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Jiggle Bones BETA

Verlet-integration soft-body physics for skirts, hair, capes, accessories. The motion that animation engines usually skip.

Tier: Builder Pro Status: 🔵 BETA — works on common rigs, edge cases continue tuning Menu: Physics → ✨ Jiggle Bones

What it does​

When a character moves, secondary motion comes along for the ride: hair sways, skirts swing, capes flutter, jewelry bobs. These chains of bones are attached to the body but should react to body motion with their own delayed, dampened, physically-plausible response — not just rigid-follow the parent bone.

Most browser-based animation tools skip this entirely — it's a hard problem to solve in real time. Kinetiq ships it built-in, powered by Kinetiq Physics, our in-engine Verlet soft-body simulator.

The result: when your character moves, their skirt fabric trails realistically. When they stop, the skirt settles with a brief overshoot and decay. When they spin, hair swings out by inertia and follows back. All in the browser. All in real time.

How it works (Verlet integration)​

Verlet integration is a numerical method for simulating particles connected by stiffness constraints. Each "jiggle bone" gets:

  • A particle at its position (the bone's tip in world space).
  • A stiffness constraint to its parent particle (the chain).
  • An inertia term — the difference between current and previous positions, which carries velocity automatically.
  • A gravity term — pulls particles downward.
  • A damping term — bleeds energy out so the chain settles.

Per frame, the simulation:

  1. Computes new positions by extrapolating from velocity (Verlet step).
  2. Applies gravity.
  3. Iteratively enforces stiffness constraints so the chain stays connected.
  4. Updates the bone rotations to match the new particle positions.

The result is a chain that swings, settles, and reacts naturally to the parent body without any keyframe data — pure physics layered on top of the underlying animation.

Configuring jiggle chains​

The Jiggle Bones panel surfaces a chain editor where you mark bones as "jiggle":

Kinetiq Engine Jiggle Bones panel — anime-style avatar with long hair and skirt in the viewport on the left, panel on the right titled 'Jiggle Bones — Kinetiq Physics, 94 active wigglers'. Six preset chips (Hair, Breasts, Butt, Tail, Cape, Skirt) plus Custom; an active Hair group with sliders for Stiffness 0.50, Gravity 1.40, Air Drag 0.68, Blend 1.00; a list of bones in the group (HairRoot, Hairfront1L, Hairfront1R, etc.) and an Auto-Suggest button

The Jiggle Bones panel. Start from a preset (Hair, Breasts, Butt, Tail, Cape, Skirt) for a one-click setup, or build a Custom group. The four sliders shape every chain in the group: Stiffness (how rigid), Gravity (how heavy), Air Drag (how much it slows global motion), and Blend (how much of the simulation bleeds into the bake — 0 = pure animation, 1 = pure physics). Everything updates live in the viewport as you drag.

  1. Open Physics → Jiggle Bones.
  2. The hierarchy view lists all bones in the active avatar.
  3. For each bone you want to jiggle (e.g. the skirt cone, hair tendrils):
    • Toggle the Jiggle flag.
    • Adjust per-chain parameters:
      • Stiffness (0.0 - 1.0) — how rigidly the chain follows its rest shape. Higher = stiffer, less swing. Lower = looser, more swing.
      • Damping (0.0 - 1.0) — how quickly motion decays. Higher = motion dies off fast. Lower = chain swings longer.
      • Gravity (per-chain or global) — how strongly the chain falls. Higher = realistic skirt heaviness. Lower = floaty hair.
      • Max Stretch — limits how much the chain can elongate beyond rest length. Critical for preventing rubber-band artifacts.

The defaults are tuned for medium-weight cloth (skirt-like). Adjust per chain depending on material:

  • Heavy fabric (long coat) → low stiffness, high damping, high gravity.
  • Light fabric (silk, hair) → higher stiffness (more snappy), lower gravity.
  • Metal accessories (chains, beads) → high stiffness, low damping, high gravity.

Live preview while authoring​

Jiggle Bones runs live during playback. Toggle a bone to jiggle, hit play — the chain immediately starts simulating. Adjust stiffness — the chain's response changes mid-playback. There's no bake step required for the preview; bake only happens at export.

Known limitations (still BETA)​

The Verlet implementation is solid for the common skirt/hair/cape case but has known limitations:

No collision (yet)​

Chains don't collide with the avatar's body or other chains. A skirt that should rest against legs may pass through them. We're tracking integration with cannon-es or Rapier3D for real collision support — likely 8-15 hours of work, deferred to post-launch (May 2026+) to avoid bundle-size impact on free users.

No constraint to body curves​

The chain stiffness is to its own rest shape, not to a curve guide on the body. So a hair chain doesn't "follow" the back of the head as the head turns; it just dangles from its root.

Single-chain at a time​

The current UI focuses on configuring one chain at a time. Bulk-configuring many chains across a complex character (full hair + dual ponytails + skirt + scarf) is workable but tedious. We're planning a bulk-configure mode post-launch.

Performance on dense rigs​

30+ jiggle bones running simultaneously will tax low-end GPUs. The simulation is CPU-side TypeScript, so it doesn't help to have a beefy GPU. If frame rate drops, reduce the iteration count in the chain config or disable some chains during preview (re-enable only at bake/export).

Workflow — adding jiggle to a skirt​

  1. Import an avatar with a skirt rig (e.g. an SL Bento Female with skirt bones).
  2. Open Physics → Jiggle Bones.
  3. In the hierarchy, mark the skirt root bone and its chain (typically mSkirt0 through mSkirt9 for SL).
  4. Set per-chain values:
    • Stiffness: 0.3 (loose enough to swing meaningfully)
    • Damping: 0.4 (settle in ~1 second)
    • Gravity: enabled, default magnitude
  5. Toggle the panel ON. The simulation kicks in immediately.
  6. Play a walk or turn animation. The skirt should swing as the avatar moves.
  7. Tune by ear: too floaty? Raise stiffness. Too stiff? Lower it. Settles too fast? Lower damping.
  8. Once it feels right, the configuration is saved with the avatar; export bakes the simulated motion into the animation track.

Edge cases​

Avatar's skirt mesh isn't bone-driven​

Some avatar meshes have cloth/skirt geometry that's not weight-painted to a bone chain — instead, it's a separate mesh with morph targets or simulated separately. Jiggle Bones only animates bones. If your skirt isn't bone-driven, this tool can't help directly; you'd need to weight-paint the mesh to a bone chain first.

Hair with multiple branches​

A character with two ponytails or split hair tendrils requires multiple chains — one per branch. Configure each separately.

Performance with many chains​

20-30 chains running simultaneously on a complex character may dip below 60 FPS on low-end devices. Reduce the iteration count or temporarily disable non-critical chains during preview.

  • Foot Locking — main body motion stays locked while jiggle layers on top
  • Quality Score — Quality Score doesn't currently score jiggle bones (the BETA status reflects this)
  • Roadmap — see "Three.js physics engine integration for Jiggle Bones full parity" — the cannon-es / Rapier3D upgrade for real collision support