static void Main(string[] args)
{
  Bitmap bitmap = new Bitmap(Environment.CurrentDirectory + "//1.jpg");
 
  for (int i = 0; i < bitmap.Width; i++)
  {
    for (int j = 0; j < bitmap.Height; j++)
    {
      //取图片当前的像素点
      var color = bitmap.GetPixel(i, j);
 
      var gray = (int)(color.R * 0.299 + color.G * 0.587 + color.B * 0.114);
 
      //重新设置当前的像素点
      bitmap.SetPixel(i, j, Color.FromArgb(gray, gray, gray));
    }
  }
 
  bitmap.Save(Environment.CurrentDirectory + "//2.jpg");
}

static void Main(string[] args)
    {
      Bitmap bitmap = new Bitmap(Environment.CurrentDirectory + "//1.jpg");
 
      //定义锁定bitmap的rect的指定范围区域
      Rectangle rect = new Rectangle(0, 0, bitmap.Width, bitmap.Height);
 
      //加锁区域像素
      var bitmapData = bitmap.LockBits(rect, ImageLockMode.ReadWrite, bitmap.PixelFormat);
 
      //位图的首地址
      var ptr = bitmapData.Scan0;
 
      //stride：扫描行
      int len = bitmapData.Stride * bitmap.Height;
 
      var bytes = new byte[len];
 
      //锁定区域的像素值copy到byte数组中
      Marshal.Copy(ptr, bytes, 0, len);
 
      for (int i = 0; i < bitmap.Height; i++)
      {
        for (int j = 0; j < bitmap.Width * 3; j = j + 3)
        {
          var color = bytes[i * bitmapData.Stride + j + 2] * 0.299
                + bytes[i * bitmapData.Stride + j + 1] * 0.597
                + bytes[i * bitmapData.Stride + j] * 0.114;
 
          bytes[i * bitmapData.Stride + j]
               = bytes[i * bitmapData.Stride + j + 1]
               = bytes[i * bitmapData.Stride + j + 2] = (byte)color;
        }
      }
 
      //copy回位图
      Marshal.Copy(bytes, 0, ptr, len);
 
      //解锁
      bitmap.UnlockBits(bitmapData);
 
      bitmap.Save(Environment.CurrentDirectory + "//3.jpg");
    }