def blend0(d=0.0, u=1.0, s=1.0):
    """
       blending function trapezoid
       d = delta x = xabs - xdr
       u = uncertainty radius of xabs estimate error
       s = tuning scale factor

       returns blend
    """
    d = float(abs(d))
    u = float(abs(u))
    s = float(abs(s))
    v = d - u #offset by radius

    if v >= s:  #first so if s == 0 catches here so no divide by zero below
        b = 0.0
    elif v <= 0.0:
        b = 1.0
    else: # 0 < v < s
        b = 1.0 - (v / s)

    return b