public static void Bake(Mesh mesh, Target target)
{
    var verts = mesh.vertices;
    var tris = mesh.triangles;
    var normals = mesh.normals;
    var tangents = mesh.tangents;
    if (tangents == null || tangents.Length != verts.Length)
    {
        mesh.RecalculateTangents();
        tangents = mesh.tangents;
    }

    // 1) 按位置分组：硬边处同一个位置有好几个顶点（各自带不同法线）
    var groups = new Dictionary<Vector3Int, Vector3>();
    Vector3Int Key(Vector3 p) => new Vector3Int(Mathf.RoundToInt(p.x * 1e4f), Mathf.RoundToInt(p.y * 1e4f), Mathf.RoundToInt(p.z * 1e4f));

    // 2) 累加每个三角形的面法线，用“顶点处的夹角”加权（比按面积加权更不受三角剖分影响）
    for (int t = 0; t < tris.Length; t += 3)
    {
        Vector3 p0 = verts[tris[t]], p1 = verts[tris[t + 1]], p2 = verts[tris[t + 2]];
        Vector3 fn = Vector3.Cross(p1 - p0, p2 - p0).normalized;
        for (int k = 0; k < 3; k++)
        {
            Vector3 a = verts[tris[t + k]], b = verts[tris[t + (k + 1) % 3]], c = verts[tris[t + (k + 2) % 3]];
            float angle = Vector3.Angle(b - a, c - a) * Mathf.Deg2Rad;
            var key = Key(a);
            groups.TryGetValue(key, out var sum);
            groups[key] = sum + fn * angle;
        }
    }

    // 3) 写回
    var outTangents = new Vector4[verts.Length];
    var outColors = new Color[verts.Length];
    var oldColors = mesh.colors;
    for (int i = 0; i < verts.Length; i++)
    {
        Vector3 smooth = groups[Key(verts[i])].normalized;
        if (target == Target.Tangent)
        {
            outTangents[i] = new Vector4(smooth.x, smooth.y, smooth.z, 1);
        }
        else
        {
            // 物体空间 → 该顶点自己的切线空间（T, B, N）
            Vector3 n = normals[i];
            Vector3 tg = tangents[i];
            Vector3 bt = Vector3.Cross(n, tg) * tangents[i].w;
            var ts = new Vector3(Vector3.Dot(smooth, tg), Vector3.Dot(smooth, bt), Vector3.Dot(smooth, n));
            float alpha = oldColors != null && oldColors.Length == verts.Length ? oldColors[i].a : 1f;  // A 通道留给线宽遮罩
            outColors[i] = new Color(ts.x * 0.5f + 0.5f, ts.y * 0.5f + 0.5f, ts.z * 0.5f + 0.5f, alpha);
        }
    }
    if (target == Target.Tangent) mesh.tangents = outTangents;
    else mesh.colors = outColors;
}

[MenuItem("Cookbook/Ch14/Bake Smooth Normals (Tangent)")]
static void BakeToTangent() => BakeSelection(Target.Tangent);

[MenuItem("Cookbook/Ch14/Bake Smooth Normals (Vertex Color)")]
static void BakeToColor() => BakeSelection(Target.VertexColor);

static void BakeSelection(Target target)
{
    foreach (var obj in Selection.objects)
    {
        string path = AssetDatabase.GetAssetPath(obj);
        var meshes = new List<Mesh>();
        if (obj is Mesh m) meshes.Add(m);
        else foreach (var a in AssetDatabase.LoadAllAssetsAtPath(path)) if (a is Mesh mm) meshes.Add(mm);

        foreach (var src in meshes)
        {
            var copy = Object.Instantiate(src);   // 不改原始模型，另存一份
            copy.name = src.name + "_SmoothNormals";
            Bake(copy, target);
            string dir = System.IO.Path.GetDirectoryName(path);
            string outPath = AssetDatabase.GenerateUniqueAssetPath($"{dir}/{copy.name}.asset");
            AssetDatabase.CreateAsset(copy, outPath);
            Debug.Log($"[Ch14] 平滑法线已烘焙到 {target}：{outPath}");
        }
    }
    AssetDatabase.SaveAssets();
}

[MenuItem("Cookbook/Ch14/Bake Smooth Normals (Tangent)", true)]
[MenuItem("Cookbook/Ch14/Bake Smooth Normals (Vertex Color)", true)]
static bool Validate() => Selection.objects.Length > 0;