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Every Ed25519 signature WAI defines is verified strictly, in order: a canonical key, a key not of small order, S < L, R not of small order, and the cofactorless equation with R compared byte for byte. OER/2's chain signature keeps the rule oer-receipts §5 settles. jwp-receipts §3.1 step 3, http-delivery's playlist receipt and packaging's receipt cite it instead of restating it; the worlds, avatars, feeds, spatial-audio, haptics, volumetric, score, film, provenance, ternary and quantum receipt texts cite it where they name their signature. CHANGELOG records the sweep. Co-Authored-By: Claude Opus 5.5 <[email protected]>
116 lines
5.1 KiB
Markdown
116 lines
5.1 KiB
Markdown
# WAI Extension: Volumetric / 4D Splats
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> Status: Draft. Phase 5 of the cargo roadmap (the frontier). A scene
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> that is volumetric *and* moves: ship a Gaussian cloud as per-Gaussian
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> keyframed tracks, the sink interpolates the cloud at any frame and
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> renders it with the EWA pipeline. Conformance is
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> **frame-state-equivalence**. Reference impl: the `splat4d` feature of
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> `wai-rs`; corpus: `volumetric-conformance/`; live sink:
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> wai.transaction.science/volumetric.
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Keywords **MUST**, **MUST NOT**, **SHOULD**, **MAY** are RFC 2119/8174.
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## 1. Scope and model
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A `wai.splat.4d` object is a **temporal Gaussian splat**: the static
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splat (`wai.splat.spz`) is a single anisotropic-Gaussian cloud; this
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extension gives each Gaussian a keyframed track — position, scale,
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rotation, color, opacity over time — and the sink reconstructs the cloud
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at any frame by interpolating the tracks in the `wai.det.fixed64` floor.
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It is lever 3 (instructions-at-the-sink) for volumetric capture: the wire
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carries the trajectories, the sink reconstructs the frame.
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In scope: the container, the per-Gaussian keyframe model, the
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deterministic reconstruct contract, the **frame-state-equivalence**
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conformance criterion, and the receipt profile.
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Out of scope: a transport; the render (EWA splatting, depth sort, alpha
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blend — the sink's, exactly as the static splat's rasterization is); a
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codec for the cloud bytes (a real capture is authored by an upstream
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3DGS/4DGS pipeline; this extension carries and verifies its trajectories,
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it does not fit Gaussians to video).
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Relationship to the core spec: mirrors haptics / spatial audio. The
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reconstruct runs in the `wai.det.fixed64` floor; `media` takes the
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informational value `"volumetric"`; `wai.splat.4d` is authoritative.
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## 2. Container (`W4DS`)
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```
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"W4DS" | u16 n_sections | section table (u8 kind, u32 off, u32 len) | sections
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```
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| kind | section | required |
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|---|---|---|
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| `0x01` | contract (canonical JSON) | REQUIRED |
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| `0x02` | Gaussian tracks | REQUIRED |
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### 2.1 Contract
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```json
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{ "frame_rate": 30, "n_frames": 13, "n_gaussians": 2,
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"numeric": "wai.det.fixed64" }
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```
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### 2.2 Gaussian tracks
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`n_gaussians u32`, then per Gaussian: `n_keys u16`, then
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`n × ( frame u64 | pos[3] | scale[3] | rot[4] | color[3] | opacity )`,
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the 14 attributes as `i64` Fx in that field order. Each track MUST be
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ascending by `frame`. `color` and `opacity` are in `[0, 1]`; `rot` is a
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quaternion `(x, y, z, w)`.
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## 3. Reconstruct contract
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For each Gaussian, at frame `f`: each of the 14 attributes is the `Fx`
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linear interpolation of the track's keyframes at `f`; before the first /
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after the last key the endpoint holds. The canonical reconstruction of a
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frame is `n_gaussians u32 | per-Gaussian 14×i64` in the field order
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above. The floor stays trig-free — rotations interpolate component-wise
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(nlerp without renormalization, deterministic); the sink normalizes the
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quaternion for rendering, which is presentation.
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## 4. Conformance — frame-state-equivalence
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> Given the same `wai.splat.4d`, a conforming sink MUST reconstruct the
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> identical Gaussian set at every frame, hence the identical BLAKE3 — on
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> every machine, no tolerance parameter.
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```
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frame_hash = BLAKE3("wai:splat4d-frame\x01" || frame_le || canonical frame bytes)
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clip_hash = BLAKE3("wai:splat4d-clip\x01" || contract_bytes || tracks)
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```
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`frame_hash` is the temporal counterpart of the static splat's content
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hash; `clip_hash` is the frame-independent identity.
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## 5. Fidelity and size (honest)
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A splat **approximates** a captured scene, so `wai.splat.4d` is tiered
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`Lossy` like `wai.splat.spz` — even though the per-frame reconstruction
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from the tracks is *frame-state-exact*. Volumetric is **heavy**: a real
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cloud is megabytes per frame. The win here is the same as the static
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splat — content-addressing, dedup, one signed envelope, and a verifiable
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per-frame state — **not** payload reduction. We state that plainly rather
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than imply a compression claim the format does not make.
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## 6. Receipt (JWP profile)
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`clip_hash` is the identity; the group's objects are per-frame
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reconstruction blocks (`frame_hash` per frame), Merkle-bound; one
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Ed25519 signature (verified as SPEC §7 *Signatures* requires) over clip_hash + root + `work` (= n_frames ×
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n_gaussians) + carried `joules_micro`; `parent_receipt_hash` chains — the
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worlds / audio / haptics receipt shape reused. The figure's acquisition class rides beside it as an optional signed label
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([energy-measurement](energy-measurement.md) §2.8): absent, the receipt keeps
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its legacy bytes and the figure is unlabelled; present, it is inside the
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signature. The reference meter labels every figure it measures
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`OnChipCounter`, with its declared uncertainty.
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## Appendix
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Frame-state-equivalence is sample-equivalence with the haptic sample
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replaced by a whole Gaussian cloud, and the static splat extended to
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time. The same fixed-point floor that makes a world replay, a mix sum, an
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actuator buzz, a feed converge, and a body pose now makes a volumetric
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frame reconstruct — one floor, six cargo classes, one verification. The
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representation is heavy and approximate; the *reconstruction* of it is
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exact and auditable, and that is the honest thing the standard adds.
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