/gltf-feature-sparse.ts	New preParse feature that materializes sparse accessors into appended binary data and rewrites accessors to point at it.
packages/babylon-lite/src/loader-gltf/gltf-feature-registry.ts	Registers the sparse feature with a JSON predicate and positions it in the pre-parse order.
lab/public/bundle/manifest/scene99.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene73.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene7.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene5.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene47.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene41.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene39.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene37.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene35.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene34.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene33.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene32.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene31.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene30.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene29.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene28.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene27.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene260.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene257.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene255.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene254.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene253.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene251.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene247.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene246.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene245.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene244.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene243.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene242.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene241.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene240.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene219.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene218.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene212.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene211.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene210.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene178.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene176.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene175.json	Updates runtime chunk hashes after rebuild.
lab/public/bundle/manifest/scene174.json	Updates runtime chunk hashes after rebuild.
lab/public/bundle/manifest/scene171.json	Updates runtime chunk hashes after rebuild.
lab/public/bundle/manifest/scene164.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene158.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene157.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene152.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene149.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene148.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene147.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene146.json	Updates measured gzip size after rebuild.
lab/public/bundle/manifest/scene144.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene12.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene115.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene112.json	Updates runtime chunk hashes after rebuild.
lab/public/bundle/manifest/scene11.json	Updates measured bundle size and registry chunk hash after feature-registry changes.
lab/public/bundle/manifest/scene105.json	Updates runtime chunk hashes after rebuild.
lab/public/bundle/manifest/scene104.json	Updates measured bundle size and registry chunk hash after feature-registry changes.

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[bjsplat]

Bundle Size Changes

Increases

Package	Current	Master	Change
Scene 12 — PBR Shader Balls scene12	104 KB	103 KB	+1 KB
Scene 28 — Clearcoat glTF scene28	87 KB	86 KB	+1 KB
Scene 144 — Bloom Post-process scene144	111 KB	110 KB	+1 KB
Scene 157 — glTF Weighted Animation Blend scene157	96 KB	95 KB	+1 KB

Sizes rounded to nearest KB. Run pnpm build:bundle-scenes locally to verify.

[bjsplat]

Bundle Size Changes

Increases

Package	Current	Master	Change
Scene 12 — PBR Shader Balls scene12	104 KB	103 KB	+1 KB
Scene 28 — Clearcoat glTF scene28	87 KB	86 KB	+1 KB
Scene 144 — Bloom Post-process scene144	111 KB	110 KB	+1 KB
Scene 157 — glTF Weighted Animation Blend scene157	96 KB	95 KB	+1 KB

Sizes rounded to nearest KB. Run pnpm build:bundle-scenes locally to verify.

[bjsplat]

Bundle Size Changes

Increases

Package	Current	Master	Change
Scene 32 — KHR_materials_unlit scene32	80 KB	79 KB	+1 KB
Scene 99 — Bone Control scene99	92 KB	91 KB	+1 KB
Scene 158 — Additive Animation Blend scene158	96 KB	95 KB	+1 KB
Scene 253 — AnimateAllTheThings scene253	153 KB	152 KB	+1 KB
Scene 258 — Interleaved UV scene258	83 KB	82 KB	+1 KB

Sizes rounded to nearest KB. Run pnpm build:bundle-scenes locally to verify.

[bjsplat]

Bundle Size Changes

Increases

Package	Current	Master	Change
Scene 5 — Alien Skeleton scene5	92 KB	91 KB	+1 KB
Scene 32 — KHR_materials_unlit scene32	80 KB	79 KB	+1 KB
Scene 99 — Bone Control scene99	92 KB	91 KB	+1 KB
Scene 47 — Physics Heightfield scene47	92 KB	91 KB	+1 KB
Scene 158 — Additive Animation Blend scene158	96 KB	95 KB	+1 KB
Scene 253 — AnimateAllTheThings scene253	153 KB	152 KB	+1 KB

Sizes rounded to nearest KB. Run pnpm build:bundle-scenes locally to verify.

[bjsplat]

Lab - Static Site

Open deployed site

_{Build 20260710.16 - merge @ d6494ca}

[bjsplat]

Bundle Size Changes

Increases

Package	Current	Master	Change
Scene 47 — Physics Heightfield scene47	93 KB	92 KB	+1 KB
Scene 104 — Physics Character Controller scene104	102 KB	101 KB	+1 KB
Scene 115 — Alien Picking Frame 100 scene115	127 KB	126 KB	+1 KB
Scene 254 — Signed Accessor Animation scene254	93 KB	92 KB	+1 KB

Sizes rounded to nearest KB. Run pnpm build:bundle-scenes locally to verify.