Unity3D - Model postprocessor for fixing blend shapes normals and tangents

MeshPostprocessor.cs

// Place inside Editor folder

using UnityEditor;
using UnityEngine;

namespace Code.Editor
{
    public class MeshPostprocessor : AssetPostprocessor
    {
        private void OnPostprocessModel(GameObject g)
        {
            var smrs = g.GetComponentsInChildren<SkinnedMeshRenderer>();
            for (int i = 0; i < smrs.Length; i++)
            {
                FixShapes(smrs[i]);
            }
        }

        
        
        private struct ShapeData
        {
            public string Name;
            public float Weight;
            public Vector3[] DeltaVertices;
            public Vector3[] DeltaNormals;
            public Vector3[] DeltaTangents;
        }

        private static void FixShapes(SkinnedMeshRenderer smr)
        {
            // Looks OK, but maybe should be increased
            const float eps = float.Epsilon;
            
            var mesh = smr.sharedMesh;

            var vertexCount = mesh.vertexCount;
            var shapeCount = mesh.blendShapeCount;
            var shapeDataArray = new ShapeData[shapeCount];
        
            for (int i = 0; i < shapeCount; i++)
            {
                shapeDataArray[i].Name = mesh.GetBlendShapeName(i);
                // Add support for multiple frames in one shape?
                shapeDataArray[i].Weight = mesh.GetBlendShapeFrameWeight(i, 0);
                shapeDataArray[i].DeltaVertices = new Vector3[vertexCount];
                shapeDataArray[i].DeltaNormals  = new Vector3[vertexCount];
                shapeDataArray[i].DeltaTangents = new Vector3[vertexCount];
                mesh.GetBlendShapeFrameVertices(i, 0, shapeDataArray[i].DeltaVertices, shapeDataArray[i].DeltaNormals, shapeDataArray[i].DeltaTangents);
            
                // Clean "empty" vertices
                for (int j = 0; j < vertexCount; j++)
                {
                    if (Vector3.SqrMagnitude(shapeDataArray[i].DeltaVertices[j]) < eps)
                    {
                        shapeDataArray[i].DeltaVertices[j] = Vector3.zero;
                        shapeDataArray[i].DeltaNormals [j] = Vector3.zero;
                        shapeDataArray[i].DeltaTangents[j] = Vector3.zero;
                    }
                }
            }

            mesh.ClearBlendShapes();
        
            // Add all shapes back
            for (int i = 0; i < shapeDataArray.Length; i++)
            {
                mesh.AddBlendShapeFrame(shapeDataArray[i].Name, shapeDataArray[i].Weight, shapeDataArray[i].DeltaVertices, shapeDataArray[i].DeltaNormals, shapeDataArray[i].DeltaTangents);
            }
        }
        
    }
}

Pic: blend shape moves vertices on body, hand vertices shouldn't change

This postprocessor can help fix blend shape artifacts. Basically if you import model with blend shape, some vertices with zero changes will generate garbage data, resulting in incorrect normals and tangents.

If you still having this artifact, try to increasing "eps" value