Flipped Bitmap from Twain

Question

I'm currently getting scanned pages through Twain and transforming the pages to Bitmap, using the BitmapRenderer of twaindotnet project, as described in this post.

My scanner allows me to scan recto and verso. When I scan recto only pages, it works like a charm: the generated bitmaps are perfect. But when it scans recto-verso, the bitmap are flipped . Sometimes vertically, sometimes horizontally.

I can't use the Bitmap.RotateFlip() method because the effect doesn't concern each picture, but only when recto-verso pages.

I've tried the Bitmap.FromHbitmap() described here or the default constructor, but it throws an error related to GDI+.

I'm pretty sure the issue is where the bitmap is converted from the pointer, in the BitmapRenderer class. Here is the code (I did not include the Dispose() methods for clarity purpose) :

public class BitmapRenderer : IDisposable
{
    private readonly IntPtr _picturePointer;
    private readonly IntPtr _bitmapPointer;
    private readonly IntPtr _pixelInfoPointer;
    private Rectangle _rectangle;
    private readonly BitmapInfoHeader _bitmapInfo;

    /// <summary>
    /// Initializes a new instance of the <see cref="BitmapRenderer"/> class.
    /// </summary>
    /// <param name="picturePointer_">The picture pointer.</param>
    public BitmapRenderer(IntPtr picturePointer_)
    {
        _picturePointer = picturePointer_;
        _bitmapPointer = Kernel32Native.GlobalLock(picturePointer_);

        _bitmapInfo = new BitmapInfoHeader();
        Marshal.PtrToStructure(_bitmapPointer, _bitmapInfo);

        _rectangle = new Rectangle();
        _rectangle.X = _rectangle.Y = 0;
        _rectangle.Width = _bitmapInfo.Width;
        _rectangle.Height = _bitmapInfo.Height;

        if (_bitmapInfo.SizeImage == 0)
        {
            _bitmapInfo.SizeImage = ((((_bitmapInfo.Width*_bitmapInfo.BitCount) + 31) & ~31) >> 3)*
                                    _bitmapInfo.Height;
        }

        // The following code only works on x86
        Debug.Assert(Marshal.SizeOf(typeof (IntPtr)) == 4);

        int pixelInfoPointer = _bitmapInfo.ClrUsed;
        if ((pixelInfoPointer == 0) && (_bitmapInfo.BitCount <= 8))
            pixelInfoPointer = 1 << _bitmapInfo.BitCount;

        pixelInfoPointer = (pixelInfoPointer*4) + _bitmapInfo.Size + _bitmapPointer.ToInt32();

        _pixelInfoPointer = new IntPtr(pixelInfoPointer);
    }

    /// <summary>
    /// Renders to bitmap.
    /// </summary>
    /// <returns></returns>
    public Bitmap RenderToBitmap()
    {
        Bitmap bitmap = new Bitmap(_rectangle.Width, _rectangle.Height);

        using (Graphics graphics = Graphics.FromImage(bitmap))
        {
            IntPtr hdc = graphics.GetHdc();
            try
            {
                Gdi32Native.SetDIBitsToDevice(hdc, 0, 0, _rectangle.Width, _rectangle.Height,
                                              0, 0, 0, _rectangle.Height, _pixelInfoPointer, _bitmapPointer, 0);
            }
            finally
            {
                graphics.ReleaseHdc(hdc);
            }
        }
        bitmap.SetResolution(PpmToDpi(_bitmapInfo.XPelsPerMeter), PpmToDpi(_bitmapInfo.YPelsPerMeter));
        return bitmap;
    }

    private static float PpmToDpi(double pixelsPerMeter_)
    {
        double pixelsPerMillimeter = pixelsPerMeter_/1000.0;
        double dotsPerInch = pixelsPerMillimeter*25.4;
        return (float) Math.Round(dotsPerInch, 2);
    }

I don't understand where this is from or how to solve it.

EDIT

Well, it appears this situation is not related to twain conversion to bitmap (the issue is not from twaindotnet project at all).

It only occurs with handwritten pages. This is an automatic OCR issue. Does someone know how to disable OCR for handwritten document ?