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    This function implements the alpha estimator for random walk alpha matting
    as described in :cite:`grady2005random`.

    Parameters
    ----------
    image: numpy.ndarray
        Image with shape :math:`h\times w \times 3`
    sigma: float
        Sigma used to calculate the weights (see Equation 4 in
        :cite:`grady2005random`), defaults to :math:`0.033`
    radius: int
        Radius of local window size, defaults to :math:`1`, i.e. only adjacent
        pixels are considered. The size of the local window is given as
        :math:`(2 r + 1)^2`, where :math:`r` denotes the radius. A larger radius
        might lead to violated color line constraints, but also favors further
        propagation of information within the image.
    regularization: float
        Regularization strength, defaults to :math:`10^{-8}`. Strong
        regularization improves convergence but results in smoother alpha matte.

    Returns
    -------
    L: scipy.sparse.spmatrix
        Matting Laplacian
    Nr   )r   )regularization)r   r    scipysparseZ
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