在C＃中将数字个人指纹位图图像设置为8位深度灰度

Question

我想将指纹图像的位深度从初始的32位深度设置为8位。这就是我尝试过的

    public override void OnComplete(object Capture, string ReaderSerialNumber, DPFP.Sample Sample)
    {
        Bitmap picture = new Bitmap(500,500,PixelFormat.Format8bppIndexed);
        var bitmapData = picture.LockBits(new Rectangle(Point.Empty, picture.Size),ImageLockMode.ReadWrite, picture.PixelFormat);
        Random random = new Random();
        byte[] buffer = new byte[picture.Width * picture.Height];
        random.NextBytes(buffer);
        Marshal.Copy(buffer, 0, bitmapData.Scan0, buffer.Length);
        picture.UnlockBits(bitmapData);
        MakeReport("The fingerprint  was captured.");
        SetPrompt("Scan the same fingerprint again.");
        MemoryStream ms = new MemoryStream();
        picture.SetResolution(500, 500);
        picture.Save(ms, ImageFormat.Bmp);
        fingerImage = ms.ToArray(); 
        Capturer.StopCapture();
        ((Func<object, Task<object>>)_input.event_handler)(fingerImage).Start();
    }

使用上面的代码可以根据需要将图像设置为8位，但是我无法按预期再次获得原始指纹图像。请指导