particles-physics

What this skill does

# Particle Physics

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

## Quick Start

```tsx
// Simple gravity + velocity
useFrame((_, delta) => {
  for (let i = 0; i < count; i++) {
    // Apply gravity
    velocities[i * 3 + 1] -= 9.8 * delta;
    
    // Update position
    positions[i * 3] += velocities[i * 3] * delta;
    positions[i * 3 + 1] += velocities[i * 3 + 1] * delta;
    positions[i * 3 + 2] += velocities[i * 3 + 2] * delta;
  }
  geometry.attributes.position.needsUpdate = true;
});
```

## Force Types

### Gravity (Constant Force)

```tsx
function applyGravity(
  velocities: Float32Array,
  count: number,
  gravity: THREE.Vector3,
  delta: number
) {
  for (let i = 0; i < count; i++) {
    velocities[i * 3] += gravity.x * delta;
    velocities[i * 3 + 1] += gravity.y * delta;
    velocities[i * 3 + 2] += gravity.z * delta;
  }
}

// Usage
const gravity = new THREE.Vector3(0, -9.8, 0);
applyGravity(velocities, count, gravity, delta);
```

### Wind (Directional + Noise)

```tsx
function applyWind(
  velocities: Float32Array,
  positions: Float32Array,
  count: number,
  direction: THREE.Vector3,
  strength: number,
  turbulence: number,
  time: number,
  delta: number
) {
  for (let i = 0; i < count; i++) {
    const x = positions[i * 3];
    const y = positions[i * 3 + 1];
    const z = positions[i * 3 + 2];
    
    // Base wind
    let wx = direction.x * strength;
    let wy = direction.y * strength;
    let wz = direction.z * strength;
    
    // Add turbulence (using simple noise approximation)
    const noise = Math.sin(x * 0.5 + time) * Math.cos(z * 0.5 + time);
    wx += noise * turbulence;
    wy += Math.sin(y * 0.3 + time * 1.3) * turbulence * 0.5;
    wz += Math.cos(x * 0.4 + time * 0.7) * turbulence;
    
    velocities[i * 3] += wx * delta;
    velocities[i * 3 + 1] += wy * delta;
    velocities[i * 3 + 2] += wz * delta;
  }
}
```

### Drag (Velocity Damping)

```tsx
function applyDrag(
  velocities: Float32Array,
  count: number,
  drag: number,  // 0-1, higher = more drag
  delta: number
) {
  const factor = 1 - drag * delta;
  
  for (let i = 0; i < count; i++) {
    velocities[i * 3] *= factor;
    velocities[i * 3 + 1] *= factor;
    velocities[i * 3 + 2] *= factor;
  }
}

// Quadratic drag (more realistic)
function applyQuadraticDrag(
  velocities: Float32Array,
  count: number,
  coefficient: number,
  delta: number
) {
  for (let i = 0; i < count; i++) {
    const vx = velocities[i * 3];
    const vy = velocities[i * 3 + 1];
    const vz = velocities[i * 3 + 2];
    
    const speed = Math.sqrt(vx * vx + vy * vy + vz * vz);
    if (speed > 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

```tsx
function applyAttractor(
  velocities: Float32Array,
  positions: Float32Array,
  count: number,
  attractorPos: THREE.Vector3,
  strength: number,  // Positive = attract, negative = repel
  delta: number
) {
  for (let i = 0; i < count; i++) {
    const dx = attractorPos.x - positions[i * 3];
    const dy = attractorPos.y - positions[i * 3 + 1];
    const dz = attractorPos.z - positions[i * 3 + 2];
    
    const distSq = dx * dx + dy * dy + dz * dz;
    const dist = Math.sqrt(distSq);
    
    if (dist > 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

```tsx
function applyOrbitAttractor(
  velocities: Float32Array,
  positions: Float32Array,
  count: number,
  center: THREE.Vector3,
  orbitStrength: number,
  pullStrength: number,
  delta: number
) {
  for (let i = 0; i < count; i++) {
    const dx = positions[i * 3] - center.x;
    const dy = positions[i * 3 + 1] - center.y;
    const dz = positions[i * 3 + 2] - center.z;
    
    const dist = Math.sqrt(dx * dx + dy * dy + dz * dz);
    
    if (dist > 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

```tsx
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 < count; i++) {
    const px = positions[i * 3];
    const py = positions[i * 3 + 1];
    const pz = positions[i * 3 + 2];
    
    for (const attractor of attractors) {
      const dx = attractor.position.x - px;
      const dy = attractor.position.y - py;
      const dz = attractor.position.z - pz;
      
      const dist = Math.sqrt(dx * dx + dy * dy + dz * dz);
      
      if (dist > 0.1 && dist < attractor.radius) {
        // Smooth falloff within radius
        const falloff = 1 - dist / attractor.radius;
        const force = attractor.strength * falloff * falloff;
        
        velocities[i * 3] += (dx / dist) * force * delta;
        velocities[i * 3 + 1] += (dy / dist) * force * delta;
        velocities[i * 3 + 2] += (dz / dist) * force * delta;
      }
    }
  }
}
```

## Velocity Fields

### Curl Noise Field

```tsx
// In shader (GPU)
vec3 curlNoise(vec3 p) {
  const float e = 0.1;
  
  vec3 dx = vec3(e, 0.0, 0.0);
  vec3 dy = vec3(0.0, e, 0.0);
  vec3 dz = vec3(0.0, 0.0, e);
  
  float n1 = snoise(p + dy) - snoise(p - dy);
  float n2 = snoise(p + dz) - snoise(p - dz);
  float n3 = snoise(p + dx) - snoise(p - dx);
  float n4 = snoise(p + dz) - snoise(p - dz);
  float n5 = snoise(p + dx) - snoise(p - dx);
  float n6 = snoise(p + dy) - snoise(p - dy);
  
  return normalize(vec3(n1 - n2, n3 - n4, n5 - n6));
}

// Usage in vertex shader
vec3 velocity = curlNoise(position * 0.5 + uTime * 0.1);
position += velocity * delta;
```

### Flow Field (2D/3D Grid)

```tsx
class FlowField {
  private field: THREE.Vector3[];
  private resolution: number;
  private size: number;
  
  constructor(resolution: number, size: number) {
    this.resolution = resolution;
    this.size = size;
    this.field = [];
    
    for (let i = 0; i < resolution ** 3; i++) {
      this.field.push(new THREE.Vector3());
    }
  }
  
  // Generate field from noise
  generate(time: number, scale: number) {
    for (let x = 0; x < this.resolution; x++) {
      for (let y = 0; y < this.resolution; y++) {
        for (let z = 0; z < this.resolution; z++) {
          const index = x + y * this.resolution + z * this.resolution * this.resolution;
          
          // Use noise to generate flow direction
          const wx = x / this.resolution * scale;
          const wy = y / this.resolution * scale;
          const wz = z / this.resolution * scale;
          
          const angle1 = noise3D(wx, wy, wz + time) * Math.PI * 2;
          const angle2 = noise3D(wx + 100, wy, wz + time) * Math.PI * 2;
          
          this.field[index].set(
            Math.cos(angle1) * Math.cos(angle2),
            Math.sin(angle2),
            Math.sin(angle1) * Math.cos(angle2)
          );
        }
      }
    }
  }
  
  // Sample field at position
  sample(position: THREE.Vector3): THREE.Vector3 {
    const halfSize = this.size / 2;
    
    const x = Math.floor(((position.x + halfSize) / this.size) * this.resolution);
    co