UnityEditor上でMeshにUV2-UV4を追加する。

UVCreator.cs

using System.Linq;
using System.Text.RegularExpressions;
using UnityEditor;
using UnityEngine;

namespace Onoty3D
{
    enum UVType
    {
        //UV1,
        UV2,
        UV3,
        UV4,
    }
    public class UVCreator
    {
#if UNITY_EDITOR
        [MenuItem("Onoty3D/CreateUV2(from SkinnedMeshRenderer)")]
        private static void CreateUV2()
        {
            UVCreator.Create(UVType.UV2);
        }

        [MenuItem("Onoty3D/CreateUV3(from SkinnedMeshRenderer)")]
        private static void CreateUV3()
        {
            UVCreator.Create(UVType.UV3);
        }

        [MenuItem("Onoty3D/CreateUV4(from SkinnedMeshRenderer)")]
        private static void CreateUV4()
        {
            UVCreator.Create(UVType.UV4);
        }

        private static void Create(UVType uv)
        {
            var hasSkinnedMesh = false;
            var skinnedMeshRenderer = default(SkinnedMeshRenderer);

            if (Selection.gameObjects.Length != 0)
            {
                var gameObject = Selection.gameObjects[0];
                skinnedMeshRenderer = gameObject.GetComponentInChildren<SkinnedMeshRenderer>();
                if (skinnedMeshRenderer != null)
                {
                    hasSkinnedMesh = true;
                }
            }

            //選択したオブジェクトにSkinnedMeshRendererが無かったら終了
            if (!hasSkinnedMesh)
            {
                EditorUtility.DisplayDialog(
                "Select SkinnedMeshRender",
                "SkinnedMeshRenderを含むオブジェクトを選択してね！",
                "Ok");
                return;
            }

            var path = EditorUtility.SaveFilePanel(
                "Save File",
                "Assets",
                Selection.gameObjects[0].name + uv.ToString(),
                "asset");

            if (string.IsNullOrEmpty(path))
            {
                return;
            }

            var match = Regex.Match(path, @"Assets/.*");
            path = match.Value;

            //メッシュのクローンをSkinnedMeshRendererにセット
            var mesh = skinnedMeshRenderer.sharedMesh;
            mesh = GameObject.Instantiate(mesh);

            var vertices = mesh.vertices;
            var uvs = new Vector2[vertices.Length];

            var minX = vertices.Min(x => x.x);
            var maxX = vertices.Max(x => x.x);
            var minY = vertices.Min(x => x.y);
            var maxY = vertices.Max(x => x.y);
            var scaleX = Mathf.Abs(maxX - minX);
            var scaleY = Mathf.Abs(maxY - minY);
            
            for (int i = 0; i < uvs.Length; i++)
            {
                var x = 1 - (vertices[i].x - minX) / scaleX;
                var y = (vertices[i].y - minY) / scaleY;
                uvs[i] = new Vector2(x, y);
            }

            switch (uv)
            {
                case UVType.UV2:
                    mesh.uv2 = uvs;
                    break;
                case UVType.UV3:
                    mesh.uv3 = uvs;
                    break;
                case UVType.UV4:
                    mesh.uv4 = uvs;
                    break;
                default:
                    break;
            }

            //出力
            AssetDatabase.CreateAsset(mesh, path);

            skinnedMeshRenderer.sharedMesh = mesh;
        }
#endif
    }
}