C / C++ / MFC

void MyScrolledWindow::OnPaint(wxPaintEvent& event)
{       
    wxPaintDC dc(this);   

    // Only paint if we have a valid bitmap
    if(m_theBitmap && m_theBitmap->Ok())
    {
        // Get the outer bounds of the region of the window that has been damaged.
        const wxRect rectUpdate = GetUpdateRegion().GetBox();
        // Translate the device coordinates of the upper left corner of the invalid region to logical coordinates.
        const wxPoint ptVirt = CalcUnscrolledPosition(rectUpdate.GetTopLeft());
        // The memory device context we are using for double buffering.
        wxMemoryDC mdc;       
        // Select the bitmap into the device context so we are ready for double buffering.
        mdc.SelectObject(*m_theBitmap);       
       
        // Get the size of the client area.
        const wxRect rcClient = GetClientSize();

        // This will be the point of where to draw the upper left corner of our image.
        wxPoint upperLeft;

        // If the client area is bigger than the bitmap in either width or height...
        if(rcClient.GetWidth() > m_theBitmap->GetWidth() || rcClient.GetHeight() > m_theBitmap->GetHeight())
        {
            // ...draw the background...
            PaintBackground(dc);

            // ... and get the width and height of the image...
            wxCoord w = m_theBitmap->GetWidth();
            wxCoord h = m_theBitmap->GetHeight();           

            // ...so we can use that for calculating the position of the upper left corner of the image.
            int x = wxMax(0, (rcClient.width-w)/2);
            int y = wxMax(0, (rcClient.height-h)/2);

            // Save the just calculated coordinates.
            upperLeft.x = x;
            upperLeft.y = y;           
        }
        else
        {
            // We have a huge image, so we simple paint it at the top left of the client region.
            upperLeft = rectUpdate.GetTopLeft();
        }

        // Now switch the device context so we show the bitmap.
        dc.Blit(upperLeft, rectUpdate.GetSize(), &mdc, ptVirt, wxCOPY);
        // Select in a null object to clear the memory device context.
        mdc.SelectObject(wxNullBitmap);
    }
    else
    {
        // We have no valid bitmap so simply paint the background.
        PaintBackground(dc);       
    }
}