open-standards/wai/extensions/avatars.md

WAI Extension: Avatars (Digital Humans)

Status: Draft. Phase 4 of the cargo roadmap. Motion as instructions: ship a signed stream of joint transforms — a skeleton, not a video of a body — and the sink skins a mesh by the deterministic pose. An avatar is a skeletal world, so conformance is pose-equivalence, a special case of replay-equivalence. Reference impl: the world feature of wai-rs (sim wai.world.sim.skeletal0); corpus: avatar-conformance/; live sink: wai.transaction.science/avatar. Keywords MUST, MUST NOT, SHOULD, MAY are RFC 2119/8174.

1. Scope and model

A wai.avatar.pose object is a skeletal world: the same WWLD container, op log, canonical ordering, causality, sealing, state hash, and receipt chain as interactive-worlds (see interactive-worlds.md), with the simulation step reduced to the identity (as with state-feeds). The bones are nodes; an animation / expression op stream sets joint transforms; nothing is integrated. A telepresence stream is a live world of one avatar — the op log that drives a multiplayer world drives a body.

In scope: the bone model, the registered skeletal sim (wai.world.sim.skeletal0) and its mutation kinds, the statement that conformance is worlds' replay-equivalence over the pose, and the binding to a wai.mesh.glb for skinning.

Out of scope: everything interactive-worlds already defines (normative here unchanged); the skinning algorithm and forward-kinematics composition (sink presentation — a pure function of the hashed pose + the hierarchy); a transport; blend-shape / morph-target semantics beyond joint transforms (a later registration may add them).

Relationship to the core spec: a registration under interactive-worlds, not a new container. media takes the value "avatar"; the authoritative capability is wai.avatar.pose, carried in a WWLD whose contract names sim = wai.world.sim.skeletal0. The avatar's mesh is a content-addressed wai.mesh.glb asset (its armature + skin weights), bound to the rig like any other presentation asset.

2. The bone

A node is a bone: a parent reference plus a local TRS transform. Canonical field order (and therefore hash order) is exactly:

field	type	meaning
parent	[u8; 16]	parent bone's node id, or all-zero for a root
translation	[Fx; 3]	local translation (Q32.32)
rotation	[Fx; 4]	local rotation quaternion (x, y, z, w)
scale	[Fx; 3]	local scale

The canonical node serialization is parent(16) | translation | rotation | scale = 16 + 10×i64 = 96 bytes. The default bone is the identity (zero translation, identity quaternion, unit scale).

The floor stays trig-free: a rotation is carried as a quaternion, and the quaternion → rotation-matrix the sink uses for skinning is + − × only. Axis-angle authoring (which needs sin/cos) happens once, off-line, in whatever produces the op bytes — the canonical artifact is the Fx quaternion, and every sink replays it exactly.

3. The skeletal sim — wai.world.sim.skeletal0

Registers these mutation kinds; the step is the identity. All are LWW (a transform set rides the canonical total order — last canonical writer wins; there is no commutative transform set, so no SUM kind).

kind	payload	effect
0x0001 CREATE	parent[16] | translation[3] | rotation[4] | scale[3]	insert a bone; target MUST equal mint_node_id(actor, lamport)
0x0002 DELETE	(empty)	remove the bone
0x0010 SET_ROTATION	rotation[4]	set a joint's local rotation (the animation driver)
0x0011 SET_TRANSLATION	translation[3]	set a bone's local translation
0x0012 SET_SCALE	scale[3]	set a bone's local scale

An op targeting an absent bone is a deterministic no-op (interactive-worlds §6.4). parent need not name a present bone — a missing parent is treated as a root at FK time; the hash depends only on the stored values, so existence is irrelevant to conformance.

4. Conformance — pose-equivalence

Given the same avatar container and the same signed op set, a conforming sink MUST compute the identical BLAKE3 state hash at every checkpoint — the per-bone local transforms, in node-id order — on every machine, no tolerance parameter.

This is interactive-worlds' criterion verbatim, applied to the bone state. The forward-kinematics composition into world space and the skinning of the mesh produce identical results on any two sinks that agree on the pose (a pure function of the hashed locals + the hierarchy), so they are legitimately left to presentation and excluded from the hash — exactly as wai.mesh.glb geometry is.

5. Receipt (JWP profile)

The interactive-worlds session receipt (jwp-receipts.md) applies unchanged: world identity over contract + tick-0 graph; the group's objects are the op batches, Merkle-bound; one Ed25519 seal over world hash + root + checkpoints + exact signed work + carried joules_micro; parent_receipt_hash chains a performance's seals into a timeline.

Appendix

An avatar is the highest-bandwidth capture reduced to its instructions: a body in motion is a rig plus a stream of joint angles, and the standard carries the rig and the angles, not the pixels. Pose-equivalence makes the motion auditable — two parties to a call hold the same signed skeleton — while the appearance stays the sink's, skinned from a shared mesh. Sight (worlds), sound (mixdown), touch (sample), live state (feed), and now a moving body (pose) are one floor, one verification.