public static Bitmap ChangeGray(Bitmap b) 
 { 
  BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, 
   PixelFormat.Format24bppRgb); 
  int stride = bmData.Stride;   // 扫描的宽度 
  
  
  unsafe 
  { 
   byte* p = (byte*)bmData.Scan0.ToPointer(); // 获取图像首地址 
   int nOffset = stride - b.Width * 3;  // 实际宽度与系统宽度的距离 
   byte red, green, blue; 
   for (int y = 0; y < b.Height; ++y) 
   { 
    for (int x = 0; x < b.Width; ++x) 
    { 
     blue = p[0]; 
     green = p[1]; 
     red = p[2]; 
  
     p[0] = p[1] = p[2] = (byte)(.299 * red + .587 * green + .114 * blue); // 转换公式 
     p += 3;  // 跳过3个字节处理下个像素点 
    } 
    p += nOffset; // 加上间隔 
   } 
  } 
  b.UnlockBits(bmData); // 解锁 
  return b; 
 }