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    Estimate alpha from an input image and an input trimap using Closed-Form Alpha Matting as proposed by :cite:`levin2007closed`.

    Parameters
    ----------
    image: numpy.ndarray
        Image with shape :math:`h \times  w \times d` for which the alpha matte should be estimated
    trimap: numpy.ndarray
        Trimap with shape :math:`h \times  w` of the image
    preconditioner: function or scipy.sparse.linalg.LinearOperator
        Function or sparse matrix that applies the preconditioner to a vector (default: ichol)
    laplacian_kwargs: dictionary
        Arguments passed to the :code:`cf_laplacian` function
    cg_kwargs: dictionary
        Arguments passed to the :code:`cg` solver
    is_known: numpy.ndarray
        Binary mask of pixels for which to compute the laplacian matrix.
        Providing this parameter might improve performance if few pixels are unknown.

    Returns
    -------
    alpha: numpy.ndarray
        Estimated alpha matte

    Example
    -------
    >>> from pymatting import *
    >>> image = load_image("data/lemur/lemur.png", "RGB")
    >>> trimap = load_image("data/lemur/lemur_trimap.png", "GRAY")
    >>> alpha = estimate_alpha_cf(
    ...     image,
    ...     trimap,
    ...     laplacian_kwargs={"epsilon": 1e-6},
    ...     cg_kwargs={"maxiter":2000})
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