OnTwos

Stop-motion / stepped animation in Unity. A self-contained asset implementing PCHIP-based motion quantization for animator-driven rigs, procedural rigs, and physics ragdolls.

The idea: sample the underlying continuous motion (animator output, any bone-driving system, or rigidbody trajectory), fit it with monotone cubic splines, and replay it in stepped frames chosen by an arc-length-weighted deviation threshold. The result reads as hand-animated stop-motion rather than smooth linear interpolation.

What you get

• AnimationStepper — applies stepping to any bone hierarchy each frame. Two modes:
  • AnimatorDriven (default) — reads Animator output. Detects state transitions and flushes held poses automatically so new states start clean.
  • AnySource — reads whatever localRotation the bones have each frame. No Animator required. Works with IK rigs, script-driven bones, cloth results baked to transforms, motion matching, audio-reactive bones, or anything else that writes to bone transforms directly.
• RagdollStepper — snapshots a physics ragdoll, builds a transform-only visual proxy, hides the source, and replays the snapshot stream in stepped frames. Settles when motion drops below threshold; wakes if hit again.
• OnTwosProfile — a ScriptableObject holding all the tuning (τ values, hold-frame bounds, settle thresholds, bone-exclusion keywords, per-bone overrides). Drop one onto an authoring component and you're done.
• RagdollStepper — exposes OnSettled and OnWoke C# events so you can hook dissolves, despawns, prop swaps, or re-enable interactions without polling. IsSettled and VisualProxy are also public properties.
• OnTwosAuthoring — single MonoBehaviour entry point. Call ActivateRagdoll() from your own code to switch from animator-driven to physics-driven stepped motion. Call Deactivate() to reverse the transition (get-up, revival, stagger recovery).

Install

Drop the OnTwos folder anywhere under Assets/ in your project. The two assembly definitions (OnTwos.Runtime, OnTwos.Editor) keep editor code out of player builds automatically.

Unity 2021.3 LTS and newer should work. Unity 6 is supported — the runtime uses #if UNITY_6000_0_OR_NEWER to select Rigidbody.linearVelocity vs the legacy Rigidbody.velocity.

Quick start

1. Assets → Create → CrunchyRagdoll → Profile to make a profile asset.
2. Add OnTwosAuthoring to a character GameObject.
3. Drag the profile onto the authoring's Profile slot.
4. AnimationStepper is added automatically on Awake when AutoBindOnAwake is true (the default). You only need to add it manually if you have turned AutoBindOnAwake off. RagdollStepper is created automatically the first time ActivateRagdoll() is called when AutoCreateProxy is true.
5. For physics-driven motion: call GetComponent<OnTwosAuthoring>().ActivateRagdoll() from your own code, or use the Activate Ragdoll button in the Inspector during Play Mode.
6. To reverse it (get-up, revival): call GetComponent<OnTwosAuthoring>().Deactivate().

Non-Animator rigs (IK, procedural, script-driven)

Set AnimationStepper.Mode to AnySource. The stepper reads whatever localRotation the bones have each LateUpdate — no Animator needed. Call FlushAllHolds() manually if your source system has discrete states and you want to prevent cross-state pose ghosting.

Folder layout

OnTwos/
├── Runtime/        — runtime code, separate asmdef, ships in player builds
├── Editor/         — custom inspectors, drawers, preview window
└── Samples~/       — demo prefab setup walkthrough (see Samples~/README.md)

Caveats

• Demo prefabs are not pre-authored. Samples~/ contains a step-by-step README. Setting up the demo prefabs manually takes about two minutes and produces something more reliable than auto-generated YAML referencing imported humanoid rigs.
• Visual proxy is mandatory for the ragdoll path. Writing localRotation directly to non-kinematic bone Rigidbodies causes CharacterJoint constraint error to accumulate, and the bodies drift sideways under gravity. The RagdollStepper always builds a proxy at the scene root with all physics and OnTwos components stripped, and writes the stepped pose there.

Algorithm summary

For each tracked bone, every frame:

1. Sample the source value (rotation / position / animator pose / any source).
2. Append to a MonotoneCubicSampler rolling buffer; fit a Fritsch-Carlson PCHIP spline over the recent window.
3. Walk the spline forward — emit a new "snap" when accumulated deviation (rotation degrees or position meters) exceeds τ, with candidate placements weighted by arc length.
4. HoldFrameScheduler gates the snap on elapsed time, from a StepRate in poses per second (12 = the classic "on twos"), so the cadence is identical at any framerate. CadenceJitter sets how far bones may drift off that shared beat — 0 locks the whole rig in step.
5. Apply the chosen snap pose to the visible bones; everything else holds.

For ragdolls, samples come from Rigidbody world transforms rather than Animator output, and stepped poses are written to the visual proxy rather than the source bones.

License

MIT — see below.

The algorithm, architecture, and design of OnTwos are original work by the author. C# implementation was produced with AI assistance.

MIT License

Copyright (c) 2026 Yusuf Hosam

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

Where to look

• Profile inspector: open any OnTwosProfile asset.
• Live telemetry: Window → CrunchyRagdoll → Preview (during Play Mode).
• Per-component foldouts: select an authoring or stepper in the scene.