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Robust Recursive Envelope Operators for Fast Retinex

Shaked, Doron; Keshet, Renato


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Abstract: The effort to make efficient Retinex engines motivated the research reported in this paper. In most Retinex- type algorithms the computational engine is some (possibly non-linear) variant of a large support low- pass filter. The presented work relies on the fact that recursive (2D IIR) filtering is an efficient method to implement such large support linear space invariant filters. We propose a generalization of recursive filters to non-liner operators, and their use in the Retinex core. In particular, we pay attention to two kinds of non-linearities, which are important in the context of Retinex type algorithms. First, the output should be an upper or lower envelope of the original image, and secondly, the operator should be robust in the sense that it does not use information from across an edge in the image. In an effort to reach stable algorithm parameters, namely parameters that would perform reasonably well for all images, we developed scale invariant operators, in which the nature of the obtained envelope is invariant to the size of the input image. Operators presented in this paper provide Retinex enhancement that is comparable to the best alternatives, and can be computed in about half the time.

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