Performs update of the displayed image on the GUI canvas, using the
        supplied device context.  If drawDC is None, a ClientDC will be used to
        redraw the image.

def gui_repaint(self, drawDC=None):
        """
        Performs update of the displayed image on the GUI canvas, using the
        supplied device context.  If drawDC is None, a ClientDC will be used to
        redraw the image.
        """
        DEBUG_MSG("gui_repaint()", 1, self)
        if self.IsShownOnScreen():
            if drawDC is None:
                drawDC=wx.ClientDC(self)

            #drawDC.BeginDrawing()
            drawDC.DrawBitmap(self.bitmap, 0, 0)
            #drawDC.EndDrawing()
            #wx.GetApp().Yield()
        else:
            pass