将彩色图像转换为8位灰度

Question

我正在编写一个应用程序，假设它可以在灰度图像上运行。我在转换输入图像到8位灰度级图像时遇到了麻烦。我编写的方法是将输入图像转换为8位，但它的输出远不像灰度图像。

    private Bitmap ConvertToGrayScale(Bitmap bitmap)
    {
        Int32 bytesPerPixel = Image.GetPixelFormatSize(bitmap.PixelFormat) / 8;
        if (bytesPerPixel == 1)
            return bitmap;            

        Bitmap grayscaleBitmap = new Bitmap(bitmap.Width, bitmap.Height,
            PixelFormat.Format8bppIndexed);
        Byte[] pixelData = GetPixelData(bitmap);
        Byte[] grayScalePixelData = new byte[grayscaleBitmap.Width * grayscaleBitmap.Height];

        for (int i = 0; i < grayScalePixelData.Length; i++)
        {
            var pixelValue = (Byte)
                    (pixelData[i * bytesPerPixel] * 0.3 + pixelData[i * bytesPerPixel + 1] * 0.59 +
                     pixelData[i * bytesPerPixel + 2] * 0.11);

            grayScalePixelData[i] = pixelValue;
        }

        Rectangle rectangle = new Rectangle(0, 0, grayscaleBitmap.Width, grayscaleBitmap.Height);
        BitmapData grayscaleBitmapData = grayscaleBitmap.LockBits(rectangle, ImageLockMode.WriteOnly,
            grayscaleBitmap.PixelFormat);
        IntPtr pointer = grayscaleBitmapData.Scan0;
        Marshal.Copy(grayScalePixelData, 0, pointer, grayScalePixelData.Length);
        grayscaleBitmap.UnlockBits(grayscaleBitmapData);
        grayscaleBitmap.Save(@"D:\gray.jpg");

        return grayscaleBitmap;
    }

有谁可以指出我做错了什么？