Animations and Tweens in Decentraland

Authoring split

Animator, Tween, and TweenSequence are all supported in main-entities.ts — declare the entity, its visual components, and its initial animation state in the literal. Switch clips or replace tweens at runtime in src/index.ts via getMutable.

When to Use Which Animation Approach

| Need | Approach | When | |------|----------|------| | Play animation baked into a .glb model | Animator | Character walks, door opens, flag waves — any animation created in Blender/Maya | | Move/rotate/scale an entity smoothly | Tween | Sliding doors, floating platforms, growing objects — procedural A-to-B motion | | Chain multiple animations in sequence | TweenSequence | Patrol paths, multi-step doors, complex choreography | | Continuous per-frame control | engine.addSystem() | Physics-like motion, following a target, custom easing |

Decision flow:

1. Does the .glb model already have the animation? → Animator
2. Is it a simple move/rotate/scale between two values? → Tween
3. Do you need frame-by-frame control or custom math? → System with dt

GLTF Animations (Animator)

Declare the character and its animation states in main-entities.ts. The states array is part of the Animator shape and is JSON-compatible.

// main-entities.ts
import type { Scene } from '@dcl/sdk/scene-types'

export const scene = {
  character: {
    components: {
      Transform: { position: { x: 8, y: 0, z: 8 } },
      GltfContainer: { src: 'models/character.glb' },
      Animator: {
        states: [
          { clip: 'idle', playing: true, loop: true, speed: 1 },
          { clip: 'walk', playing: false, loop: true, speed: 1 },
          { clip: 'attack', playing: false, loop: false, speed: 1.5 }
        ]
      }
    }
  }
} satisfies Scene

Trigger and switch animations at runtime in src/index.ts:

import { engine, Animator } from '@dcl/sdk/ecs'

export function main() {
  const character = engine.getEntityOrNullByName('character')
  if (!character) return

  // Play a specific animation
  Animator.playSingleAnimation(character, 'walk')

  // Stop all animations
  // Animator.stopAllAnimations(character)
}

Switching Animations

import { Entity, Animator } from '@dcl/sdk/ecs'

function playAnimation(entity: Entity, clipName: string) {
  const animator = Animator.getMutable(entity)
  for (const state of animator.states) state.playing = false
  const state = animator.states.find(s => s.clip === clipName)
  if (state) state.playing = true
}

Tweens

Tween and TweenSequence are JSON-compatible. The mode is a $case-tagged union; positions/quaternions are plain object literals; easing is a numeric enum.

Move

// main-entities.ts
sliding_box: {
  components: {
    Transform: { position: { x: 2, y: 1, z: 8 } },
    MeshRenderer: { mesh: { $case: 'box', box: { uvs: [] } } },
    Tween: {
      duration: 2000,  // milliseconds
      easingFunction: 6,  // EasingFunction.EF_EASESINE
      mode: {
        $case: 'move',
        move: { start: { x: 2, y: 1, z: 8 }, end: { x: 14, y: 1, z: 8 } }
      }
    }
  }
}

Common easing values: 0 = EF_LINEAR, 1 = EF_EASEINQUAD, 2 = EF_EASEOUTQUAD, 3 = EF_EASEQUAD, 6 = EF_EASESINE. To replace or stop a tween at runtime, use Tween.createOrReplace / Tween.deleteFrom on the named entity in src/index.ts.

Rotate

// main-entities.ts — quaternions are { x, y, z, w } literals.
// 360° around Y end-state is { x: 0, y: 1, z: 0, w: 0 }
spinning_obj: {
  components: {
    Transform: { position: { x: 8, y: 1, z: 8 } },
    MeshRenderer: { mesh: { $case: 'box', box: { uvs: [] } } },
    Tween: {
      duration: 3000,
      easingFunction: 0,  // EF_LINEAR
      mode: {
        $case: 'rotate',
        rotate: {
          start: { x: 0, y: 0, z: 0, w: 1 },
          end: { x: 0, y: 1, z: 0, w: 0 }
        }
      }
    }
  }
}

For exact-degree rotations without hand-computing quaternions, leave the tween out of main-entities.ts and create it at runtime in src/index.ts where Quaternion.fromEulerDegrees() is available:

// src/index.ts — runtime tween authoring (helpers allowed)
import { engine, Tween, EasingFunction } from '@dcl/sdk/ecs'
import { Quaternion } from '@dcl/sdk/math'

export function main() {
  const box = engine.getEntityOrNullByName('spinning_obj')
  if (!box) return
  Tween.createOrReplace(box, {
    mode: Tween.Mode.Rotate({
      start: Quaternion.fromEulerDegrees(0, 0, 0),
      end: Quaternion.fromEulerDegrees(0, 360, 0)
    }),
    duration: 3000,
    easingFunction: EasingFunction.EF_LINEAR
  })
}

Scale

// main-entities.ts
grow_box: {
  components: {
    Transform: { position: { x: 8, y: 1, z: 8 } },
    MeshRenderer: { mesh: { $case: 'box', box: { uvs: [] } } },
    Tween: {
      duration: 1000,
      easingFunction: 14,  // EF_EASEOUTBOUNCE
      mode: {
        $case: 'scale',
        scale: { start: { x: 1, y: 1, z: 1 }, end: { x: 2, y: 2, z: 2 } }
      }
    }
  }
}

Tween Sequences (Chained Animations)

Chain multiple tweens to play one after another. TweenSequence is supported in main-entities.ts:

// main-entities.ts
patrol_box: {
  components: {
    Transform: { position: { x: 2, y: 1, z: 8 } },
    MeshRenderer: { mesh: { $case: 'box', box: { uvs: [] } } },
    Tween: {
      duration: 2000,
      easingFunction: 6,  // EF_EASESINE
      mode: { $case: 'move', move: { start: { x: 2, y: 1, z: 8 }, end: { x: 14, y: 1, z: 8 } } }
    },
    TweenSequence: {
      loop: 0,  // TweenLoop.TL_RESTART (1 = TL_YOYO)
      sequence: [
        {
          duration: 2000,
          easingFunction: 6,
          mode: { $case: 'move', move: { start: { x: 14, y: 1, z: 8 }, end: { x: 2, y: 1, z: 8 } } }
        }
      ]
    }
  }
}

Easing Functions

Inside main-entities.ts use the integer values (left). In src/index.ts you can reference EasingFunction.<NAME> directly.

| value | enum | shape | |---|---|---| | 0 | EF_LINEAR | Constant speed | | 1 | EF_EASEINQUAD | Quadratic in | | 2 | EF_EASEOUTQUAD | Quadratic out | | 3 | EF_EASEQUAD | Quadratic in-out | | 4 | EF_EASEINSINE | Sinusoidal in | | 5 | EF_EASEOUTSINE | Sinusoidal out | | 6 | EF_EASESINE | Sinusoidal in-out (smooth) | | 7 | EF_EASEINEXPO | Exponential in | | 8 | EF_EASEOUTEXPO | Exponential out | | 9 | EF_EASEEXPO | Exponential in-out | | 10 | EF_EASEINELASTIC | Elastic in | | 11 | EF_EASEOUTELASTIC | Elastic out | | 12 | EF_EASEELASTIC | Elastic in-out | | 13 | EF_EASEINBOUNCE | Bounce in | | 14 | EF_EASEOUTBOUNCE | Bounce out | | 15 | EF_EASEBOUNCE | Bounce in-out | | 16 | EF_EASEINCUBIC | Cubic in | | 17 | EF_EASEOUTCUBIC | Cubic out | | 18 | EF_EASECUBIC | Cubic in-out | | 19 | EF_EASEINQUART | Quartic in | | 20 | EF_EASEOUTQUART | Quartic out | | 21 | EF_EASEQUART | Quartic in-out | | 22 | EF_EASEINQUINT | Quintic in | | 23 | EF_EASEOUTQUINT | Quintic out | | 24 | EF_EASEQUINT | Quintic in-out | | 25 | EF_EASEINCIRC | Circular in | | 26 | EF_EASEOUTCIRC | Circular out | | 27 | EF_EASECIRC | Circular in-out | | 28 | EF_EASEINBACK | Overshoot in | | 29 | EF_EASEOUTBACK | Overshoot out | | 30 | EF_EASEBACK | Overshoot in-out |

Custom Animation Systems

For complex animations, create a system:

// Continuous rotation system
function spinSystem(dt: number) {
  for (const [entity] of engine.getEntitiesWith(Transform, Spinner)) {
    const transform = Transform.getMutable(entity)
    const spinner = Spinner.get(entity)
    // Rotate around Y axis
    const currentRotation = Quaternion.toEulerAngles(transform.rotation)
    transform.rotation = Quaternion.fromEulerDegrees(
      currentRotation.x,
      currentRotation.y + spinner.speed * dt,
      currentRotation.z
    )
  }
}

engine.addSystem(spinSystem)

Tween Helper Methods

Use shorthand helpers that create or replace the Tween component directly on the entity:

import { Tween, EasingFunction } from '@dcl/sdk/ecs'

// Move — signature: Tween.setMove(entity, start, end, duration, easingFunction?)
Tween.setMove(entity,
  Vector3.create(0, 1, 0), Vector3.create(0, 3, 0),
  1500, EasingFunction.EF_EASEINBOUNCE
)

// Rotate — signature: Tween.setRotate(entity, start, end, duration, easingFunction?)
Tween.setRotate(entity,
  Quaternion.fromEulerDegrees(0, 0, 0), Quaternion.fromEulerDegrees(0, 180, 0),
  2000, EasingFunction.EF_EASEOUTQUAD
)

// Scale — signature: Tween.setScale(entity, start, end, duration, easingFunction?)
Tween.setScale(entity,
  Vector3.One(), Vector3.create(2, 2, 2),
  1000, EasingFunction.EF_LINEAR
)

// Combined Move + Rotate + Scale in one Tween
Tween.setMoveRotateScale(entity, {
  positionStart: Vector3.create(0, 0, 0), positionEnd: Vector3.create(0, 5, 0),
  rotationStart: Quaternion.fromEulerDegrees(0, 0, 0), rotationEnd: Quaternion.fromEulerDegrees(0, 360, 0),
  scaleStart: Vector3.One(), scaleEnd: Vector3.create(0.5, 0.5, 0.5)
}, 2000, EasingFunction.EF_EASEINOUTCUBIC)

// Continuous spin — applies a per-frame Quaternion delta. No end state.
Tween.setRotateContinuous(entity, Quaternion.fromEulerDegrees(0, -1, 0), 700)

// Continuous move — applies a per-frame Vector3 delta scaled by speed.
Tween.setMoveContinuous(entity, Vector3.create(0, 0, 1), 0.5)

*Continuous variants don't end — they apply a constant delta until the Tween is removed (Tween.deleteFrom(entity)).

Yoyo Loop Mode

TL_YOYO reverses the tween at each end instead of restarting:

TweenSequence.create(entity, {
  sequence: [{ duration: 1000, ... }],
  loop: TweenLoop.TL_YOYO
})

Detecting Tween Completion

Use tweenSystem.tweenCompleted() to check if a tween finished this frame:

engine.addSystem(() => {
  if (tweenSystem.tweenCompleted(entity)) {
    console.log('Tween finished on', entity)
  }
})

Animator Extras

Additional Animator features:

// Get a specific clip to modify
const clip = Animator.getClip(entity, 'Walk')

// shouldReset: restart animation from beginning when re-triggered
Animator.playSingleAnimation(entity, 'Attack', true) // resets to start

// weight: blend between animations (0.0 to 1.0)
const anim = Animator.getMutable(entity)
anim.states[0].weight = 0.5  // blend walk at 50%
anim.states[1].weight = 0.5  // blend idle at 50%

Morph Targets (Blend Shapes)

If a GLTF model exports morph targets (e.g., facial expressions, shape blends), drive them via the weights array on each Animator.state. Indices match the order morph targets were exported. Values range 0–1.

const anim = Animator.getMutable(entity)
const state = anim.states.find(s => s.clip === 'Idle')
if (state) {
  state.weights = state.weights ?? []
  state.weights[0] = 0.8   // morph target 0 at 80%
  state.weights[1] = 0.3   // morph target 1 at 30%
}

Troubleshooting

| Problem | Cause | Solution | |---------|-------|----------| | GLTF animation not playing | Wrong clip name in Animator.states | Open the .glb in a viewer (e.g., Blender) to find exact clip names — they are case-sensitive | | Animator component has no effect | Entity missing GltfContainer | Animator only works on entities that have a loaded GLTF model | | Tween doesn't move | Start and end positions are the same | Verify start and end values differ in Tween.Mode.Move() | | Tween plays once then stops | No TweenSequence with loop | Add TweenSequence.create(entity, { sequence: [], loop: TweenLoop.TL_YOYO }) for back-and-forth | | Animation jitters or stutters | Creating new Tween every frame | Only create Tween once, not inside a system — use tweenSystem.tweenCompleted() to chain |

Need 3D models to animate? See the add-3d-models skill for loading GLTF models that contain animation clips.

Best Practices

• Use Tweens for simple A-to-B animations (doors, platforms, UI elements)
• Use Animator for character/model animations baked into GLTF files
• Use Systems for continuous or physics-based animations
• Tween durations are in milliseconds (1000 = 1 second)
• Combine move + rotate tweens by applying them to parent/child entities
• For looping: use TweenSequence with loop: TweenLoop.TL_RESTART