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Smoothing nonlinear conjugate gradient method for image restoration using nonsmooth nonconvex minimization. (English) Zbl 1200.65031
Summary: Image restoration problems are often converted into large-scale, nonsmooth, and nonconvex optimization problems. Most existing minimization methods are not efficient for solving such problems. It is well known that nonlinear conjugate gradient methods are preferred to solve large-scale smooth optimization problems due to their simplicity, low storage, practical computation efficiency, and nice convergence properties. In this paper, we propose a smoothing nonlinear conjugate gradient method where an intelligent scheme is used to update the smoothing parameter at each iteration and guarantees that any accumulation point of a sequence generated by this method is a Clarke stationary point of the nonsmooth and nonconvex optimization problem. Moreover, we present a class of smoothing functions and show their approximation properties. This method is easy to implement without adding any new variables. Three image restoration problems with different pixels and different regularization terms are used in numerical tests. Experimental results and comparison with the continuation method in [M. Nikolova et al., SIAM J. Imaging Sci. 1, No. 1, 2–25 (2008; Zbl 1207.94017)] show the efficiency of the proposed method.
MSC:
65F22Ill-posedness, regularization (numerical linear algebra)
65F10Iterative methods for linear systems
65K05Mathematical programming (numerical methods)
Software:
GradSamp; L-BFGS