C#实现24/16位图像转8位灰度处理

class To8BitImage
    {
        /// <summary>
        /// Convet图形为索引图形
        /// </summary>
        /// <param name="p_SourceImage">原始图形</param>
        /// <param name="p_PixelFormat">格式 之支持 Format4bppIndexed Format1bppIndexed Format8bppIndexed</param>
        /// <param name="p_Dithering">原始图形</param>
        /// <returns>图形</returns>
        public static Bitmap Convert(Bitmap p_SourceImage, bool p_Dithering)
        {
            int _Width = p_SourceImage.Width;
            int _Height = p_SourceImage.Height;

            PixelFormat _SetPixFormat = PixelFormat.Format8bppIndexed;

            //创建颜色索引表 为2个颜色
            Bitmap _NewBitmap = new Bitmap(_Width, _Height, _SetPixFormat);

            ColorPalette _Palette = _NewBitmap.Palette;//LoadPalette(p_SourceImage,256);



            //重新获取一个原始图形 色彩设置为24色 R G B
            Bitmap _Source = new Bitmap(_Width, _Height, PixelFormat.Format24bppRgb);
            Graphics _Graphics = Graphics.FromImage(_Source);
            _Graphics.DrawImage(p_SourceImage, 0, 0, _Width, _Height);
            _Graphics.Dispose();

            //获取两个图形的数据
            BitmapData _SourceData = _Source.LockBits(new Rectangle(0, 0, _Width, _Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
            BitmapData _NewData = _NewBitmap.LockBits(new Rectangle(0, 0, _Width, _Height), ImageLockMode.WriteOnly, _SetPixFormat);

            //复制出图形数据的
            byte[] _SourceByte = new byte[_SourceData.Stride * _Height];
            byte[] _NewByte = new byte[_NewData.Stride * _Height];
            Marshal.Copy(_SourceData.Scan0, _SourceByte, 0, _SourceByte.Length);
            Marshal.Copy(_NewData.Scan0, _NewByte, 0, _NewByte.Length);

            int _SourceIndex = 0;
            int _NewIndex = 0;
            //颜色表列
            Dictionary<int, byte> _ColorTable = new Dictionary<int, byte>();

            Color _SetColor;

            for (int i = 0; i != _Height; i++)
            {
                _NewIndex = i * _NewData.Stride;
                _SourceIndex = i * _SourceData.Stride;
                for (int z = 0; z != _Width; z++)
                {
                    #region 获取色彩索引
                    //获取原始图形色彩
                    _SetColor = Color.FromArgb(0, _SourceByte[_SourceIndex + 2], _SourceByte[_SourceIndex + 1], _SourceByte[_SourceIndex]);
                    _SourceIndex += 3;

                    byte _TableIndex = 0;
                    if (!(_ColorTable.TryGetValue(_SetColor.ToArgb(), out _TableIndex)))