加载的位图与jpeg中保存的位图不同（c＃）

Question

我正在使用以下方法从jpeg文件（JpegToBitmap）加载位图：

private static Bitmap JpegToBitmap(string fileName)
    {

        FileStream jpg = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read);

        JpegBitmapDecoder ldDecoder = new JpegBitmapDecoder(jpg, BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.Default);
        BitmapFrame lfFrame = ldDecoder.Frames[0];
        Bitmap lbmpBitmap = new Bitmap(lfFrame.PixelWidth, lfFrame.PixelHeight);
        Rectangle lrRect = new Rectangle(0, 0, lbmpBitmap.Width, lbmpBitmap.Height);
        BitmapData lbdData = lbmpBitmap.LockBits(lrRect, ImageLockMode.WriteOnly, (lfFrame.Format.BitsPerPixel == 24 ? PixelFormat.Format24bppRgb : PixelFormat.Format32bppArgb));
        lfFrame.CopyPixels(System.Windows.Int32Rect.Empty, lbdData.Scan0, lbdData.Height * lbdData.Stride, lbdData.Stride);
        lbmpBitmap.UnlockBits(lbdData);

        return lbmpBitmap;
    }

并将位图保存到jpeg文件：

 private static void SaveToJpeg(Bitmap bitmap, string filePath)
    {
        EncoderParameters encoderParameters = new EncoderParameters(1);
        encoderParameters.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100L);
        bitmap.Save(filePath, GetEncoder(ImageFormat.Jpeg), encoderParameters);
    }

    public static ImageCodecInfo GetEncoder(ImageFormat format)
    {
        ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();

        foreach (ImageCodecInfo codec in codecs)
        {
            if (codec.FormatID == format.Guid)
            {
                return codec;
            }
        }

        return null;
    }

位图像素略有不同，但这种差异对我很重要：

Screenshot of values in matrix of colors of bitmap before save

Screenshot of values in same matrix of colors of bitmap after load

这个矩阵只是由标准位图得到的二维颜色数组.GetPixel（x，y）在一个循环中。

public ColorMatrix(Bitmap bitmap, int currHeight, int currWidth)
    {
        matrix = new Color[8, 8];
        for (int y = 0; y < 8; y++)
        {
            for (int x = 0; x < 8; x++)
            {
                matrix[y, x] = bitmap.GetPixel(x + currWidth, y + currHeight);
            }
        }
    }

所以，问题是：如何正确加载保存的位图（以jpeg / png或其他格式）？