GLTFExporter and FBXLoader Morph Target Mismatch Issue

You've identified a key incompatibility between how the FBXLoader creates morph target animation tracks and how the GLTFExporter expects them to be structured.

The Issue

The problem occurs because:

1. FBXLoader creates morph target animation tracks using numeric indices in the track name:

   JAVASCRIPTCopied
   return new NumberKeyframeTrack(
     rawTracks.modelName + '.morphTargetInfluences[' + morphNum + ']', 
     curves.times, 
     values
   );

2. GLTFExporter expects the track's property index to be a string name that exists in the morphTargetDictionary:

   JAVASCRIPTCopied
   const targetIndex = sourceTrackNode.morphTargetDictionary[sourceTrackBinding.propertyIndex];
   
   if (targetIndex === undefined) {
     throw new Error('THREE.GLTFExporter: Morph target name not found: ' + sourceTrackBinding.propertyIndex);
   }

When the exporter tries to look up a numeric index (like 0) in the morphTargetDictionary (which contains named keys like browDownLeft), it fails to find a match and throws the error.

Workaround Solution

You can create a pre-processing step before exporting to fix the animation tracks:

JAVASCRIPTCopied
function fixMorphTargetTracks(animations, object) {
  animations.forEach(animation => {
    animation.tracks = animation.tracks.map(track => {
      // Check if this is a morph target track
      if (track.name.includes('.morphTargetInfluences[')) {
        const parts = track.name.split('.morphTargetInfluences[');
        const objectName = parts[0];
        const indexStr = parts[1].replace(']', '');
        const index = parseInt(indexStr);
        
        // Find the target object
        const targetObject = object.getObjectByName(objectName);
        
        if (targetObject && targetObject.morphTargetDictionary) {
          // Find the morph target name that corresponds to this index
          const morphName = Object.keys(targetObject.morphTargetDictionary)
            .find(key => targetObject.morphTargetDictionary[key] === index);
          
          if (morphName) {
            // Create a new track with the name instead of the index
            return new THREE.NumberKeyframeTrack(
              `${objectName}.morphTargetInfluences[${morphName}]`,
              track.times,
              track.values
            );
          }
        }
      }
      return track;
    });
  });
  
  return animations;
}

// Usage before exporting:
const fixedAnimations = fixMorphTargetTracks(animations, scene);
gltfExporter.parse(scene, function(gltf) {
  // Export successful
}, {
  animations: fixedAnimations,
  // other options...
});

This is indeed a mismatch in the expected formats between the two components. The FBXLoader creates tracks with numeric indices, while the GLTFExporter expects named indices that match keys in the morphTargetDictionary.

This issue highlights a design inconsistency in the Three.js addons that could be addressed in future versions of the library.