Local Contrast Enhancement Using 2-Dimensional Recursive Filters

An unsharp masking type local contrast enhancement method that uses spatially adaptive 2-dimensional (2-D) recursive filters is presented. The proposed 2-D recursive filters are a family of exponential smoothing filters. Three different forms are proposed: separable extension of the 1-dimensional (1-D) case, non-separable filter that uses 4-neighbors, and non-separable filter that uses 8-neighbors. The isotropy of the transfer function improves when moved from the first to the last; hence, better visual quality is obtained in the real video experiments. The computational complexity of the proposed recursive filters, unlike non-recursive filters, is independent of the desired cutoff frequency. This independence enables different levels of smoothing simply by spatially adapting the recursion coefficients without any additional computation