Physics & Simulation

GPU-accelerated cloth, rope, fluid volumes, and rigid body physics. All solvers run on compute shaders via vGPU.

Cloth Simulation

Mass-spring cloth solver in engine/sim/cloth/. Each cloth is a grid of particles connected by structural, shear, and bending springs. The solver runs on GPU compute with configurable:

• Stiffness — Spring force coefficient
• Damping — Velocity damping factor
• Gravity — Per-cloth gravity override
• Wind — Directional force with turbulence noise
• Pin constraints — Fixed vertices (corners, edges, arbitrary)
• Collision — Sphere and plane collision response

Rope Physics

Position-Based Dynamics (PBD) rope solver in engine/sim/particles/RopePhysics.js. Ropes are chains of particles with distance constraints solved iteratively.

Rope Features

• PBD constraints — Distance constraints with configurable compliance
• Material properties — Stiffness, damping, mass per unit length (RopeMaterial.js)
• Particle interaction — Ropes interact with the particle system (RopeParticleInteraction.js)
• GPU rendering — Smooth tube rendering with normals and lighting (RopeGPURenderer.js)

Fluid Simulation

Two approaches to fluid simulation:

Lagrangian (SPH)

Smoothed Particle Hydrodynamics via the particle system. Each fluid particle carries density, pressure, and viscosity. The SPH solver computes inter-particle forces using a neighbor grid. See ParticleSPH.js.

Eulerian (Grid-Based)

ParticleEulerianFluid.js provides a grid-based fluid solver using the Navier-Stokes equations. Suitable for contained volumes (pools, rivers). Uses pressure projection and velocity advection on a 3D grid.

Rigid Body Physics

The ECS PhysicsBody component drives rigid body simulation:

• Body types — Dynamic, kinematic, static
• Collider shapes — Sphere, box, capsule, mesh (via Collider component)
• Forces — Gravity, impulses, torques
• Constraints — Distance, hinge, ball-socket

PBD Ragdoll

Position-Based Dynamics ragdoll solver for character physics. Joint limits, bone chains, and muscle constraints. Integrates with the AGI system for learning-based locomotion.

Simulation Update

SimulationUpdate.js (30K lines) orchestrates all simulation systems per frame:

1. ECS physics system tick (rigid bodies, colliders)
2. Cloth solver step (GPU compute)
3. Rope constraint solving (PBD iterations)
4. Particle simulation (main compute + advanced subsystems)
5. Fluid pressure solve (if active)

GPU Physics Engine

Key Files