fit image to window

def fit_to_window(self):
        '''fit image to window'''
        state = self.state
        self.dragpos = wx.Point(0, 0)
        client_area = state.frame.GetClientSize()
        self.zoom = min(float(client_area.x) / self.img.GetWidth(),
                        float(client_area.y) / self.img.GetHeight())
        self.need_redraw = True