def pa_zoom(self, viewer, event, msg=True):
        """Interactively zoom the image by a pan gesture.
        (the back end must support gestures)
        """
        event = self._pa_synth_scroll_event(event)
        if event.state != 'move':
            return False
        self._sc_zoom(viewer, event, msg=msg, origin=None)
        return True