softy

Spring-damper systems and Verlet integration for games -- ropes, cloth, and soft bodies.

Live Demo -- try it in your browser (compiled to WebAssembly)

Overview

• Analytical springs -- closed-form critically/under/over-damped spring-damper systems (no Euler drift)
• Verlet dynamics -- position-based integration with implicit velocity
• Constraint solver -- distance, pin, angle, and bounds constraints with iterative relaxation
• Ropes & cloth -- VerletChain and VerletGrid with tear support
• Soft bodies -- 2D pressure-based deformable shapes
• Generic -- works with any float/vector type via Float and Vec traits
• Deterministic -- reproducible results across runs
• no_std compatible -- works in embedded and WASM environments

Quick Start

use softy::Spring1D;

// Critically damped spring: smoothly moves from 0.0 toward 10.0
let mut spring = Spring1D::<f32>::critically_damped(0.0, 10.0, 5.0);

// Advance by 1/60th of a second
spring.update(1.0 / 60.0);
println!("value: {}", spring.value()); // smoothly approaches 10.0

Features

• Spring types -- critically damped, underdamped (oscillating), and overdamped; 1D/2D/3D type aliases
• Verlet particles -- position-based dynamics with gravity, damping, and mass
• Constraint solver -- configurable iteration count and relaxation
  • DistanceConstraint -- maintain distance between particles
  • PinConstraint -- pin a particle to a fixed point
  • AngleConstraint -- limit angle between particle triplets
  • BoundsConstraint -- keep particles within a region
• VerletChain -- rope simulation with configurable segment count and stiffness
• VerletGrid -- cloth simulation with structural, shear, and bend links; supports tearing
• SoftBody -- 2D pressure-based soft body with internal volume preservation
• StepObserver -- trait for monitoring physics steps (debugging, profiling)
• no_std -- depends only on alloc; uses libm for math

Feature Flags

Flag	Default	Description
std	off	Enables standard library use

API Overview

Type	Role
Spring<V>	Analytical spring-damper (generic over vector)
Spring1D/2D/3D	Dimensional type aliases for Spring
DampingMode	Critical / underdamped / overdamped selection
Particle	Verlet particle with position, mass, pinning
ConstraintSolver	Iterative constraint relaxation solver
DistanceConstraint	Maintains distance between two particles
PinConstraint	Pins a particle to a world-space point
AngleConstraint	Limits angle formed by three particles
BoundsConstraint	Keeps particles inside a rectangular region
VerletChain	Rope / chain built from particles + distance constraints
VerletGrid	Cloth grid with optional tearing
SoftBody	2D pressure-based deformable body
SolverConfig	Iteration count, relaxation, sub-stepping
StepObserver	Trait to observe each physics step
PhysicsError	Error type for invalid configurations

License

Licensed under either of

• Apache License, Version 2.0
• MIT License

at your option.