Handling Root Motion in Three.js Animations

Based on your issue with Mixamo animations that include root transforms, I can provide some guidance on how to handle this in Three.js.

The problem you're experiencing is common when working with animations that include root motion - as the weight decreases during blending, the model returns to its original position because the root transform's influence is being reduced.

Possible Solutions

1. Using AnimationObjectGroup

One approach is to use the AnimationObjectGroup class, which allows you to treat multiple objects as a single entity for animation purposes:

JAVASCRIPTCopied
// Create an animation object group
const animGroup = new THREE.AnimationObjectGroup(skinnedMesh, parentObject);

// Create the mixer with the group instead of just the mesh
const mixer = new THREE.AnimationMixer(animGroup);

// Create your action as normal
const action = mixer.clipAction(animationClip);

2. Extracting Root Motion

Another approach is to extract the root motion from the animation and apply it separately:

JAVASCRIPTCopied
function extractRootMotion(mixer, action, skinnedMesh, parentObject) {
    // Get the root bone
    const rootBone = skinnedMesh.skeleton.bones[0]; // Assuming first bone is root
    
    // Store original position before update
    const originalPosition = rootBone.position.clone();
    const originalQuaternion = rootBone.quaternion.clone();
    
    // Update animation
    mixer.update(0.016);
    
    // Calculate delta
    const deltaPosition = rootBone.position.clone().sub(originalPosition);
    
    // Apply delta to parent object
    parentObject.position.add(deltaPosition);
    
    // For rotation, you would need to handle quaternion differences
    // This is more complex and might require additional logic
}

3. Using AnimationUtils

You might also consider using AnimationUtils to modify your animation clips before using them:

JAVASCRIPTCopied
// Clone the clip to avoid modifying the original
const modifiedClip = THREE.AnimationUtils.clone(originalClip);

// Process the tracks to redirect root motion
// This would require custom logic to identify and modify root tracks

Recommended Approach

The most robust solution would likely involve:

1. Identifying the tracks in your animation clip that affect the root bone
2. Creating a system that captures the final state of an animation before blending out
3. Applying that final state to your parent object's transform
4. Then allowing the skinned mesh animation to blend normally

This is a complex problem that often requires a custom solution tailored to your specific animation setup. You might need to implement a state machine that tracks animation transitions and preserves positional/rotational changes.

Let me know if you'd like more specific guidance on any of these approaches!