def backward(self,pixx,pixy):
        """ Transform pixx,pixy positions from the output frame back onto their
            original positions in the input frame.
        """
        skyx,skyy = self.output.wcs_pix2world(pixx,pixy,self.origin)
        result = self.input.all_world2pix(skyx,skyy,self.origin)
        return result