Particles Physics

Particle Physics

Apply forces, fields, and constraints to create dynamic particle motion.

Quick Start

// Simple gravity + velocity
useFrame((_, delta) => {
  for (let i = 0; i  0) {
      const dragForce = coefficient * speed * speed;
      const factor = Math.max(0, 1 - (dragForce * delta) / speed);
      
      velocities[i * 3] *= factor;
      velocities[i * 3 + 1] *= factor;
      velocities[i * 3 + 2] *= factor;
    }
  }
}

Attractors & Repulsors

Point Attractor

function applyAttractor(
  velocities: Float32Array,
  positions: Float32Array,
  count: number,
  attractorPos: THREE.Vector3,
  strength: number,  // Positive = attract, negative = repel
  delta: number
) {
  for (let i = 0; i  0.1) {  // Avoid division by zero
      // Inverse square falloff
      const force = strength / distSq;
      
      velocities[i * 3] += (dx / dist) * force * delta;
      velocities[i * 3 + 1] += (dy / dist) * force * delta;
      velocities[i * 3 + 2] += (dz / dist) * force * delta;
    }
  }
}

Orbit Attractor

function applyOrbitAttractor(
  velocities: Float32Array,
  positions: Float32Array,
  count: number,
  center: THREE.Vector3,
  orbitStrength: number,
  pullStrength: number,
  delta: number
) {
  for (let i = 0; i  0.1) {
      // Tangential force (orbit)
      const tx = -dz / dist;
      const tz = dx / dist;
      
      velocities[i * 3] += tx * orbitStrength * delta;
      velocities[i * 3 + 2] += tz * orbitStrength * delta;
      
      // Radial force (pull toward center)
      velocities[i * 3] -= (dx / dist) * pullStrength * delta;
      velocities[i * 3 + 1] -= (dy / dist) * pullStrength * delta;
      velocities[i * 3 + 2] -= (dz / dist) * pullStrength * delta;
    }
  }
}

Multiple Attractors

interface Attractor {
  position: THREE.Vector3;
  strength: number;
  radius: number;  // Influence radius
}

function applyAttractors(
  velocities: Float32Array,
  positions: Float32Array,
  count: number,
  attractors: Attractor[],
  delta: number
) {
  for (let i = 0; i  0.1 && dist  0.1) {
      // Tangent direction (cross product with axis)
      const tx = axis.y * pz - axis.z * py;
      const ty = axis.z * px - axis.x * pz;
      const tz = axis.x * py - axis.y * px;
      
      const tLen = Math.sqrt(tx * tx + ty * ty + tz * tz);
      const force = strength * Math.exp(-dist * falloff);
      
      velocities[i * 3] += (tx / tLen) * force * delta;
      velocities[i * 3 + 1] += (ty / tLen) * force * delta;
      velocities[i * 3 + 2] += (tz / tLen) * force * delta;
    }
  }
}

Turbulence

Simplex-Based Turbulence

// GPU turbulence in vertex shader
vec3 turbulence(vec3 p, float time, float scale, int octaves) {
  vec3 result = vec3(0.0);
  float amplitude = 1.0;
  float frequency = scale;
  
  for (int i = 0; i  radius : dist  0) {
      const nx = dx / dist;
      const ny = dy / dist;
      const nz = dz / dist;
      
      // Move to surface
      const targetDist = inside ? radius : radius;
      positions[i * 3] = center.x + nx * targetDist;
      positions[i * 3 + 1] = center.y + ny * targetDist;
      positions[i * 3 + 2] = center.z + nz * targetDist;
      
      // Reflect velocity
      const dot = velocities[i * 3] * nx + velocities[i * 3 + 1] * ny + velocities[i * 3 + 2] * nz;
      velocities[i * 3] = (velocities[i * 3] - 2 * dot * nx) * bounce;
      velocities[i * 3 + 1] = (velocities[i * 3 + 1] - 2 * dot * ny) * bounce;
      velocities[i * 3 + 2] = (velocities[i * 3 + 2] - 2 * dot * nz) * bounce;
    }
  }
}

Integration Methods

Euler (Simple)

// Fastest, least accurate
position += velocity * delta;
velocity += acceleration * delta;

Verlet (Better for constraints)

// Store previous position
const newPos = position * 2 - prevPosition + acceleration * delta * delta;
prevPosition = position;
position = newPos;

RK4 (Most accurate)

// Runge-Kutta 4th order (for high precision)
function rk4(position: number, velocity: number, acceleration: (p: number, v: number) => number, dt: number) {
  const k1v = acceleration(position, velocity);
  const k1x = velocity;
  
  const k2v = acceleration(position + k1x * dt/2, velocity + k1v * dt/2);
  const k2x = velocity + k1v * dt/2;
  
  const k3v = acceleration(position + k2x * dt/2, velocity + k2v * dt/2);
  const k3x = velocity + k2v * dt/2;
  
  const k4v = acceleration(position + k3x * dt, velocity + k3v * dt);
  const k4x = velocity + k3v * dt;
  
  return {
    position: position + (k1x + 2*k2x + 2*k3x + k4x) * dt / 6,
    velocity: velocity + (k1v + 2*k2v + 2*k3v + k4v) * dt / 6
  };
}

File Structure

particles-physics/
├── SKILL.md
├── references/
│   ├── forces.md             # All force types
│   └── integration.md        # Integration methods comparison
└── scripts/
    ├── forces/
    │   ├── gravity.ts        # Gravity implementations
    │   ├── attractors.ts     # Point/orbit attractors
    │   └── fields.ts         # Flow/velocity fields
    └── collision/
        ├── planes.ts         # Plane collision
        └── shapes.ts         # Sphere, box collision

Reference

• references/forces.md — Complete force implementations
• references/integration.md — When to use which integration method

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Bbeierle12
• Source: Bbeierle12/Skill-MCP-Claude
• License: MIT

Install and usage instructions live in the source repository linked above.