The "failed to merge bones" error in Unreal Engine typically occurs during the import process of a skeletal mesh when the engine cannot properly combine the bone hierarchies of the incoming mesh with the existing skeleton asset. Here are some common causes:
Incompatible Skeletons: The skeletal mesh being imported may have a bone hierarchy that doesn't match the target skeleton in Unreal Engine. Bones may have different names, missing parents, or additional bones that do not exist in the target skeleton.
Incorrect Bone Weights: Skinning issues where vertices are weighted to bones that don't exist in the target skeleton can lead to this error.
Import Settings: Sometimes, incorrect import settings can cause Unreal Engine to interpret the bone hierarchy incorrectly.
FBX Export Issues: Problems with the exported FBX file from 3D modeling software could result in an incompatible bone structure.
Software Version Mismatch: Using outdated or mismatched versions of Unreal Engine and the 3D modeling software can also lead to such errors.
Solution - Here's How To Resolve It
To resolve the "failed to merge bones" error, you can follow these troubleshooting steps:
Check Bone Names and Hierarchy: Ensure that the bone names and hierarchy in your 3D modeling software exactly match those of the target skeleton in Unreal Engine.
Re-skin Mesh: Make sure all vertices are appropriately skinned to valid bones, and no vertices are weighted to non-existent bones in the target skeleton.
Review Import Settings: During import, double-check settings related to skeletal meshes and rigs. Use the 'Skeleton' dropdown to select the correct skeleton and ensure 'Use T0 As Ref Pose' is checked if needed.
FBX Version Compatibility: Export the FBX file using a version compatible with your Unreal Engine version. Ensure that you are following best practices for exporting FBX files from your 3D modeling software.
Update Software: Update both Unreal Engine and your 3D modeling software to their latest versions to avoid compatibility issues.
If the error persists after attempting these solutions, consider consulting the documentation for Unreal Engine as well as any logs provided during the import process for additional insights into the specific cause of the error.