从具有负步幅的位图复制像素数据?

我一直在寻找将位图转换为 8bpp 的最快方法.我找到了两种方法:

I was looking for the fastest way to convert a Bitmap to 8bpp. I found 2 ways:

1.

        public static System.Drawing.Image ConvertTo8bpp(Bitmap oldbmp)
    {
        using (var ms = new MemoryStream())
        {
            oldbmp.Save(ms, ImageFormat.Gif);
            ms.Position = 0;
            return System.Drawing.Image.FromStream(ms);
        }
    }

2. http://www.wischik.com/lu/programmer/1bpp.html

但是:1. 结果质量非常低(托盘不良)

But: 1. Results in a very low quality result (bad pallet)

和 2 给了我一个负步幅的位图，当我尝试锁定位并将数据复制到字节数组时，我得到一个异常:尝试读取或写入受保护的内存.这通常表明其他内存已损坏.

and 2 gives me a Bitmap with negative stride, when I try to lockbits and copy the data to a byte array I get an exception: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

        BitmapData bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadWrite, bmp.PixelFormat);

        this.stride = bmpData.Stride;
        this.bytesPerPixel = GetBytesPerPixel(bmp.PixelFormat);
        int length = bmpData.Stride * bmp.Height;
        if (this.stride < 0)
            this.data = new byte[-length];
        else
            this.data = new byte[length];
        Marshal.Copy(bmpData.Scan0, data, 0, length);

        //Unlock the bitmap
        bmp.UnlockBits(bmpData);

我怎样才能让 2 取得积极的进展?或者如何使用负步幅的锁位复制数据?

How can I make 2 gives a positive stride? Or how can I copy data using lockbits of a negative stride??