How It Works | Notion

All physics simulation runs only on the server. Clients never simulate physics. Instead, they receive finished poses from the server and blend them smoothly. Active ragdoll blending is fully integrated; the server merges animation targets and physics results into a single authoritative state. The result: zero desync, zero prediction complexity.

Who Does What?

Role	Responsibility
Server	Simulates all bodies, constraints, and collisions. Blends animation pose with physics forces to produce the final authoritative pose. Generates snapshots at a fixed interval.
Client	Buffers the two latest snapshots, interpolates between them, and applies the resulting bone transforms to the skeletal mesh. No local physics simulation.
Snapshot System	Packs all bone transforms, velocities, and active ragdoll flags into a compact struct. Sent from server to clients via replicated property or RPC.

Step-by-Step Flow

Server ticks physics
- Solves ragdoll bodies, constraints, and collisions in the physics sub-step.
- The active ragdoll solver blends the animation target pose with physics forces, producing the authority pose.
Snapshot is captured
- Final bone transforms, physics state (active/unpowered), and recovery progress are packed into a single struct.
- Snapshots are taken at a configurable rate (e.g., every 2–3 physics sub-steps).
Snapshot is sent to clients
- Uses a lightweight, compressed replication path.
- Only the newest snapshot is kept for interpolation; late or duplicate snapshots are discarded.
Client buffers and interpolates
- Stores the two most recent snapshots.
- Each frame, interpolates bone transforms (linear or cubic) based on time since the last snapshot.
- Removes jitter and keeps motion smooth even at low update rates.
Skeletal mesh is updated
- Interpolated transforms are applied directly to the character mesh.
- No physics runs on the client; the mesh is purely a visual copy of server authority.
Active ragdoll recovery loop
- When the server detects the ragdoll is stable (e.g., after impact), it gradually blends the physics pose back toward the animation pose.
- This blend is included in snapshots and presented seamlessly on all clients.

Key Difference from Default Unreal Engine

Default Unreal Behaviour	This System
Physics simulated client-side, server only activates ragdoll.	Physics runs 100% on the server.
Each client gets different results, leading to desync.	Single source of truth; all clients see the exact same state.
Active ragdoll motors/constraints often run locally, adding prediction and correction overhead.	All motors and blending are resolved on the server; clients receive the finished pose.
Frame rate drops or network lag directly affect physics behaviour.	Client conditions are irrelevant — snapshots are authoritative and deterministic.

What You Gain

Zero physics desync — One authority. Every player sees the same ragdoll state.
Jitter-free, smooth visuals — Interpolation keeps motion fluid even under network fluctuation.
Predictable active ragdoll — Server-side blend is replicated exactly; get-up animations play identically for all clients.