为什么有的save是24位而有的是8位

  private void button1_Click(object sender, EventArgs e)

        {

           if (curBitmap != null)

            {



                //curBitmap为我定义的 一个载入图片 为 form的私有成员变量

                //新的我要的8位图像容器

                Bitmap bit = new Bitmap(curBitmap.Width,curBitmap.Height,PixelFormat.Format8bppIndexed);

              //可读写的方式锁定全部原图像

                BitmapData data = curBitmap.LockBits(new Rectangle(0, 0, curBitmap.Width,curBitmap.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

                //得到首地址

                IntPtr ptr1 = data.Scan0;

                //可读写的方式锁定全部容器

                BitmapData data2 = bit.LockBits(new Rectangle(0, 0, bit.Width, bit.Height), ImageLockMode.ReadWrite, PixelFormat.Format8bppIndexed);

                //得到首地址

                IntPtr ptr2 = data2.Scan0;

                //计算24位图像的字节数

                int bytes = curBitmap.Width * curBitmap.Height * 3;

                //定义位图数组

                byte[] grayValues = new byte[bytes];

                //复制被锁定的图像到该数组

                System.Runtime.InteropServices.Marshal.Copy(ptr1, grayValues, 0, bytes);

                //同上

                int byte8 = curBitmap.Width * curBitmap.Height;

                byte [] gray8Values=new byte[byte8];

                System.Runtime.InteropServices.Marshal.Copy(ptr2, gray8Values, 0, byte8);



                for (int i = 0,n=0; i < bytes;i+=3 ,n++)

                {

                    //灰度变换

                    double colorTemp = grayValues[i + 2] * 0.299 + grayValues[i + 1] * 0.589 + grayValues[i] * 0.114;

                    //这个是测试用的可以忽略  这是原图24位

                    grayValues[i + 2] = grayValues[i + 1] = grayValues[i] = (byte)colorTemp;

                    //将灰度值赋值给 容器    这是新图8位

                    gray8Values[n] = (byte)colorTemp;

                }

                //送回数组

                System.Runtime.InteropServices.Marshal.Copy( gray8Values, 0,ptr2, byte8);

                System.Runtime.InteropServices.Marshal.Copy( grayValues, 0,ptr1, bytes);

                //解锁

                curBitmap.UnlockBits(data);

                bit.UnlockBits(data2);

                //调色板

                ColorPalette palette = bit.Palette;

                //调色板结构体数组  填充  

                for (int i = 0; i != palette.Entries.Length; i++)

                {

                    palette.Entries[i] = Color.FromArgb(i, i, i);//(说实话这步为啥这麽做我也不知道)

                }

                bit.Palette = palette;



                String path2 = System.AppDomain.CurrentDomain.BaseDirectory;

                String str = path2 + "bb.bmp";

                Bitmap b = bit.Clone(new Rectangle(0, 0, bit.Width, bit.Height), PixelFormat.Format8bppIndexed);

                b.Save(@"H:\C#\bb.bmp", ImageFormat.Bmp);// b.Save(@str, ImageFormat.Jpeg);

                b.Dispose();

                bit.Dispose();

                Invalidate();



            }

        

        }

 private void save_Click(object sender, EventArgs e)

        {

            if(curBitmap == null)

            {

                return;

            }

            SaveFileDialog saveDlg = new SaveFileDialog();

            saveDlg.Title = "保存为";

            saveDlg.OverwritePrompt = true;

            saveDlg.Filter =

                "BMP文件 (*.bmp) | *.bmp|" +

                "Gif文件 (*.gif) | *.gif|" +

                "JPEG文件 (*.jpg) | *.jpg|" +

                "PNG文件 (*.png) | *.png";

            saveDlg.ShowHelp = true;

            if(saveDlg.ShowDialog() == DialogResult.OK)

            {

                string fileName = saveDlg.FileName;

                string strFilExtn = fileName.Remove(0, fileName.Length - 3);

                switch (strFilExtn)

                {

                    case "bmp":

                        curBitmap.Save(fileName, System.Drawing.Imaging.ImageFormat.Bmp);

                        break;

                    case "jpg":

                        curBitmap.Save(fileName, System.Drawing.Imaging.ImageFormat.Jpeg);

                        break;

                    case "gif":

                        curBitmap.Save(fileName, System.Drawing.Imaging.ImageFormat.Gif);

                        break;

                    case "tif":

                        curBitmap.Save(fileName, System.Drawing.Imaging.ImageFormat.Tiff);

                        break;

                    case "png":

                        curBitmap.Save(fileName, System.Drawing.Imaging.ImageFormat.Png);

                        break;

                    default:

                        break;

                }

            }

        }