Fast and robust wavelet-based dynamic range compression with local contrast enhancement

In this paper, a new wavelet-based dynamic range compression algorithm is proposed to improve the visual quality of digital images captured in the high dynamic range scenes with non-uniform lighting conditions. Wavelet transform is used especially for dimension reduction such that a dynamic range compression with local contrast enhancement algorithm is applied only to the approximation coefficients which are obtained by low-pass filtering and down-sampling the original intensity image. The normalized approximation coefficients are transformed using a hyperbolic sine curve and the contrast enhancement is realized by tuning the magnitude of the each coefficient with respect to surrounding coefficients. The transformed coefficients are then de-normalized to their original range. The detail coefficients are also modified to prevent the edge deformation. The inverse wavelet transform is carried out resulting in a low dynamic range and contrast enhanced intensity image. A color restoration process based on relationship between spectral bands and the luminance of the original image is applied to convert the enhanced intensity image back to a color image.