docs(dances): dance1 motion description from Blender kinematic analysis

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/dances/dance1.md

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+# dance1 — motion description
+
+Kinematic description of the `dance1` clip (`dancegen/dance1.glb`, clip `Dance1`),
+derived by sampling world-space bone positions across the whole clip in Blender —
+not a subjective guess. Duration **30.0 s**, retargeted onto the Quaternius rig.
+
+## One-line read
+An energetic, **arm-forward** full-body dance: big symmetric sweeping arm gestures
+carry the performance, over busy footwork and pronounced level changes, while the
+whole body ranges around a ~1.2 × 1.7 m footprint. It never settles — continuous,
+flowing motion for the full 30 seconds.
+
+## What actually moves (path length over 30 s = how much a joint travels)
+
+| Rank | Body part | Travel | Read |
+|---|---|---|---|
+| 1 | **Both hands** | ~49–50 m each | The stars of the dance — huge, near-constant sweeps |
+| 2 | Forearms | ~32–33 m | Big arcs from the elbow reinforce the hand work |
+| 3 | Shins | ~25–26 m | Busy legs — steps, lifts, direction changes |
+| 4 | Upper arms / feet | ~18–21 m | Full-shoulder arm swings; active but grounded feet |
+| 5 | Head / spine / pelvis | ~16–17 m | Whole torso rides along — leans and level changes, not stiff |
+
+Hands move at roughly **1.7 m/s average** with constant direction changes, and each
+hand's reach spans ~1.7–2.5 m per axis — meaning full-wingspan sweeps that go both
+**high** (≈1.5–1.7 m vertical range) and **wide**. This is expansive, expressive
+upper-body choreography, not tight or contained.
+
+## Character traits
+- **Highly symmetric arms** — left/right hand travel matches at a **0.99 ratio**. The
+  arm choreography is mirrored/synchronized, the hallmark of a choreographed or
+  group-style routine (consistent with Polynesian dance vocabulary).
+- **Slight left-foot bias** — feet symmetry is 0.89 (left foot a bit busier), so the
+  legwork isn't perfectly mirrored; there's a lead side.
+- **Big level changes** — pelvis vertical range is **0.94 m**. The dancer drops and
+  rises a lot (deep bends / crouches into extensions), which gives the dance its
+  dynamic, punchy feel rather than staying at one height.
+- **Travels the space** — pelvis moves over a **1.2 × 1.7 m** horizontal area (16 m of
+  total travel), so it's not planted-in-place; expect stepping and repositioning.
+
+## Rhythm / tempo (caveat)
+Autocorrelation of the most-active joint's vertical motion peaks around **~205 bpm**,
+which is almost certainly the **double-time harmonic** of the real musical pulse —
+halving it gives **~100–103 bpm**, a lively mid-tempo. Treat this as approximate: the
+trustworthy tempo comes from the **source reference video's** music, not the retargeted
+motion. The `bpm: 100` in `dance1.json` is a hand-set placeholder that happens to land
+in the same neighborhood.
+
+## If scoring a song to it
+Aim for a **~100 bpm** groove with a strong backbeat and room for **sweeping,
+sustained phrases** (to match the long arm arcs) punctuated by **accents on the level
+drops** (the pelvis dips). Symmetric, repeating 4- or 8-bar structures will sit well
+against the mirrored, cyclic choreography. Confirm the real tempo against the source
+footage before locking it in.
+
+---
+*Generated 2026-07-15 from `exchange/converted-glb/dance1.glb` via Blender headless
+motion sampling. Method: per-frame world-space bone positions → path length, bounding
+range, L/R symmetry, and vertical-signal autocorrelation for tempo.*