AssetPostProcessorReorderBones.cs

using UnityEngine;
using UnityEditor;
using System.Collections.Generic;
using System.Linq;
 
#if UNITY_EDITOR
 
public class AssetPostProcessorReorderBones : AssetPostprocessor
{
    void OnPostprocessModel(GameObject g)
    {
        this.Process(g);
    }
 
    void Process(GameObject g)
    {
        SkinnedMeshRenderer smr = g.GetComponentInChildren<SkinnedMeshRenderer>();
        if (smr == null)
        {
            Debug.LogWarning("Unable to find Renderer" + smr.name);
            return;
        }
 
        //list of bones
        List<Transform> boneTransforms = smr.bones.ToList();
 
        //sort based on hierarchy
        boneTransforms.Sort(CompareTransform);
 
        //record bone index mappings (richardf advice)
        //build a Dictionary<int, int> that records the old bone index => new bone index mappings,
        //then run through every vertex and just do boneIndexN = dict[boneIndexN] for each weight on each vertex.
        var remap = new Dictionary<int, int>();
        for (int i = 0; i < smr.bones.Length; i++)
        {
            remap[i] = boneTransforms.IndexOf(smr.bones[i]);
        }
 
        //remap bone weight indexes
        BoneWeight[] bw = smr.sharedMesh.boneWeights;
        for (int i = 0; i < bw.Length; i++)
        {
            bw[i].boneIndex0 = remap[bw[i].boneIndex0];
            bw[i].boneIndex1 = remap[bw[i].boneIndex1];
            bw[i].boneIndex2 = remap[bw[i].boneIndex2];
            bw[i].boneIndex3 = remap[bw[i].boneIndex3];
        }
 
        //remap bindposes
        var bp = new Matrix4x4[smr.sharedMesh.bindposes.Length];
        for (int i = 0; i < bp.Length; i++)
        {
            bp[remap[i]] = smr.sharedMesh.bindposes[i];
        }
 
        //assign new data
        smr.bones = boneTransforms.ToArray();
        smr.sharedMesh.boneWeights = bw;
        smr.sharedMesh.bindposes = bp;
    }
 
    private static int CompareTransform(Transform A, Transform B)
    {
        if (B.IsChildOf(A))
        {
            return -1;
        }
 
        if (A.IsChildOf(B))
        {
            return -1;
        }
 
        return 0;
    }
}
 
#endif