def __substract(I, M):
    """Substract the image M to the image of I. See
    https://docs.gimp.org/en/gimp-concepts-layer-modes.html
    and
    https://github.com/jamieowen/glsl-blend/blob/master/substract.glsl
    Renormalized images are assumed, i.e. colors range = [0, 1]

    Parameters
    ----------
    I : np.ndarray
        Left side operator
    M : np.ndarray
        Right side operator

    Returns
    -------
    E : np.ndarray
        Substracted image
    """
    E = I - M
    E[E < 0] = 0
    return E
