I've done animations mostly for Unity and there isn't an exact standard way you need to name bones or other elements. I always name the first bone in the rig's hierarchy "root" and it's good practice to do so. However the root bone is always under the rig object, and is not the top-most in file's hierarchy. Above it is the rig object that houses the bones. Beside the rig are the meshes.
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*character_file *mesh_body *mesh_head *mesh_clothes *rig *root *hips *legs *etc.
In a general-purpose engine like Unity, the bone for root-motion can be configured through the editor's interface. For specialized OpenMW, this probably won't be customizable so a strict requirement for naming and hierarchy is needed. Perhaps saying there needs to be a root bone that is immediate child of the rig, armature, skeleton object.