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Images captured in poorly lit conditions are often corrupted by acquisition
noise. Leveraging recent advances in graph-based regularization, we propose a
fast Retinex-based restoration scheme that denoises and contrast-enhances an
image. Specifically, by Retinex theory we first assume that each image pixel is
a multiplication of its reflectance and illumination components. We next assume
that the reflectance and illumination components are piecewise constant (PWC)
and continuous piecewise planar (PWP) signals, which can be recovered via graph
Laplacian regularizer (GLR) and gradient graph Laplacian regularizer (GGLR)
respectively. We formulate quadratic objectives regularized by GLR and GGLR,
which are minimized alternately until convergence by solving linear systems --
with improved condition numbers via proposed preconditioners -- via conjugate
gradient (CG) efficiently. Experimental results show that our algorithm
achieves competitive visual image quality while reducing computation complexity
noticeably.
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