Threejs Animation

# Three.js Animation

## Quick Start

```javascript
import * as THREE from "three";

// Simple procedural animation
const clock = new THREE.Clock();

function animate() {
  const delta = clock.getDelta();
  const elapsed = clock.getElapsedTime();

  mesh.rotation.y += delta;
  mesh.position.y = Math.sin(elapsed) * 0.5;

  requestAnimationFrame(animate);
  renderer.render(scene, camera);
}
animate();
```

## Animation System Overview

Three.js animation system has three main components:

1. **AnimationClip** - Container for keyframe data
2. **AnimationMixer** - Plays animations on a root object
3. **AnimationAction** - Controls playback of a clip

## AnimationClip

Stores keyframe animation data.

```javascript
// Create animation clip
const times = [0, 1, 2]; // Keyframe times (seconds)
const values = [0, 1, 0]; // Values at each keyframe

const track = new THREE.NumberKeyframeTrack(
  ".position[y]", // Property path
  times,
  values,
);

const clip = new THREE.AnimationClip("bounce", 2, [track]);
```

### KeyframeTrack Types

```javascript
// Number track (single value)
new THREE.NumberKeyframeTrack(".opacity", times, [1, 0]);
new THREE.NumberKeyframeTrack(".material.opacity", times, [1, 0]);

// Vector track (position, scale)
new THREE.VectorKeyframeTrack(".position", times, [
  0,
  0,
  0, // t=0
  1,
  2,
  0, // t=1
  0,
  0,
  0, // t=2
]);

// Quaternion track (rotation)
const q1 = new THREE.Quaternion().setFromEuler(new THREE.Euler(0, 0, 0));
const q2 = new THREE.Quaternion().setFromEuler(new THREE.Euler(0, Math.PI, 0));
new THREE.QuaternionKeyframeTrack(
  ".quaternion",
  [0, 1],
  [q1.x, q1.y, q1.z, q1.w, q2.x, q2.y, q2.z, q2.w],
);

// Color track
new THREE.ColorKeyframeTrack(".material.color", times, [
  1,
  0,
  0, // red
  0,
  1,
  0, // green
  0,
  0,
  1, // blue
]);

// Boolean track
new THREE.BooleanKeyframeTrack(".visible", [0, 0.5, 1], [true, false, true]);

// String track (for morph targets)
new THREE.StringKeyframeTrack(
  ".morphTargetInfluences[smile]",
  [0, 1],
  ["0", "1"],
);
```

### Interpolation Modes

```javascript
const track = new THREE.VectorKeyframeTrack(".position", times, values);

// Interpolation
track.setInterpolation(THREE.InterpolateLinear); // Default
track.setInterpolation(THREE.InterpolateSmooth); // Cubic spline
track.setInterpolation(THREE.InterpolateDiscrete); // Step function
```

## AnimationMixer

Plays animations on an object and its descendants.

```javascript
const mixer = new THREE.AnimationMixer(model);

// Create action from clip
const action = mixer.clipAction(clip);
action.play();

// Update in animation loop
function animate() {
  const delta = clock.getDelta();
  mixer.update(delta); // Required!

  requestAnimationFrame(animate);
  renderer.render(scene, camera);
}
```

### Mixer Events

```javascript
mixer.addEventListener("finished", (e) => {
  console.log("Animation finished:", e.action.getClip().name);
});

mixer.addEventListener("loop", (e) => {
  console.log("Animation looped:", e.action.getClip().name);
});
```

## AnimationAction

Controls playback of an animation clip.

```javascript
const action = mixer.clipAction(clip);

// Playback control
action.play();
action.stop();
action.reset();
action.halt(fadeOutDuration);

// Playback state
action.isRunning();
action.isScheduled();

// Time control
action.time = 0.5; // Current time
action.timeScale = 1; // Playback speed (negative = reverse)
action.paused = false;

// Weight (for blending)
action.weight = 1; // 0-1, contribution to final pose
action.setEffectiveWeight(1);

// Loop modes
action.loop = THREE.LoopRepeat; // Default: loop forever
action.loop = THREE.LoopOnce; // Play once and stop
action.loop = THREE.LoopPingPong; // Alternate forward/backward
actio