public Bitmap PCluster(Bitmap a)
{
  try
  {
   Rectangle rect = new Rectangle(0, 0, a.Width, a.Height);
   System.Drawing.Imaging.BitmapData bmpData = a.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
   int stride = bmpData.Stride;
   unsafe
   {
     byte* pIn = (byte*)bmpData.Scan0.ToPointer();
     byte* P;
     int R, G, B;
     for (int y = 0; y < a.Height; y++)
     {
     for (int x = 0; x < a.Width; x++)
     {
       P = pIn;
       B = P[0];
       G = P[1];
       R = P[2];
       P[0] = (byte)(B & 192); //屏蔽末6位
       P[1] = (byte)(G & 192);
       P[2] = (byte)(R & 192);
       pIn += 3;
     }
     pIn += stride - a.Width * 3;
     }
   }
   a.UnlockBits(bmpData);
   return a;
  }
  catch (Exception e)
  {
   MessageBox.Show(e.Message.ToString());
   return null;
  }
}