threejs-impl-animation

Create one AnimationMixer with the loaded gltf.scene as root, then call mixer.clipAction(clip).play() for each clip in gltf.animations. You must call mixer.update(clock.getDelta()) every frame in the render loop or animations stay frozen.

Call reset() on the incoming action, set its timeScale and weight to 1, then use crossFadeFrom(currentAction, duration, true) followed by play(). Store the current action reference so the next transition fades from the correct source.

The most common cause is a missing mixer.update(delta) call in the render loop; the mixer does not advance time on its own. Also verify you used THREE.Clock for delta rather than a fixed value, and that play() was called once, not every frame.

Without clampWhenFinished = true, a LoopOnce action resets to its initial state when it completes. Set action.clampWhenFinished = true before calling play() so the model holds the final pose.

No, multiple mixers on one root object cause conflicting property writes, producing jittering or broken animation. Use a single mixer per model and blend clips with weight and crossfade methods instead.

Create a NumberKeyframeTrack targeting 'meshName.material.opacity' and add it to an AnimationClip. You must set material.transparent = true first, otherwise opacity values below 1 have no visual effect during rendering.