private static unsafe void InvertBitmap(Bitmap bmp) {
            BitmapData data = bmp.LockBits(new Rectangle(0, 0, bmp.Width,
              bmp.Height), ImageLockMode.ReadWrite, bmp.PixelFormat);
 
            int byteCount = bmp.Height * data.Stride;
            byte* pt = (byte*)data.Scan0.ToPointer();
 
            for (int i = 0; i < byteCount; i++, pt++) {
                *pt = (byte) (255 - *pt);
            }
 
            bmp.UnlockBits(data);    
        }