PIL Image of current window screen. (the window must be on the top)
        reference: https://msdn.microsoft.com/en-us/library/dd183402(v=vs.85).aspx

def screen(self):
        """PIL Image of current window screen. (the window must be on the top)
        reference: https://msdn.microsoft.com/en-us/library/dd183402(v=vs.85).aspx"""
        # opengl windows cannot get from it's hwnd, so we use the screen
        hwnd = win32gui.GetDesktopWindow()

        # get window size and offset
        left, top, right, bottom = self.rect
        width, height = right-left, bottom-top

        # the device context of the window 
        hdcwin = win32gui.GetWindowDC(hwnd)
        # make a temporary dc
        hdcmem = win32gui.CreateCompatibleDC(hdcwin)
        # make a temporary bitmap in memory, this is a PyHANDLE object
        hbmp = win32gui.CreateCompatibleBitmap(hdcwin, width, height)
        # select bitmap for temporary dc
        win32gui.SelectObject(hdcmem, hbmp)
        # copy bits to temporary dc
        win32gui.BitBlt(hdcmem, 0, 0, width, height, 
                        hdcwin, left, top, win32con.SRCCOPY)
        # check the bitmap object infomation
        bmp = win32gui.GetObject(hbmp)

        bi = BITMAPINFOHEADER()
        bi.biSize = ctypes.sizeof(BITMAPINFOHEADER)
        bi.biWidth = bmp.bmWidth
        bi.biHeight = bmp.bmHeight
        bi.biPlanes = bmp.bmPlanes
        bi.biBitCount = bmp.bmBitsPixel
        bi.biCompression = 0 # BI_RGB
        bi.biSizeImage = 0
        bi.biXPelsPerMeter = 0
        bi.biYPelsPerMeter = 0
        bi.biClrUsed = 0
        bi.biClrImportant = 0

        # calculate total size for bits
        pixel = bmp.bmBitsPixel
        size = ((bmp.bmWidth * pixel + pixel - 1)/pixel) * 4 * bmp.bmHeight
        buf = (ctypes.c_char * size)()

        # read bits into buffer
        windll.gdi32.GetDIBits(hdcmem, hbmp.handle, 0, bmp.bmHeight, buf, ctypes.byref(bi), win32con.DIB_RGB_COLORS)

        # make a PIL Image
        img = Image.frombuffer('RGB', (bmp.bmWidth, bmp.bmHeight), buf, 'raw', 'BGRX', 0, 1)
        img = img.transpose(Image.FLIP_TOP_BOTTOM)

        # cleanup
        win32gui.DeleteObject(hbmp)
        win32gui.DeleteObject(hdcmem)
        win32gui.ReleaseDC(hwnd, hdcwin)

        return img