def norm_to_index(z, s):
    """Compute the index from a normalized and centered coordinate.

    Parameters
    ----------
    z : float
        normalized and centered coordinate
    s : int
        shape of one axis. Eg x_index = norm_to_index(x, w) 

    Returns
    -------
    float
        float value of the index.
    """
    return z*float(s) - .5