// TextureUtils.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;
using System.IO;

public class TextureUtils : MonoBehaviour
{
 [MenuItem("Tools/GenGrayTexture")]
 public static void GenGrayTexture()
 {
  // 獲取選中的圖片
  var textures = Selection.GetFiltered<Texture2D>(SelectionMode.DeepAssets);
  foreach (var t in textures)
  {
   var path = AssetDatabase.GetAssetPath(t);

   // 如果提示圖片不可讀，需要設置一下isReadable為true, 操作完記得再設置為false
   var imp = AssetImporter.GetAtPath(path) as TextureImporter;
   imp.isReadable = true;
   AssetDatabase.ImportAsset(path);

   var newTexture = new Texture2D(t.width, t.height, TextureFormat.RGBA32, false);
   var colors = t.GetPixels();
   var targetColors = newTexture.GetPixels();

   for (int i = 0, len = colors.Length; i < len; ++i)
   {
    var c = colors[i];
    // 顏色值計算，rgb去平均值
    var v = (c.r + c.g + c.b) / 3f;
    targetColors[i] = new Color(v, v, v, c.a);
   }
   newTexture.SetPixels(targetColors);
   string fname = path.Split('.')[0] + "_gray.png";
   File.WriteAllBytes(fname, newTexture.EncodeToPNG());

   imp.isReadable = false;
   AssetDatabase.Refresh();
  }
 }
}

var files = Directory.GetFiles("D:\\path", "*.*", SearchOption.AllDirectories);
foreach(var f in files)
{
 if(!f.EndsWith(".png") && !f.EndsWith(".jpg")) continue;
 // TODO...
 
}