import math


def _is_close(a, b, alpha=0.01):
    """ Check if two blobs are close to one another

    :param a: first blob. This is larger than b.
    :param b: second blob. This is smaller than a.
    :param alpha: The amount of overlap allowed between blobs
    :returns: if blobs are close
    """
    if alpha < 0 or alpha > 1:
        raise ValueError("Value of alpha must be between 0 and 1.")

    ay, ax, ar = a
    by, bx, br = b

    d = math.sqrt((ax - bx)**2 + (ay - by)**2)
    return d <= ar - br*alpha