Troubleshooting Eye Mesh Animation in Skinned Models

Based on your description, you're experiencing an issue where the eye meshes aren't following their associated bones in your skinned model, despite the bones themselves moving correctly. This is a common issue when working with complex skinned meshes that share skeletons.

Potential Causes and Solutions

1. Bind Matrix Issues

When multiple skinned meshes share the same skeleton, the bind matrices need to be correctly set for each mesh. The bind matrix defines the relationship between the mesh and the skeleton in the rest pose.

JAVASCRIPTCopied
// After loading your model, you might need to update the bind matrix for the eye meshes
eyeMesh.bindMatrix.copy(eyeMesh.matrixWorld);
eyeMesh.bindMatrixInverse.copy(eyeMesh.bindMatrix).invert();

2. Skeleton Sharing Implementation

Make sure both meshes are properly sharing the same skeleton instance:

JAVASCRIPTCopied
// Ensure both meshes reference the same skeleton
eyeMesh.skeleton = bodyMesh.skeleton;

3. Bone Influence Verification

Even though you've verified the skinIndex and skinWeight attributes, double-check that the weights are significant enough:

JAVASCRIPTCopied
// Log the weights for the eye bones to verify they have sufficient influence
const eyeRBoneIndex = skeleton.bones.findIndex(bone => bone.name === 'DEF-eyeR');
const eyeLBoneIndex = skeleton.bones.findIndex(bone => bone.name === 'DEF-eyeL');

// Check if these indices appear in the skinIndex attribute with meaningful weights

4. Update Matrix World Chain

Ensure the entire transformation chain is being updated:

JAVASCRIPTCopied
// Make sure to update the world matrices after any transformations
model.updateWorldMatrix(true, true);

5. Animation System Conflicts

If you're using multiple animation systems or mixing shape keys with skeletal animation, they might be conflicting:

JAVASCRIPTCopied
// Make sure your animation mixer is properly set up
const mixer = new THREE.AnimationMixer(model);
const action = mixer.clipAction(animations[0]);
action.play();

// Update in your animation loop
mixer.update(delta);

6. Parent-Child Relationship

Check if the eye meshes are properly parented in the scene hierarchy:

JAVASCRIPTCopied
// The eyes should be siblings to the body mesh, not children of it
scene.add(bodyMesh);
scene.add(eyeMesh);

Next Steps

1. Examine the bone hierarchy in your model to ensure the eye bones are properly connected to the head/skull bones
2. Try isolating the eye meshes and their bones in a separate test to verify they work independently
3. Check if there are any transformations being applied to the eye meshes that might override the skeletal animation

Without being able to examine your specific model file, these are general approaches to troubleshooting this type of issue with skinned meshes in Three.js.