threejs-animation

name	threejs-animation
allowed-tools	Read, Glob
description	Three.js animation - keyframe animation, skeletal animation, morph targets, animation mixing. Use when animating objects, playing GLTF animations, creating procedural motion, or blending animations.

Three.js Animation

Iron Law

Always call mixer.update(delta) in the animation loop — the AnimationMixer does not advance on its own, even when actions are playing.

Quick Start

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.

// 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

// 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

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.

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

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.

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
action.repetitions = 3; // Number of loops (Infinity default)

// Clamping
action.clampWhenFinished = true; // Hold last frame when done

// Blending
action.blendMode = THREE.NormalAnimationBlendMode;
action.blendMode = THREE.AdditiveAnimationBlendMode;

Fade In/Out

// Fade in
action.reset().fadeIn(0.5).play();

// Fade out
action.fadeOut(0.5);

// Crossfade between animations
const action1 = mixer.clipAction(clip1);
const action2 = mixer.clipAction(clip2);

action1.play();

// Later, crossfade to action2
action1.crossFadeTo(action2, 0.5, true);
action2.play();

Loading GLTF Animations

Most common source of skeletal animations.

import { GLTFLoader } from "three/examples/jsm/loaders/GLTFLoader.js";

const loader = new GLTFLoader();
loader.load("model.glb", (gltf) => {
  const model = gltf.scene;
  scene.add(model);

  // Create mixer
  const mixer = new THREE.AnimationMixer(model);

  // Get all clips
  const clips = gltf.animations;
  console.log(
    "Available animations:",
    clips.map((c) => c.name),
  );

  // Play first animation
  if (clips.length > 0) {
    const action = mixer.clipAction(clips[0]);
    action.play();
  }

  // Play specific animation by name
  const walkClip = THREE.AnimationClip.findByName(clips, "Walk");
  if (walkClip) {
    mixer.clipAction(walkClip).play();
  }

  // Store mixer for update loop
  window.mixer = mixer;
});

// Animation loop
function animate() {
  const delta = clock.getDelta();
  if (window.mixer) window.mixer.update(delta);

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

Skeletal Animation

Skeleton and Bones

// Access skeleton from skinned mesh
const skinnedMesh = model.getObjectByProperty("type", "SkinnedMesh");
const skeleton = skinnedMesh.skeleton;

// Access bones
skeleton.bones.forEach((bone) => {
  console.log(bone.name, bone.position, bone.rotation);
});

// Find specific bone by name
const headBone = skeleton.bones.find((b) => b.name === "Head");
if (headBone) headBone.rotation.y = Math.PI / 4; // Turn head

// Skeleton helper
const helper = new THREE.SkeletonHelper(model);
scene.add(helper);

Programmatic Bone Animation

function animate() {
  const time = clock.getElapsedTime();

  // Animate bone
  const headBone = skeleton.bones.find((b) => b.name === "Head");
  if (headBone) {
    headBone.rotation.y = Math.sin(time) * 0.3;
  }

  // Update mixer if also playing clips
  mixer.update(clock.getDelta());
}

Bone Attachments

// Attach object to bone
const weapon = new THREE.Mesh(weaponGeometry, weaponMaterial);
const handBone = skeleton.bones.find((b) => b.name === "RightHand");
if (handBone) handBone.add(weapon);

// Offset attachment
weapon.position.set(0, 0, 0.5);
weapon.rotation.set(0, Math.PI / 2, 0);

Morph Targets

Blend between different mesh shapes.

// Morph targets are stored in geometry
const geometry = mesh.geometry;
console.log("Morph attributes:", Object.keys(geometry.morphAttributes));

// Access morph target influences
mesh.morphTargetInfluences; // Array of weights
mesh.morphTargetDictionary; // Name -> index mapping

// Set morph target by index
mesh.morphTargetInfluences[0] = 0.5;

// Set by name
const smileIndex = mesh.morphTargetDictionary["smile"];
mesh.morphTargetInfluences[smileIndex] = 1;

Animating Morph Targets

// Procedural
function animate() {
  const t = clock.getElapsedTime();
  mesh.morphTargetInfluences[0] = (Math.sin(t) + 1) / 2;
}

// With keyframe animation
const track = new THREE.NumberKeyframeTrack(
  ".morphTargetInfluences[smile]",
  [0, 0.5, 1],
  [0, 1, 0],
);
const clip = new THREE.AnimationClip("smile", 1, [track]);
mixer.clipAction(clip).play();

Animation Blending

Mix multiple animations together.

// Setup actions
const idleAction = mixer.clipAction(idleClip);
const walkAction = mixer.clipAction(walkClip);
const runAction = mixer.clipAction(runClip);

// Play all with different weights
idleAction.play();
walkAction.play();
runAction.play();

// Set initial weights
idleAction.setEffectiveWeight(1);
walkAction.setEffectiveWeight(0);
runAction.setEffectiveWeight(0);

// Blend based on speed
function updateAnimations(speed) {
  if (speed < 0.1) {
    idleAction.setEffectiveWeight(1);
    walkAction.setEffectiveWeight(0);
    runAction.setEffectiveWeight(0);
  } else if (speed < 5) {
    const t = speed / 5;
    idleAction.setEffectiveWeight(1 - t);
    walkAction.setEffectiveWeight(t);
    runAction.setEffectiveWeight(0);
  } else {
    const t = Math.min((speed - 5) / 5, 1);
    idleAction.setEffectiveWeight(0);
    walkAction.setEffectiveWeight(1 - t);
    runAction.setEffectiveWeight(t);
  }
}

Additive Blending

// Base pose
const baseAction = mixer.clipAction(baseClip);
baseAction.play();

// Additive layer (e.g., breathing)
const additiveAction = mixer.clipAction(additiveClip);
additiveAction.blendMode = THREE.AdditiveAnimationBlendMode;
additiveAction.play();

// Convert clip to additive
THREE.AnimationUtils.makeClipAdditive(additiveClip);

References

• references/advanced-patterns.md — Smooth damping, spring physics, oscillation patterns, animation utilities, custom interpolation, and performance optimization

See Also

• threejs-loaders - Loading animated GLTF models
• threejs-fundamentals - Clock and animation loop
• threejs-shaders - Vertex animation in shaders