Reference

Posecode Protocol Specification v0.4

Protocol Version: 0.4 Language Version: 0.4 Document Status: Normative Specification

Posecode is a small text language for describing a single person's kinematic movement so it can be rendered as an animated 3D figure in a web browser.

This document is the normative language and IR contract (protocol specification). The LLM authoring guide (or llm-authoring.md) is an optional, task-oriented aid. It is self-contained, but it MUST NOT define syntax or behavior that differs from this normative specification.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 / BCP 14.

It is to human movement what Mermaid is to diagrams. A human, animation tool, or an LLM writes a compact document; a client-side parser and renderer turn it into a moving mannequin. The source describes semantic movement phases rather than 3D matrices.

• Version keyword: documents declare nothing; this is posecode 0.4.
• Compatibility: v0.4 parsers continue to accept v0.3/v0.2 documents and the v0.1 easing aliases.
• File extension: .posecode
• Compute model: parsing and all 3D math run on the client (Three.js). Authoring may be manual, tool-driven, or LLM-assisted.

Table of Contents

1. Grammar
2. Bones and Joints
3. Actions
4. Configured Range-of-Motion Limits
5. Contact Semantics: ground-lock, reach, pin, and grip
6. Rendering and Kinematic Model
7. Intermediate Representation (IR)

1. Grammar

Posecode is line- and indentation-oriented. Comments start with # or //.

document   = header { directive } ;
header     = "posecode" kind STRING ;
kind       = "exercise" | "stretch" | "posture" ;
directive  = rig | avatar | prop | pose | clip | step | repeat ;
rig        = "rig" "humanoid" ;
avatar     = "avatar" ("avatar1"|"avatar2"|"avatar3") ;
prop       = "prop" ("chair"|"wall"|"bar"|"box"|"dip-bars") ;
pose       = "pose" "start" "=" startPose [ ":" { startOverride } ] ;
startOverride = jointTarget ;                         (* indented; sparse overlay, not a phase *)
startPose  = "neutral"|"standing"|"first-position"|"plank"|"supine"|"prone"|"seated" ;
clip       = "clip" STRING ;                            (* optional mocap clip; renderer may retarget & blend *)
repeat     = "repeat" NUMBER ;
step       = "step" STRING DURATION timingMode ":" { child } ;
timingMode = "flow" | "settle" | "drive" | "snap" | "linear" ;
child      = jointTarget | groundLock | reach | pin | grip | turn | travel | cue ;
jointTarget= joint ":" action [ NUMBER ] ;
groundLock = "ground-lock" ":" floorContact { "," floorContact } ;
floorContact = "hands" | "forearms" | "feet" | "back"
             | ( "hand" | "elbow" | "foot" ) "_" ( "left" | "right" )
             | ( "left" | "right" ) ( "hand" | "forearm" | "elbow" | "foot" ) ;
reach      = "reach" ":" effector target ;             (* effector → world target via ROM-constrained IK *)
pin        = "pin" ":" effector anchor ;               (* move the BODY to one primary contact *)
grip       = "grip" ":" handEffector gripAnchor ;      (* hand contact on a bar or rails *)
handEffector = "hands" | "hand_left" | "hand_right" ;
gripAnchor = "bar" | "bars" ;
turn       = "turn" ":" NUMBER ;                       (* face this yaw (deg) by phase end *)
travel     = "travel" ":" NUMBER NUMBER ;              (* move to this x z (metres) by phase end *)
cue        = "cue" STRING ;                              (* display-only coaching text *)
DURATION   = NUMBER "s" ;                                (* e.g. 2s, 1.5s *)

A step is one phase of the movement. Phases run in sequence; within a phase, all joint targets apply concurrently.

A built-in start pose can be customized with an indented override block:

  pose start = standing:
    shoulders: flex 20
    elbow_left: flex 35

The existing one-line form remains valid. Start-pose targets use the same joint/action vocabulary, channel mirroring, compatibility checks, and ROM clamping as phase targets, but they do not create a phase or consume time. They sparsely overlay the selected built-in pose: omitted channels keep the built-in value, while hold neutral resets all three channels on that joint. The composed start pose is the deterministic animation start and loop-reset pose. Contacts, root travel/turn, cues, and other step-only directives are not valid inside this block.

A document may contain at most one pose start declaration. A second declaration is an error rather than a replacement, so an earlier scoped block cannot be bypassed before its joint/action vocabulary is validated.

The header kind, rig, props, start poses, joints, actions, effectors, targets, and timing modes are closed vocabularies. Unknown values are errors; the parser does not accept a plausible-looking word and leave it for the renderer to ignore.

Contact target vocabularies are capability-specific. reach accepts floor, a rig body landmark, or an anchor from a declared prop. pin accepts only fixed world anchors (floor or a declared prop anchor), because translating the root cannot pin one body landmark to another landmark that moves with that same root. grip accepts only anchors supplied by a declared bar or dip-bars prop. Grouped grip: hands ... uses the bare bar / bars anchor and expands to separate left/right anchors; an explicitly sided grip anchor is valid only with the matching single-hand effector.

Contact mechanisms

ground-lock, pin, and grip are mutually exclusive root-solving families within one phase. Choose one primary support family, then express compatible additional contacts with independent reach constraints. The parser rejects conflicting combinations instead of silently choosing a solver order.

cue is display-only coaching metadata. The parser stores it in the IR and a viewer may show it alongside the current phase, but it does not alter joint targets, contacts, timing, range validation, or rendering solves. A cue should describe only motion that the phase's executable directives actually encode.

Ground locks accept the groups hands, forearms, and feet, the axial surface contact back, plus the single-side forms hand_left|hand_right, elbow_left|elbow_right, and foot_left|foot_right. Human-readable left foot, right foot, left hand, and related forms normalize to the canonical side-specific names.

Timing modes describe how motion crosses the phase boundary:

Legacy v0.1 spellings remain valid and normalize to canonical modes: ease-in → drive, ease-out → settle, and ease-in-out → settle. New documents should use the canonical v0.2 names.

2. Joints

Plural names move both sides symmetrically (left-side bones mirror the Y and Z axes). Use a singular *_left / *_right name to move one side. Each finger is a single curl (flex) joint; the thumb also takes abduct/adduct.

3. Actions

Each action maps to a rotation about one local axis. Degrees are absolute targets for that direction (a phase that doesn't mention a joint leaves it where the previous phase left it).

hinge is a hip hinge: applied to the pelvis, it pivots the torso forward over the hip line while the legs stay planted and vertical (the renderer counter-rotates the hips). Use it, not spinal flex, for a flat-back forward bend: deadlift, bent-over row, good-morning, or a bow.

rotate-in / rotate-out describe internal and external rotation only on a shoulder or hip. Use twist-left / twist-right for axial bones; older axial uses of rotate-in/out remain readable during the compatibility window but emit an authoring hint.

Each authored action changes one Euler channel. Other channels on that joint carry forward from the prior phase. hold neutral is the deliberate exception: it resets all three channels, so it is suitable for an explicit recovery.

Forearm roll is authored on elbows or its anatomical alias forearms because the wrist itself does not pronate. With upright arms at the sides, forearms: pronate 80 faces the palms inward toward the thighs; pronate 0 faces them forward. The final world-facing direction also depends on shoulder and elbow pose, so use supinate / pronate as anatomical rotation rather than as an absolute world-space palm constraint. Because targets are absolute, pronate 0 and supinate 0 name the same zero-angle reference; the action name selects the direction only when the authored magnitude is greater than zero.

Coordinate convention: rest pose is standing, arms at sides, facing +Z. The renderer's mannequin is built in this same convention so the parser's resolved Euler angles apply directly.

4. Configured range-of-motion limits

Every authored angle is clamped to the rig's configured per-axis limits before rendering; a clamp emits a warning (it does not fail the parse). Reach IK and contact orientation are constrained against the same per-axis boxes. These are simplified reference limits for visualization, not a medical or whole-movement safety guarantee. Selected ceilings (degrees):

Note: These are general literature values, not medical advice. Consult a qualified professional for physiotherapy or exercise prescription.

A pelvis hinge counter-rotates the hips in the renderer. The validator therefore also checks the combined hinge + carried hip-flexion result and clamps the newly authored channel when their composed local hip angle would exceed 135°.

5. Contact Semantics: ground-lock, reach, pin, and grip

Posecode provides four distinct contact mechanisms. Choosing the correct mechanism is essential for predictable motion:

The Fundamental Rule: Pin Moves the Body; Reach Moves the Limb

1. pin moves the body: The solver translates the figure's entire floating root in 3D space so that the primary effector sits precisely on the target anchor. The rest of the body carries with this translation.
2. reach moves the limb: The solver runs Cyclic Coordinate Descent (CCD) along the limb hierarchy (e.g. shoulder → elbow → wrist). The root position remains unmoved. If the target is beyond kinematic reach or would violate configured range-of-motion limits, the limb reaches as far as legal angles allow and records an explicit diagnostic residual.

Visual Comparison: Reach vs. Pin on the Floor

Consider a standing figure where you request the left hand to contact the floor:

• Under reach: hand_left floor: The figure's pelvis and stance remain upright in place. The left shoulder and elbow flex and extend toward the floor. Because a standing human cannot reach the floor without bending at the waist or knees, the hand reaches downward as far as legal joint angles allow. The body does not sink or drop to the floor. An unresolved reach residual diagnostic is produced if the floor cannot be contacted within configured ROM.
• Under pin: hand_left floor: The solver translates the entire figure downward and forward until the left hand rests on the floor anchor. The torso, pelvis, and legs translate downward along with the root.

Multi-Contact Constraints: Single Pin Invariant

A step phase MUST NOT declare more than one pin. Because each pin computes an absolute world-space translation for the single floating root, declaring two pins creates an over-constrained, conflicting system with no well-defined root position.

Handling two simultaneous ground contacts:

• When both feet are planted on the floor during movement (e.g. squats or demi-plié), you MUST use ground-lock: feet (or ground-lock: foot_left, foot_right), NOT two pins.
• To contact an anchor while another point is grounded or pinned, declare one primary support (ground-lock or pin) and add independent secondary contacts using reach: <effector> <target>.

Minimal Valid and Conflicting Examples

1. ground-lock

• Valid (minimal):

step "Squat" 1.5s settle:
  hips: flex 70
  knees: flex 80
  ground-lock: feet

• Invalid / Conflicting: Declaring an unsupported contact name or pairing ground-lock with a conflicting pin:

# Error: 'head' is not a recognised ground-lock contact
ground-lock: head
# Error: conflicting root solvers in a single phase
ground-lock: feet
pin: knee_left floor

2. reach

• Valid (minimal):

step "Touch toe" 1s flow:
  pelvis: hinge 70
  reach: hand_left ankle_left

• Invalid / Conflicting: Targeting an undeclared prop anchor:

# Error: 'box' anchor used but 'prop box' was not declared in header
reach: hand_right box

3. pin

• Valid (minimal):

step "Kneel" 1.2s settle:
  knees: flex 90
  pin: knee_left floor

• Invalid / Conflicting: Declaring two pins in the same phase:

# Error: a phase accepts at most one pin
pin: knee_left floor
pin: knee_right floor

4. grip

• Valid (minimal):

posecode exercise "Bar hang"
  rig humanoid
  prop bar
  pose start = standing

  step "Hang" 2s settle:
    shoulders: flex 170
    grip: hands bar
  repeat 1

• Invalid / Conflicting: Using grip without declaring the required prop:

# Error: 'grip: hands bar' requires 'prop bar' directive
step "Hang" 1s settle:
  grip: hands bar

The cue Directive: Display-Only Coaching Text

The cue directive provides human-readable coaching text (e.g. cue "Keep knees aligned with toes").

• Display-only: A cue is strictly visual, display-only coaching metadata and text rendered in viewers, logs, or UI cards.
• Zero kinematic effect: A cue MUST NOT affect joint rotations, IK solvers, contact resolution, range-of-motion clamping, phase timing, or validation. Parsers and renderers ignore cues when calculating kinematics.
• Never infer motion from cues: Movement authors and LLM prompt systems MUST NOT rely on cues to imply or fabricate motion; all joint and spatial behavior MUST be explicitly declared in the kinematics directives.

6. Rendering and Kinematic Model

1. Forward kinematics: each phase sets joint angles; the renderer uses C1-continuous quaternion splines between keyframes, shaped by the destination phase's timing mode.
2. Grounding: the figure is dropped so its lowest point rests on the floor (a bounding-box drop), which grounds standing, plank, and the lying/seated poses alike.
3. Ground-lock: contacts listed in ground-lock (hands, forearms, feet, or the per-side aliases hand_left|hand_right, elbow_left|elbow_right, foot_left|foot_right) stay planted while the body moves. back holds the pelvis-to-ribcage surface on the floor for supine work such as dead bugs. Unsupported contact names are line-anchored validation errors.
4. Reach-IK: a reach: line drives an effector (hand_*, fist_*, elbow_*, knee_*, or foot_*, plus their supported groups) to a world target via Cyclic Coordinate Descent (CCD) over the arm/leg chain. A target is a body landmark bone (e.g. ankle_left), the keyword floor, or a prop anchor (bar, seat, wall). The solve is ROM-constrained: each iteration clamps every chain joint into its §4 configured range-of-motion limits (expressed as a per-axis box in the bone's local Euler frame), so a reach toward an unreachable target settles on the closest pose available within that configured joint-angle box; solved angles obey the same limits as authored ones. The viewer records a post-solve residual for every active reach. A reach target is not reported as reached merely because its syntax parsed: missing, unsupported, and geometrically unreachable reach targets remain explicit diagnostics. A palm or fist declared against the floor also presents its matching contact surface to the floor. For a palm, the solver may redistribute incompatible authored roll into a legal forearm/wrist frame; the explicit floor contact takes priority, and every adjusted joint remains inside the same configured ROM.
5. Props: prop chair|wall|bar|box|dip-bars adds a scene object at a fixed default placement (chair/wall behind, bar overhead, box in front, dip bars either side); its named anchors (seat, wall, bar, box, bars) become reach, pin, or grip targets. Selected prop surfaces declare sampled blocking faces (the wall's surface, the chair's backrest and seat edge, the box's near face). A bounded contact pass reduces penetration, either by translating the whole figure out along the face normal (a wall-sit slides down the wall's *surface*, feet walking forward, instead of the torso hinging through the slab) or by bending the offending limb's hip clear, ROM-clamped like every other solve. Limbs pinned, gripped, or reached to a prop anchor are that phase's declared support and are exempt (a foot standing on the box top is not "inside" the box).
6. Pins: pin: <effector> <anchor> translates the whole figure so one primary effector sits on the anchor. Where ground-lock preserves an already planted floor support and reach moves a limb to a target, a pin moves the body. Typical uses include pin: knee_left floor, pin: foot_right box, and pin: pelvis floor. A phase accepts one pin because each pin translates the same floating root; express additional simultaneous contacts with independent reach constraints. Use grip instead of hand pins for a two-handed bar or rail contact.
7. Grips: grip: hands bar|bars is the dedicated two-hand contact for an overhead bar or dip rails; side-specific hand_left / hand_right forms are also available. A grip resolves independent left/right anchors, uses arm IK for each hand, orients the terminal contact, and closes the fingers. The matching prop must be declared. Use grips, rather than hand pins, for hangs, pull-ups, and dips.
8. Spatial choreography: turn: <deg> rotates the figure's facing (yaw about vertical) and travel: <x> <z> moves it across the floor (world metres from the load spot). Both are absolute targets carried across phases (like joint angles) and both return home on the loop wrap, so a box-step traces a square back to start and a pirouette spins a full turn. They layer under grounding (feet still rest on the floor) and power pirouettes, grapevines, traveling combos, and walk cycles. Standing poses only: combining with lying/seated bases (whose root is already tilted) is out of scope.
9. Looping: the timeline loops base → phases → base; repeat is the rep count surfaced to the UI.

When a mocap clip is active, the renderer selects the take containing the most actual bone motion (rather than blindly choosing the longest embedded take), retargets and blends it, then restores solved terminal contacts on the visible character. Mocap therefore cannot overwrite a planted sole or active grip.

Start poses: neutral, standing, first-position (ballet turnout), plank, supine (face-up), prone (face-down), seated (long-sit on the floor). Append : and indented joint targets to define a custom start pose as a sparse overlay on any of these built-ins.

IK note: Three.js's bundled CCDIKSolver targets SkinnedMesh; the Posecode mannequin is rigid capsule segments, so Posecode implements CCD directly over the Object3D bone hierarchy (posecode-render/ik.ts) for both ground-lock and reach. Selected limb-vs-body and body-vs-prop penetrations are reduced with sampled, bounded correction passes; this is not comprehensive collision detection or a physics simulation. Two-person/dual-IK and figure-vs-figure collision remain deferred (research §5.2, §6.2).

7. Intermediate Representation (IR)

parse(source) returns { ir, warnings, errors }. The IR is renderer-agnostic; angles are in degrees.

interface PosecodeIR {
  version: string;          // "0.4"
  kind: string;             // "exercise" | "stretch" | "posture"
  name: string;
  rig: string;              // "humanoid"
  avatar?: string;          // "avatar1" | "avatar2" | "avatar3"
  startPose?: string;       // "plank" | "standing" | ...
  startPoseOverrides?: {    // sparse, ROM-clamped overlay on startPose
    boneId: string;
    euler: { x: number; y: number; z: number };
    axes?: ("x" | "y" | "z")[];
  }[];
  repeat: number;
  phases: {
    name: string;
    durationSec: number;
    easing: "flow" | "settle" | "drive" | "snap" | "linear";
    targets: {
      boneId: string;
      euler: { x: number; y: number; z: number };
      axes?: ("x" | "y" | "z")[]; // channels explicitly authored this phase
    }[];
    groundLock: string[];   // ["hands","feet"], ["foot_right"], or ["back"]
    reaches: { effector: string; target: string }[];
    pins: { effector: string; anchor: string }[];
    grips: { effector: string; anchor: string }[];
    turnDeg?: number;
    travel?: { x: number; z: number };
    cue?: string;
  }[];
}

See llm-authoring.md for the prompt that teaches an LLM to write Posecode, and examples/ for complete documents.