new_float_type = {
    # preserved types
    np.float32().dtype.char : np.float32,
    np.float64().dtype.char : np.float64,
    np.complex64().dtype.char : np.complex64,
    np.complex128().dtype.char : np.complex128,
    # altered types
    np.float16().dtype.char : np.float32,
    'g' : np.float64      # np.float128 ; doesn't exist on windows
    'G' : np.complex128   # np.complex256 ; doesn't exist on windows
}


def _supported_float_type(dtype, allow_complex=False):
    """Return an appropriate floating-point dtype for a given dtype.

    float32, float64, complex64, complex128 are preserved.
    float16 is promoted to float32.
    complex256 is demoted to complex128.
    Other types are cast to float64.

    Paramters
    ---------
    dtype : np.dtype or Iterable of np.dtype
        The input dtype. If a sequence of multiple dtypes is provided, each
        dtype is first converted to a supported floating point type and the
        final dtype is then determined by applying `np.result_type` on the
        sequence of supported floating point types.
    allow_complex : bool, optional
        If False, raise a ValueError on complex-valued inputs.

    Retruns
    -------
    float_type : dtype
        Floating-point dtype for the image.
    """
    if isinstance(dtype, Iterable) and not isinstance(dtype, str):
        return np.result_type(*(_supported_float_type(d) for d in dtype))
    dtype = np.dtype(dtype)
    if not allow_complex and dtype.kind == 'c':
        raise ValueError("complex valued input is not supported")
    return new_float_type.get(dtype.char, np.float64)