Converts from Galactic coordinates to pixel indices.

        Args:
            gal (:obj:`astropy.coordinates.SkyCoord`): Galactic coordinates. Must
                store an array of coordinates (i.e., not be scalar).

        Returns:
            ``j, k, mask`` - Pixel indices of the coordinates, as well as a mask
            of in-bounds coordinates. Outputs have the same shape as the input
            coordinates.

def _gal2idx(self, gal):
        """
        Converts from Galactic coordinates to pixel indices.

        Args:
            gal (:obj:`astropy.coordinates.SkyCoord`): Galactic coordinates. Must
                store an array of coordinates (i.e., not be scalar).

        Returns:
            ``j, k, mask`` - Pixel indices of the coordinates, as well as a mask
            of in-bounds coordinates. Outputs have the same shape as the input
            coordinates.
        """

        # Make sure that l is in domain [-180 deg, 180 deg)
        l = coordinates.Longitude(gal.l, wrap_angle=180.*units.deg)

        j = (self._inv_pix_scale * (l.deg - self._l_bounds[0])).astype('i4')
        k = (self._inv_pix_scale * (gal.b.deg - self._b_bounds[0])).astype('i4')

        idx = (j < 0) | (j >= self._shape[0]) | (k < 0) | (k >= self._shape[1])

        if np.any(idx):
            j[idx] = -1
            k[idx] = -1

        return j, k, ~idx