# lib-core.puppet

Skeletal animation primitive for top-down 2D characters. Manages a
bone hierarchy with cascaded world-transforms, keyframe animation
playback (multi-channel: rot/pos/scl), multi-target look-at constraints
(slerp or rate-limit or instant-snap), procedural animation callbacks,
and a per-channel conflict guard that allows animation and procedural
layers to coexist on the same bones.

**Version:** 0.4.3
**Lib-ID:** lib-core.puppet
**Requires:** (none — pure Lua + engine.render.* + engine.asset.*)
**Tags:** animation, skeleton, puppet, character

## Topology

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## API

### `puppet.build_rig(rig_table)`

Validates a rig table (parsed from `*.rig.json`) and returns an internal
representation with `bones_by_id`, parent-resolved object references,
track-membership maps, z-sorted bone list, and optional locomotion config.
Raises on invalid data (missing parents, bones in 2 tracks, legacy
`rest.angle` field, etc.).

### `puppet.build_animation(anim_table, rig)`

Validates an animation table against a built rig. Bones referenced in
keyframes must exist and be in the animation's declared track. Raises on
unknown bones, wrong-track bones, or legacy `angle` channel name.

### `puppet.sample_animation(anim, t)`

Returns a frame `{ <bone_id> = { rot?, pos?, scl? }, ... }` sampling the
animation at time `t` with linear interpolation per channel. Wraps on
`t > duration` if `animation.loop` is true. Each channel is sampled
independently; absent channel means no override (hold-on-single-side
semantics).

### `puppet.spawn(rig, {x, y})`, `puppet.despawn(handle)`

Spawn a new puppet instance at world position (x, y). Returns an opaque
handle. `despawn` removes the instance and all its state.

### `puppet.update(dt)`, `puppet.render(handle)`

`update(dt)` runs the full pipeline for ALL spawned puppets (rest-reset
→ keyframes → look-at → procedural → compute_world → apply_locomotion).
`render(handle)` draws ONE puppet via `engine.render.draw_sprite_transform`
per non-virtual bone. Call `render` inside a `camera.begin()/finish()`
block.

### `puppet.set_look_target(handle, target_id, x, y)`, `puppet.clear_look_target(handle, target_id?)`

Set or clear a named look-at target. Bones with `look_at: "target_id"`
in the rig rotate to face the matching target position each frame. The
2-arg legacy form `set_look_target(handle, x, y)` sets the "default"
target. `clear_look_target(handle)` without a target_id clears all targets.

### `puppet.set_procedural(handle, name, callback)`, `puppet.clear_procedural(handle, name)`

Register or unregister a per-frame procedural callback. Callback signature:
`function(handle, dt)`. Inside the callback, mutate bone transforms via
`puppet.write_bone`. Callbacks are invoked once per puppet per frame, at
root-visit time (before compute_world for that frame).

### `puppet.write_bone(handle, bone_id, {rot?, pos?, scl?})`

Write one or more channels of a bone's local transform from a procedural
callback. Per-channel conflict guard: raises if the written channel is also
written by a keyframe in any active animation on that bone's track. Allows
procedural to write `rot` while an animation writes `scl` on the same bone.

### `puppet.play(handle, anim_id, {loop?, speed?})`, `puppet.stop(handle, anim_id)`, `puppet.stop_all(handle)`, `puppet.is_playing(handle, anim_id)`

Keyframe-layer controls. Animation must be registered via
`puppet.register_animation(handle, anim)` first. `speed` defaults to 1.0;
pass a value like `0.45` for a slower cadence.

### `puppet.register_animation(handle, anim)`

Binds a built animation (from `build_animation`) to a puppet instance,
making it available to `play`.

### `puppet.load_rig(path, asset_aliases?)`

Loads a rig from a JSON file and optionally resolves bone textures via
`puppet.load_textures`. Pass `engine.module.asset_aliases()` to enable
sprite-mode rendering; omit for colored-rect-only rigs.

### `puppet.load_textures(rig, asset_aliases)`

For each bone with a `texture` field, resolves the atlas-id to a
texture-handle via the asset-lib indirection. Also resolves bone anchors
from the atlas (unless the bone has an explicit `anchor` field). Caches
texture dimensions for locomotion sprite_reach computation.

### `puppet.load_animation(path, rig)`

Convenience wrapper: reads a JSON file and calls `build_animation`.

### `puppet.bone_world_transform(handle, bone_id)`

Returns `x, y, rot` — the bone's cached world-transform from the last
`puppet.update` tick. World rot is in radians. Used by procedural callbacks
to read parent bone state (e.g., `body.world.rot` for leg orientation math).

### `puppet.bone_world_scale(handle, bone_id)`

Returns `scl_x, scl_y` — the bone's cached world scale. Accounts for
`inherit_scale` flag (false → returns local.scl, not parent.scl * local.scl).

### `puppet.position(handle)`, `puppet.facing(handle)`, `puppet.move_to(handle, x, y)`

Puppet world position management. `move_to` sets position instantly and
updates `facing` (last-movement-direction angle, radians). `facing` returns
the last movement direction.

### `puppet.write_bone_test_only(handle, bone_id, angle_rad)`

Test-setup helper: directly write a bone angle bypassing the per-channel
conflict guard. Stored in `test_overrides`; applied after rest-reset and
keyframe sampling. Not for production use.

### `puppet.set_foot_sprite_reach_test_only(handle, leg_index, value)`, `puppet.foot_state_test_only(handle, leg_index)`, `puppet.bone_animated_test_only(handle, bone_id)`

Test-only access to locomotion internal state and per-frame keyframe flags.
Not for production use.

## Conventions

- World-coords + pixel-units throughout (per ADR-0031).
- Bone angles in radians internally. JSON `rest.rot` and keyframe `rot`
  are in degrees; converted on load via `math.rad`.
- `look_at_speed` is degrees/s in JSON; stored as radians/s.
- `look_at_slerp` is a unitless rate (not converted).
- `sprite_rot` is degrees in JSON; stored as radians.
- `rest.rot_min` / `rest.rot_max` are degrees in JSON; stored as radians.
- Update pipeline runs once per frame (`puppet.update(dt)`) for ALL
  spawned puppets. `render` is per-puppet so callers can interleave
  with other rendering.
- `render` must be called inside a `camera.begin()` / `camera.finish()`
  block to render in world-space.

## CHANGELOG

### v0.4.3
- `inherit_scale: false` bone flag: when false, `world.scl = local.scl`
  (parent.world.scl not multiplied). Position cascade still uses parent.scl.
  Primary use case: feet that track leg-end positionally without Y-flip
  inheritance when leg.scl_y inverts during walk cycle.

### v0.4.2
- Per-channel write_bone conflict check (was per-track). Procedural can now
  write channels (e.g., rot) that no active animation writes on the same bone
  (e.g., animation writes scl). Enables animation + procedural on same bone
  without conflict.

### v0.4.1
- target_local normalization to (-π, π] in look-at block. Fixes head-flick
  when body world.rot crosses ±π (south-facing direction).

### v0.4.0
- Multi-look-target API: `puppet.set_look_target(handle, target_id, x, y)`.
  Bones declare `look_at: "target_id"` (string). Legacy `look_at: true`
  maps to "default" target.
- Slerp smoothing: `look_at_slerp` field (unitless rate, exponential approach).
  `look_at_speed` (linear rate-limit) kept for backward-compat.
- Virtual bones: `virtual: true` — participates in compute_world, skipped
  in render.
- Legs re-parented to body (was root in v0.3.0).

### v0.3.3
- Per-bone `sprite_rot` (degrees in JSON): render-time rotation offset
  applied as `world.rot + sprite_rot`. Matches substrate puppet.c:2101
  convention.

### v0.3.2
- Persistent `look_at_state` per bone. Fixes rate-limit reset-bug where
  each frame's rest-reset wiped the accumulated look-at rotation.

### v0.3.1
- Look-at math fix: `world_angle = atan(dx, -dy)` (sprite-top convention:
  rot=0 = top faces target, not game-forward).

### v0.3.0
- BoneState multi-channel (rot/pos/scl) with world.scl_x/y
- Cascaded world-transform with scale propagation (compute_world)
- Procedural locomotion layer via rig.locomotion block (apply_locomotion)
- Schema: rest.rot (renamed from rest.angle), rest.scl added
- New APIs: bone_world_scale, set_foot_sprite_reach_test_only,
  foot_state_test_only, bone_animated_test_only

### v0.2.0
- Rig-format extension (texture, z_order, anchor, asset_pack)
- Multi-level parent-chain transform + sprite-mode render
- New API: load_textures, bone_world_transform, write_bone_test_only

### v0.1.0
- Initial release: skeleton + bones + tracks + rest + keyframe +
  procedural + look-at constraint. Footplant IK deferred.

## References

- `architecture/puppet.md` (Reference — updated to v0.4.3)
- `superpowers/specs/2026-05-18-puppet-subterrain-model-design.md` (Baseline Spec v0.4.3)
- `docs/adrs/0040-puppet-subterrain-model-pivot.md` (Pivot rationale)
- ADR-0037 (Tests-as-Libs)
- ADR-0038 (API-Doc-Convention)