WAI Extension: Avatars (Digital Humans)
Mirrored from the canonical text at commit 117bad22 ().
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
worldfeature ofwai-rs(simwai.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
(with its acquisition class as an optional signed label,
energy-measurement §2.8);
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.