asp.net(c#)将彩色图片变灰阶图片

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Drawing;
using System.Drawing.Imaging;
/* 何问起 hovertree.com */
namespace WebApplication2
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            Bitmap b = new Bitmap(Server.MapPath("01.jpg"));


            for (int x = 0; x < b.Width; x++)
            {
                for (int y = 0; y < b.Height; y++)
                {


                    Color pixel = b.GetPixel(x, y);




                    int val = (pixel.R + pixel.G + pixel.B) / 3;






                    b.SetPixel(x, y, Color.FromArgb(val, val, val));


                }
            }


            b.Save(Server.MapPath("1.jpg"));


        }
    }
}