def to_mask(self, method='exact', subpixels=5):
        """
        Return a list of `~photutils.ApertureMask` objects, one for each
        aperture position.

        Parameters
        ----------
        method : {'exact', 'center', 'subpixel'}, optional
            The method used to determine the overlap of the aperture on
            the pixel grid.  Not all options are available for all
            aperture types.  Note that the more precise methods are
            generally slower.  The following methods are available:

                * ``'exact'`` (default):
                  The the exact fractional overlap of the aperture and
                  each pixel is calculated.  The returned mask will
                  contain values between 0 and 1.

                * ``'center'``:
                  A pixel is considered to be entirely in or out of the
                  aperture depending on whether its center is in or out
                  of the aperture.  The returned mask will contain
                  values only of 0 (out) and 1 (in).

                * ``'subpixel'``:
                  A pixel is divided into subpixels (see the
                  ``subpixels`` keyword), each of which are considered
                  to be entirely in or out of the aperture depending on
                  whether its center is in or out of the aperture.  If
                  ``subpixels=1``, this method is equivalent to
                  ``'center'``.  The returned mask will contain values
                  between 0 and 1.

        subpixels : int, optional
            For the ``'subpixel'`` method, resample pixels by this factor
            in each dimension.  That is, each pixel is divided into
            ``subpixels ** 2`` subpixels.

        Returns
        -------
        mask : list of `~photutils.ApertureMask`
            A list of aperture mask objects.
        """

        use_exact, subpixels = self._translate_mask_mode(method, subpixels,
                                                         rectangle=True)

        if hasattr(self, 'w'):
            w = self.w
            h = self.h
        elif hasattr(self, 'w_out'):    # annulus
            w = self.w_out
            h = self.h_out
            h_in = self.w_in * self.h_out / self.w_out
        else:
            raise ValueError('Cannot determine the aperture radius.')

        masks = []
        for bbox, edges in zip(self.bounding_boxes, self._centered_edges):
            ny, nx = bbox.shape
            mask = rectangular_overlap_grid(edges[0], edges[1], edges[2],
                                            edges[3], nx, ny, w, h,
                                            self.theta, 0, subpixels)

            # subtract the inner circle for an annulus
            if hasattr(self, 'w_in'):
                mask -= rectangular_overlap_grid(edges[0], edges[1], edges[2],
                                                 edges[3], nx, ny, self.w_in,
                                                 h_in, self.theta, 0,
                                                 subpixels)

            masks.append(ApertureMask(mask, bbox))

        return masks
Return a list of `~photutils.ApertureMask` objects, one for each
        aperture position.

        Parameters
        ----------
        method : {'exact', 'center', 'subpixel'}, optional
            The method used to determine the overlap of the aperture on
            the pixel grid.  Not all options are available for all
            aperture types.  Note that the more precise methods are
            generally slower.  The following methods are available:

                * ``'exact'`` (default):
                  The the exact fractional overlap of the aperture and
                  each pixel is calculated.  The returned mask will
                  contain values between 0 and 1.

                * ``'center'``:
                  A pixel is considered to be entirely in or out of the
                  aperture depending on whether its center is in or out
                  of the aperture.  The returned mask will contain
                  values only of 0 (out) and 1 (in).

                * ``'subpixel'``:
                  A pixel is divided into subpixels (see the
                  ``subpixels`` keyword), each of which are considered
                  to be entirely in or out of the aperture depending on
                  whether its center is in or out of the aperture.  If
                  ``subpixels=1``, this method is equivalent to
                  ``'center'``.  The returned mask will contain values
                  between 0 and 1.

        subpixels : int, optional
            For the ``'subpixel'`` method, resample pixels by this factor
            in each dimension.  That is, each pixel is divided into
            ``subpixels ** 2`` subpixels.

        Returns
        -------
        mask : list of `~photutils.ApertureMask`
            A list of aperture mask objects.